A taxonomic and phylogenetic re-evaluation of Therizinosauria (Dinosauria: Maniraptora)

Abstract

Therizinosaurians are among the most poorly understood dinosaurs. Their unusual morphology and fragmentary fossil record has precluded a synthetic understanding of the group since their remains were first discovered over 60 years ago. Although the clade was recently substantiated as a monophyletic group of maniraptoran theropods, little foundational work has been conducted at the species level. A recent plethora of therizinosaurian discoveries – including the most complete primitive and derived members recovered to date – permits an alpha taxonomic and phylogenetic re-evaluation of the clade. The phylogenetic analysis presented is the most comprehensive yet conducted for Therizinosauria, and provides a foundation for scrutinizing previous definitions of Therizinosauria, Therizinosauroidea and Therizinosauridae. Here, support is provided for the maintenance of all three taxa; however, Therizinosauria is redefined and Falcarius is excluded from Therizinosauroidea. In addition, the previously described therizinosauroids, Beipiaosaurus, Enigmosaurus, Suzhousaurus, Segnosaurus and Therizinosaurus, are rediagnosed and photodocumented. In contrast to other analyses, the ingroup topology recovered in this study suggests intermediate (therizinosauroid) status for Neimongosaurus, Erliansaurus and Enigmosaurus (based on relatively primitive pelvic morphology), despite the derived forelimb anatomy evident in the former two taxa. Here, the large-bodied taxa Nothronychus and Nanshiungosaurus brevispinus are recovered as therizinosaurids. This discrepancy indicates a relatively complex pattern of mosaic evolution, which may ultimately be found to correlate with body-size trends in the clade. This work also reviews the chronostratigraphic and biogeographic distribution of therizinosaurian taxa and putatively referred elements and finds no compelling evidence of the clade outside of Asia and North America, nor for the referral of therizinosaurian materials from Kazakhstan to cf. Neimongosaurus. Time calibration of ingroup relationships indicates a pre-Turonian dispersal event is needed to account for the presence of therizinosaurids in the Late Cretaceous of North America and Asia; this conclusion supports previous hypotheses of a Laurasian faunal interchange event during the Albian.

Full text

Journal of Systematic Palaeontology, Vol. 8, Issue 4, December 2010, 503–543
A taxonomic and phylogenetic re-evaluation of Therizinosauria
(Dinosauria: Maniraptora)
Lindsay E. Zanno∗
The Field Museum, 1400 S Lake Shore Drive, Chicago, Illinois 60605, USA
(Received 11 May 2009; accepted 29 October 2009)
Therizinosaurians are among the most poorly understood dinosaurs. Their unusual morphology and fragmentary fossil record
has precluded a synthetic understanding of the group since their remains were first discovered over 60 years ago. Although
the clade was recently substantiated as a monophyletic group of maniraptoran theropods, little foundational work has been
conducted at the species level. A recent plethora of therizinosaurian discoveries – including the most complete primitive
and derived members recovered to date – permits an alpha taxonomic and phylogenetic re-evaluation of the clade. The
phylogenetic analysis presented is the most comprehensive yet conducted for Therizinosauria, and provides a foundation for
scrutinizing previous definitions of Therizinosauria, Therizinosauroidea and Therizinosauridae. Here, support is provided for
the maintenance of all three taxa; however, Therizinosauria is redefined and Falcarius is excluded from Therizinosauroidea.
In addition, the previously described therizinosauroids, Beipiaosaurus,Enigmosaurus,Suzhousaurus,Segnosaurus and
Therizinosaurus, are rediagnosed and photodocumented. In contrast to other analyses, the ingroup topology recovered in
this study suggests intermediate (therizinosauroid) status for Neimongosaurus,Erliansaurus and Enigmosaurus (based on
relatively primitive pelvic morphology), despite the derived forelimb anatomy evident in the former two taxa. Here, the large-
bodied taxa Nothronychus and Nanshiungosaurus brevispinus are recovered as therizinosaurids. This discrepancy indicates
a relatively complex pattern of mosaic evolution, which may ultimately be found to correlate with body-size trends in the
clade. This work also reviews the chronostratigraphic and biogeographic distribution of therizinosaurian taxa and putatively
referred elements and finds no compelling evidence of the clade outside of Asia and North America, nor for the referral
of therizinosaurian materials from Kazakhstan to cf. Neimongosaurus. Time calibration of ingroup relationships indicates
a pre-Turonian dispersal event is needed to account for the presence of therizinosaurids in the Late Cretaceous of North
America and Asia; this conclusion supports previous hypotheses of a Laurasian faunal interchange event during the Albian.
Keywords: Segnosauria; Theropoda; Dinosauria; systematics; biogeography; evolution
Introduction
Therizinosaurians comprise a poorly understood clade of
coelurosaurian dinosaurs that are largely marginalized as
evolutionary misfits in palaeoecological and systematic
studies of Theropoda. In contrast to the predatory adap-
tations exhibited by most theropod clades, therizinosauri-
ans are characterized by traits indicative of a plant-based
diet, such as: small, tightly packed, leaf-shaped cheek teeth;
medial deflection of the tooth row and development of
a lateral ‘shelf’ on the dentary; incisiform rostral denti-
tion (basal forms); development of a rostral rhampotheca
(derived taxa); a broad abdomen and pelvis, indicating
massive gut capacity; and secondary loss of cursorial adap-
tations (e.g. reversal to a functionally tetradactyl pes).
In keeping with their unusual and paradoxical anatomy,
therizinosaurians have endured one of the most convo-
luted taxonomic histories within Dinosauria. Pre-cladistic
hypotheses regarding their classification vary widely and
∗Email: lzanno@fieldmuseum.org
include placement within nearly all of the major dinosaurian
subclades. Early systematic studies were equally prob-
lematic, producing widely differing topological placement
(Fig. 1) and suffering from a historically inadequate fossil
record and an unexpected degree of convergence with other
herbivorous dinosaur clades. Several authors have provided
excellent reviews of the history of therizinosaurian classi-
fication. The most comprehensive review to date is that of
Zanno (2004); however, Barsbold & Marya´
nska (1990),
Clark et al. (1994, 2004), Marya ´
nska (1997, Xu et al.
(2001a) and Li et al. (2007) also provide excellent summa-
tions. A long history of taxonomic controversy notwith-
standing, Therizinosauria is now unequivocally regarded as
a monophyletic subclade of coelurosaurian theropods, and
all recent phylogenetic analyses that include therizinosauri-
ans corroborate this topology.
Although the hypothesis that therizinosaurians are
coelurosaurian theropods is widely accepted, few phyloge-
netic studies have been constructed with the explicit intent
ISSN 1477-2019 print / 1478-0941 online
Copyright C
2010 The Natural History Museum
DOI: 10.1080/14772019.2010.488045
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504 L. E. Zanno
Figure 1. Previously hypothesized outgroup relationships of
Therizinosauria. A, Barsbold & Maryanska (1990); B, Russell
& Dong (1993); C, Sereno (1999a), only the most inclusive clade
including Ornitholestes shown; D, Makovicky & Sues (1998); E,
Sues (1997); F, Holtz (1998); G, Rauhut (2003) (see Fig. 7 for
outgroup topologies from TWiG analyses). Some monophyletic
clades of OTUs combined into single branch in reproduced trees.
Therizinosauria shown in grey.
of testing their outgroup relationships, and fewer still have
attempted to resolve species-level ingroup relationships.
Several otherwise taxonomically comprehensive studies of
coelurosaurian phylogeny lack therizinosaurians altogether
(e.g. Gauthier 1986; Holtz 1994; Forster et al. 1998; Xu
et al. 1999a, 2000; Lu et al. 2004; Senter et al. 2004;
Kim et al. 2005). The consequence of this exclusion is that
nearly all modern hypotheses of coelurosaurian evolution
lack the data provided by the fossil record of this unique
maniraptoran group.
Recently, a plethora of therizinosaurian discoveries has
propelled a new understanding of therizinosaurian anatomy
and diversity. From their once poor fossil representation,
therizinosaurians are rapidly becoming one of the most
diverse theropod clades, with at least 14 species described
from Cretaceous deposits of the USA, China and Mongo-
lia. Additionally, several putative therizinosaurian elements
have been described from other countries in Asia, Europe
and North America. Within the past eight years alone, more
new therizinosaurian species have been documented than
were known during the preceding half-century of palaeonto-
logical research. Two recent, highly significant discoveries
are the most primitive and most complete therizinosaurian
yet known – Falcarius utahensis from the lower Cedar
Mountain Formation (Barremian), Utah (Kirkland et al.
2005; Zanno 2006, 2010; Smith et al. in press) – as well
as the most complete derived therizinosaurian recovered
to date, Nothronychus graffami (Gillette et al. 2001, 2005;
Zanno et al. 2009). Additional highlights include the first
feathered therizinosaurian (Xu et al. 1999b, 2003, 2009),
the first non-Asian therizinosaurians (Kirkland & Wolfe
2001; Kirkland et al. 2004, 2005; Zanno 2004, 2006, 2010;
Gillette et al. 2001, 2005), the first ontogenetic sample of a
therizinosaurian species (Zanno & Erickson 2006) and the
first therizinosaurian bone bed (Kirkland et al. 2004, 2005;
Zanno 2004, 2006, 2010).
The result of these critical discoveries has been a
dramatic increase in the amount of information applicable
to morphological, functional, palaeoecological and phylo-
genetic investigations of therizinosaurians. Yet, to date, few
advances have been made in our understanding of ther-
izinosaurian anatomy and evolution. The intent of this
study is to address these problems by investigating the
impact of recent therizinosaurian discoveries on the evolu-
tionary relationships of this enigmatic group of theropods.
Results pertaining to the outgroup relationships of ther-
izinosaurians and their palaeobiological and evolutionary
implications are published in a separate manuscript (Zanno
et al. 2009), which describes the North American ther-
izinosaurid No. graffami and presents a phylogeny founded
on the characters developed here. Primary goals of this
manuscript focus upon the ingroup taxonomy and system-
atics of Therizinosauria and include: (1) documentation
of current accessibility and condition of therizinosaurian
holotypes and referred materials; (2) photodocumentation
of therizinosaurian species thus far poorly represented in the
literature; (3) commentary on the utility of features previ-
ously used in therizinosaurian diagnoses and rediagnosis of
several species in light of more recent discoveries; (4) eval-
uation of previously proposed phylogenetic definitions for
Therizinosauria and Therizinosauroidea on the basis of a
new comprehensive phylogenetic analysis (Therizinosauri-
dae is redefined in Zanno et al. 2009); (5) commentary
on the validity of characters previously used to support
hypotheses of ingroup therizinosaurian relationships; and
(6) derivation of palaeobiogeographical and evolutionary
implications resulting from these findings.
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Re-evaluation of Therizinosauria 505
Institutional abbreviations
AMNH: American Museum of Natural History, New
Yo r k , U S A ; BMNH: Natural History Museum, London,
UK; CCMGE: Chernyshev’s Central Museum of Geolog-
ical Exploration, St Petersburg, Russia; CEUM: College
of Eastern Utah Prehistoric Museum, Price, Utah, USA;
FRDC: Fossil Research and Development Centre, Third
Geology and Mineral Resources Exploration Academy of
Gansu Province, Lanzhou, China; IGM: Geological Insti-
tute of the Mongolian Academy of Sciences, Ulaan Bataar,
Mongolia (=PST,GIN and PST GIN AN MNR); IVPP:
Institute of Vertebrate Palaeontology and Palaeoanthro-
pology, Chinese Academy of Sciences, Beijing, China;
IZK: Laboratory of Paleobiology, Institute of Zoology,
National Academy of Science, Alma-Ata, Kazakhstan; LH:
Long Hao Geologic and Paleontological Research Centre,
Department of Land Resources of Nei Mongol, Hohhot,
China; MCF PVPH: Museo Carmen Funes, Paleontolog´
ıa
de Vertebrados, Plaza Huincul, Argentina; MSM:Mesa
Southwest Museum, Mesa, Arizona, USA; NGMC (=
GMV): National Geological Museum of China, Beijing,
China; CMN (=MNC): Canadian Museum of Nature,
Ottawa, Ontario; PIN: Paleontological Institute, Russian
Academy of Sciences, Moscow, Russia; TMP: Royal
Tyrrell Museum of Palaeontology, Drumheller, Alberta;
UMNH: Utah Museum of Natural History, Salt Lake City,
Utah, USA; YPM: Peabody Museum of Natural History,
New Haven, Connecticut, USA; ZIN PH: Palaeoherpeto-
logical Collection, Zoological Institute, Russian Academy
of Sciences, St Petersburg, Russia.
Material and methods
Specimen availability
The following re-evaluation considers only the 15
named therizinosaurian species (Table 1). Unfortu-
nately, a large percentage of representative speci-
mens, including many holotype specimens, were inac-
cessible for the duration of this study. Materials
pertaining to Falcarius,Beipiaosaurus,Alxasaurus,No.
mckinleyi,No. graffami,Enigmosaurus,Erliansaurus,
Neimongosaurus,Segnosaurus,Therizinosaurus, Nanshi-
ungosaurus brevispinus,Suzhousaurus and several Ther-
izinosauria incertae sedis were examined first hand and/or
from casts of specimens. Although some materials were
available for study, a large portion of published specimens
of Alxasaurus,Therizinosaurus and Nanshiungosaurus
were not accessible. In addition, all specimens of the ther-
izinosaurids Erlikosaurus and ‘Nanshiungosaurus’bohlini
were inaccessible for the duration of this project; there-
fore comparisons for these taxa were made using the
literature. The taxonomy of the putative therizinosaurian
Eshanosaurus is not revisited here (see Xu et al. 2001;
Clark et al. 2004; Kirkland et al. 2005; Barrett 2009).
Although Eshanosaurus may represent a therizinosaurian,
as suggested by several authors (Zhao & Xu 1998; Xu
et al. 2001; Barrett 2009), comparative mandibles from
Segnosaurus,Erlikosaurus and Alxasaurus were unavail-
able during this study; thus the taxonomic affinity of
Eshanosaurus is not formally addressed here. Specific
information on the availability of therizinosaurian speci-
mens is detailed below.
Falcarius utahensis Kirkland et al., 2005. All UMNH
materials referred to Falcarius were included in this study.
Specimens collected by CEUM from a second locality,
although examined, are not considered here, asit has not yet
been determined if these materials are referable to Falcarius
utahensis. For additional information on available materi-
als, see the Systematic Palaeontology section below.
Beipiaosaurus inexpectus Xu et al., 1999b. The majority
of the original Beipiaosaurus inexpectus holotype mate-
rials (IVPP V11559) described by Xu et al. (1999b) are
available for study at the IVPP and remain in good condi-
tion. A caudal vertebra, ungual, a putative ischial frag-
ment, fragments of the right forelimb, and several unidenti-
fied fragments possibly representing skull material are not
currently associated with the rest of the holotype. Addi-
tional, subsequently discovered holotype materials (IVPP
V11559) described by Xu et al. (2003) are referenced from
the literature.
Alxasaurus elesitaiensis Russell & Dong, 1993. For the
duration of this study, the majority of Alxasaurus elesi-
taiensis holotype materials (IVPP 88402, two individuals)
could not be located (X. Xu pers. comm., June 2006);
thus only a fraction of the holotype individual was exam-
ined first hand. Available holotype specimens included: two
sacral? vertebrae, five dorsal vertebrae, 14 caudal verte-
brae, 10 chevrons, proximal dorsal ribs, two appendicular
fragments, distal carpal 1, an unidentified carpal (radiale?),
and two manual unguals (likely I-II and II-III). Many of
these elements were not marked with the holotype acces-
sion number IVPP 88402; however, they are consistent with
elements listed by Russell & Dong (1993).
Currently accessible referred elements from IVPP 88501
include three dorsal centra, one-and-a-half sacral vertebrae,
proximal portions of metacarpal I and II, distal portion of
manual PII-I, proximal PI-II and PIII-IV, additional manual
phalanges fragments, distal fragments of metatarsals, and
proximal and distal fragments of pedal phalanges. Several
unlabelled elements could not be confidently associated
with a particular specimen, including proximal metacarpal
I, proximal metacarpal II and proximal manual PIII-III.
Neimongosaurus yangi Zhang et al., 2001 and
Erliansaurus bellamanus Xu et al., 2002a. During this
project, the holotype of Neimongosaurus yangi (LH V0001)
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506 L. E. Zanno
Tab l e 1 . Stratigraphic, biogeographic and taxonomic data for therizinosaur species. Chronostratigraphic data from Barsbold (1983), Jerzykiewicz & Russell (1991), Wolfe &
Kirkland (1998), Swisher III et al. (1999, 2002), Shuvalov (2000), Tang et al. (2001), von Itterbeeck et al. (2005), Greenhalgh et al. (2006), Albright et al. (2007a, b) and Zhu
et al. (2007).
Species Occurrence Age Country Nominal reference Other references
Falcarius utahensis Yellow Cat Mb. Cedar
Mountain Fm.
>125 Ma Barremian USA, Utah Kirkland et al. 2005 Zanno 2006, 2010
Beipiaosaurus inexpectus Lower Yixian Fm. 125 Ma Barremian People’s Republic of China,
Liaoning
Xu et al. 1999b Xu et al. 2003
Alxasaurus elesitaiensis Bayin Gobi Fm. Albian People’s Republic of China,
Nei Mongol Zizhiqu
Russell & Dong 1993
Enigmosaurus mongoliensis Bayan Shire Fm. Cenomanian-Turonian Mongolian People’s
Republic
Barsbold 1983 Barsbold & Perle 1980
Suzhousaurus
megatheroides
Xinminpu (=Xinminbao)
Group
?Aptian-Albian People’s Republic of China,
Gansu
Li et al. 2007
Nothronychus mckinleyi Moreno Hill Fm. Middle Turonian USA, New Mexico Kirkland & Wolfe 2001
Nothronychus graffami Tropic Shale Early Turonian USA, Utah unpubl. data Albright et al. 2002;
Gillette et al. 2001, 2005
Erliansaurus bellamanus Iren Dabasu Fm. Campanian-Maastrichtian People’s Republic of China,
Nei Mongol Zizhiqu
Xu et al. 2002a
Neimongosaurus yangi Iren Dabasu Fm. Campanian-Maastrichtian People’s Republic of China,
Nei Mongol Zizhiqu
Zhang et al. 2001
“Nanshiungosaurus”
bohlini
Xinminpu (=Xinminbao)
Group
?Aptian-Albian People’s Republic of China Dong & Yu 1997
Nanshiungosaurus
brevipsinus
Yuanpu Fm. (=Nanxiong) Campanian-Maastrichtian People’s Republic of China,
Guandong
Dong 1979
Erlikosaurus andrewsi Bayan Shire Fm. Cenonmanian-Turonian Mongolian People’s
Republic
Perle 1981 Barsbold & Perle 1980;
Barsbold 1983; Clark et
al. 1994
Segnosaurus galbinensis Bayan Shire Fm. Cenonmanian-Turonian Mongolian People’s
Republic
Perle 1979 Barsbold & Perle 1980;
Barsbold 1983
Therizinosaurus
cheloniformis
Nemegt Fm. Early Maastrichtian Mongolian People’s
Republic
Maleev 1954 Barsbold 1976, 1983; Perle
1982
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Re-evaluation of Therizinosauria 507
and a portion of the holotype of Erliansaurus bellamanus
(LH V0002) were under study at the University of Chicago.
Currently unaccessioned research casts of Neimongosaurus
and Erliansaurus provided by the University of Chicago
were also used as comparative material during this project.
Several elements not noted by Zhang et al. (2001) and Xu
et al. (2002a) were included in these cast materials, yet were
not ascribed to one taxon or the other. This complication,
combined with the similar morphology of these taxa and the
presence of duplicate elements (such as three different right
femora), suggests more material from these taxa exists than
has been noted previously in the literature. This hampered
the revision of these species and is further discussed below.
Enigmosaurus mongoliensis Barsbold, 1983. The holo-
type specimen of Enigmosaurus mongoliensis (IGM
100/84) is in good condition and includes the sacrum,
ventral portions of both ilia, complete right and left pubes
and complete left ischium. A number of other fragmen-
tary elements not mentioned in the original descriptions of
the holotype are labelled with the same specimen number,
including a crushed proximal femur, a large femoral shaft
(tibia?), dorsal ribs, distal humerus, and a putative radius
and proximal ulna, all in poor condition. Although the large
femur is clearly not associated with the holotype individ-
ual, it can be assigned to Therizinosauria incertae sedis.The
remaining postcranial elements might reasonably pertain to
the holotype.
‘Nanshiungosaurus’ bohlini Dong & Yu, 1997. The
current whereabouts of the holotype of ‘Nashiungosaurus’
bohlini (IVPP V11116) are unknown (X. Xu pers. comm.,
June 2006). Therefore comparisons with ‘N.’ bohlini were
made using the descriptions and figures in Dong & Yu
(1997), which consist of the first nine cervical vertebrae
in ventral view (Dong & Yu 1997, p. 91, fig. 1) and the
10th cervical in ventral and dorsal views (Dong & Yu 1997,
p. 94, fig. 2).
Nothronychus mckinleyi Kirkland & Wolfe, 2001 and
Nothronychus graffami Zanno et al., 2009. Cast materi-
als of the holotype of No. mckinleyi (MSM P-2117) were
examined for this study, including several elements not
mentioned in Kirkland & Wolfe (2001). These elements
are detailed in the Systematic Palaeontology section below.
A second specimen of Nothronychus,No. graffami
(UMNH VP 16420), was discovered penecontemporane-
ously with No.mckinleyi and represents a new, more robust
species from a lower stratigraphic horizon. No. graffami
(Zanno et al. 2009) is the most complete therizinosaurid
known, missing only the skull, a majority of the cervi-
cal series and a few elements of the distal extremities. All
elements of No.graffami were examined at first hand.
Suzhousaurus megatherioides Li et al., 2007. All holo-
type materials of Suzhousaurus megatherioides (FRDC-
GSJB-99) were examined first hand. The referred speci-
men, FRDC-GSJB-2004–001, is referenced from Li et al.
(2008).
Nanshiungosaurus brevispinus Dong, 1979. Only the
pelvis of Nashiungosaurus brevispinus (IVPP V4731) can
be located at the IVPP (X. Xu pers. comm., June 2006). The
holotype presacral vertebrae are not accessible. The pelvis
of N. brevispinus has suffered postcollection damage and
is reconstructed with painted plaster in several areas. The
distal left preacetabular blade is missing, as are the distal
portions of the left pubis and ischium illustrated in Dong
(1979).
Two cervical and a single dorsal vertebra are illustrated
in Dong (1979, figs 1–3). Dong (1979, pl. 2) also figures
a photograph of the holotype in left lateral view and three
photographs of different vertebrae in ventral, cranial, or
caudal view. These figures, as well as written descriptions,
were used to supplement first-hand observation of inacces-
sible elements.
Erlikosaurus andrewsi Barsbold & Perle, 1980. The
holotype of Erlikosaurus andrewsi (IGM 100/1111)
includes the only well-preserved therizinosaurian skull.
Unfortunately, the skull of Erlikosaurus (as is also the
case with the mandible of Segnosaurus and Alxasaurus)is
currently on loan to A. Perle (C. Tsogtbaatar pers. comm.,
November 2006) and was not accessible during this study,
nor was any of the postcranial material of Erlikosaurus.
Thus comparisons with Erlikosaurus were made using the
detailed work of Clark et al. (1994) on the skull anatomy,
and descriptions and figures provided by Barsbold & Perle
(1980), Perle (1981) and Barsbold (1983).
Segnosaurus galbinensis Perle, 1979. There are numer-
ous problems associated with specimens referred to
Segnosaurus galbinensis, including postcollection damage,
missing elements from the holotype, incorrect identification
of referred elements, and more than one individual and/or
taxon bearing the same specimen number. Currently it is
easier to review those specimens available for study than
those that are presently inaccessible.
Accessible elements of the holotype (IGM 100/80)
include a severely damaged right ilium with sacrum missing
left sacral ribs (with damage to the preacetabular portion
such that it cannot properly be conjoined with the remainder
of the ilium), pubis missing proximal portion, and ischium
with damage to proximal portion. The holotype mandible
was not accessible during this study, nor were any forelimb,
hind limb or axial elements.
Accessible paratype elements consist of the following:
IGM 100/81, nearly complete left tibia with astragalus
and calcaneum, and left fibula (noted as right elements by
Barsbold & Perle 1980); IGM 100/82, nearly complete left
ilium, with damage to dorsal iliac blade, shaft fragment of
right femur, distal tibia with astragalus, possible distal tarsal
(unnumbered), and calcaneum; and IGM 100/83, complete
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508 L. E. Zanno
left humerus, radius, and ulna, fragmentary proximal right
humerus, damaged right radius, fragmentary cervical neural
arch.
Additional elements bearing the specimen number
100/82, yet not noted in Perle (1979), were found in the IGM
collection. These include a proximal left femur and a large
postacetabular portion of left ilium of a different individual
with ischiadic peduncle. Whereabouts of the paratype pes,
fibula, ribs, ischium and pubis (IGM 100/82), and scapulo-
coracoid and manual phalanges (IGM 100/83) are unknown
(C. Tsogtbaatar, pers. comm. November 2006).
Therizinosaurus cheloniformis Maleev, 1954. Ther-
izinosaurus cheloniformis is known from several speci-
mens, including the holotype (PIN 551-483; Maleev 1954)
and four referred specimens housed at the IGM representing
the forelimb (Barsbold 1976) and hind limb (Perle 1982).
None of these specimens were available for examination for
the duration of this study. Several Therizinosaurus speci-
mens are part of a travelling exhibit that was not on display
during this project. The whereabouts of other IGM mate-
rials are unknown (C. Tsogtbaatar pers. comm., November
2006). A privately owned cast of the right manus of Ther-
izinosaurus with unguals was used as comparative material,
although the cast appears to be a composite of IGM 100/15,
100/16, and/or 100/17. Descriptions and figures provided
by Maleev (1954), Barsbold (1976) and Perle (1982) were
also used in this study.
Taxonomic considerations
The discovery of increasingly primitive and complete ther-
izinosaurians has rendered most original diagnoses ineffec-
tive; thus nearly all therizinosaurian diagnoses are in need
of re-evaluation. A species-level taxonomic re-evaluation of
Therizinosauria is critical at this juncture because present
species diagnoses include many synapomorphic and/or
symplesiomorphic features which have hindered recent
attempts to diagnose taxa effectively (e.g. Kirkland & Wolfe
2001; Li et al. 2007).
The diagnoses given here provide updated differentiae
for known therizinosaurian species. Unfortunately, the poor
quality, fragmentary nature, and inaccessibility of many
therizinosaurian specimens proved an impediment to the
rediagnosis of several taxa. For those taxa that could not
be properly evaluated, yet are clearly in need of a viable
rediagnosis, see ‘Remarks.’ When a revised diagnosis for
a previously described taxon is published for the first time
herein, the subheading is listed as ‘Revised Diagnosis’. If a
previously proposed diagnosis is considered effective and
is maintained, a citation for the first publication of that
diagnosis is provided (e.g. ‘Diagnosis [Xu et al. 2002a]’).
Every attempt was made to maintain viable differentiae
provided by earlier authors; however, the retention of previ-
ously proposed differentiae for therizinosaurian species
generally did not prove useful. Cited characteristics within
a rediagnosis are indicative of a maintained differentia.
Wherever possible the rediagnoses presented below high-
light autapomorphic morphology; however, the fragmen-
tary nature of most therizinosaurian species and the inacces-
sibility of holotype and referred materials often precluded
the identification of enough autapomorphic features to
create a stable diagnosis. In these instances, the diag-
noses were supplemented with features differential at the
comparative ingroup level in addition to autapomorphies.
All diagnostic features listed in the diagnoses provided here
are figured as reference points for future work, except
those differentiating Falcarius, which has already been
extensively photo-documented in the literature (Kirkland
et al. 2005; Zanno 2006, 2010) and both species of the
genus Nothronychus, for which an illustrated diagnosis
has already been published (Zanno et al. 2009, electronic
supplementary material).
Several authors have proposed taxon-based or phylo-
genetic definitions for the supertaxa Therizinosauria,
Therizinosauroidea and Therizinosauridae, and their
junior subjective synonyms, Segnosaurischia, Segnosauria,
Segnosauridae, Enigmosauridae, Erlikosauridae and
Nanshiungosauridae (Perle 1979; Barsbold & Perle 1980;
Russell & Dong 1993; Dong & Yu 1997; Russell 1997;
Sereno 1998, 1999a, Xu et al. 1999b, 2001, 2002a; Ryan
& Russell 2001; Zhang et al. 2001; Clark et al. 2004; Kirk-
land et al. 2005; Zanno 2006). Yet these definitions have
never been evaluated in light of a comprehensive, species-
rich phylogeny, which should form the foundation for the
construction of a phylogenetic taxonomy de facto. Here
previously proposed definitions are scrutinized in light of
node stability and the degree of morphological differentia-
tion illustrated by character change at nodes. Phylogenetic
definitions are constructed following the rules proposed
by the International Code of Phylogenetic Nomenclature
(Phylocode), Version 4b (Cantino & de Queiroz 2007).
Neither the Phylocode (2007, Article 4.3) nor the Inter-
national Rules of Zoological Nomenclature (1999, Article
9.8) recognizes the validity of new taxonomic names and/or
definitions proposed solely via the internet (e.g. Taxon-
Search); thus definitions proposed via this format are not
considered valid. Following the Phylocode (2007, Recom-
mendation 6.1A, B), rank-based taxa are denoted by [R] in
the Systematic Palaeontology section to help avoid confu-
sion between these and phylogenetic-based taxa.
Operational taxonomic units
Base operational taxonomic units (OTUs)aretaken
from the Turner et al. (2007a) version of the
Theropod Working Group (TWiG), which includes
three therizinosaurians: Segnosaurus,Erlikosaurus and
Alxasaurus. In order to represent all currently known
therizinosaurian species, 11 OTUs were added: Ther-
izinosaurus cheloniformis,Nanshiungosaurus brevispinus,
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Re-evaluation of Therizinosauria 509
‘Nanshiungosaurus’bohlini,Suzhousaurus megatheri-
oides,No. mckinleyi,No. graffami,Enigmosaurus
mongoliensis,Erliansaurus bellamanus,Neimongosaurus
yangi,Beipiaosaurus inexpectus and Falcarius utahen-
sis. The analysis was further expanded by the addition of
Mahakala ¨
omnog¨
ovae,Dilong paradoxus,Coelurus frag-
ilis,Yanornis martini,Jeholornis prima,Protarchaeopteryx
robust us,Caenagnathus collinsi,Caenagnathus sternbergi
(but see below) and Hagryphus giganteus. The default
matrix (Appendix 1) represents a 34% increase in OTUs
over the base TWiG; however, ad hoc pruning of the matrix
during analysis resulted in the use of smaller datasets using
various subsets of available OTUs (see Results).
A large percentage of therizinosaurian materials were
inaccessible for the duration of this project. Many spec-
imens are damaged and the whereabouts of others are
unknown. A detailed review of the therizinosaurian mate-
rials examined for this analysis is provided (see Specimen
Availability) and specifics regarding potentially problem-
atic OTUs are provided below.
Therizinosaurians. With the exception of the poorly
known taxon Eshanosaurus (Xu et al. 2001), all named
therizinosaurian species are included in this analysis as
OTUs. The contention regarding the classification of
Eshanosaurus is not likely to be resolved until addi-
tional materials are recovered (see Geographic Distribution
of Therizinosauria). Furthermore, including Eshanosaurus
would fail to test currently competing hypotheses regard-
ing its evolutionary relationships because this analysis
is restricted to coelurosaurian theropods, which would
constrain Eshanosaurus to this clade (and probably Ther-
izinosauria) by default.
A syntype of the tyrannosaur Alectrosaurus olseni
(AMNH 6368) from the Iren Dabasu Formation is now
recognized as belonging to Therizinosauria (Mader &
Bradley 1989). In a phylogenetic analysis restricted to fore-
limb material, Zanno (2006) found the specimen to be
closely related to Erliansaurus, also from the Iren Dabasu
Formation, yet the fragmentary nature of the specimen and
poor preservation do not permit referral to this taxon. More-
over, compared to Erliansaurus, AMNH 6368 represents a
significantly larger and more gracile individual; thus all
that can be confidently inferred about this specimen is
that it represents an intermediate-grade therizinosaurian.
AMNH 6368 was not included within any OTU; nor was
it coded as an individual taxon in this analysis, as it
does not possess unique character combinations (Wilkinson
1995).
The fragmentary therizinosaurian ‘Nanshiungosaurus’
bohlini (IVPP V11116) was coded based on descrip-
tions and photos provided by Dong & Yu (1997, pp.
91, 94). As noted by Li et al. (2007), it is unlikely
that IVPP V11116, from the Early Cretaceous of the
Xinminpu Group (Mazongshan, Gansu Province, north-
western China), represents the genus Nanshiungosaurus
from the Late Cretaceous Yuanpu Formation (south-eastern
China); however, this specimen also cannot currently be
referred to Suzhousaurus, known from potentially correl-
ative sediments. Although IVPP V11116 is not presently
accessible for study and is not well documented in the liter-
ature, the specimen was included here as a separate OTU
in an attempt to test its phylogenetic relationships.
Perle (1982) referred the therizinosaurian hind limb
IGM 100/45 to Therizinosaurus cheloniformis based on
its stratigraphic and geographic proximity to a forelimb
referred to this taxon (IGM 100/15). Although IGM 100/45
cannot unequivocally be referred to Therizinosaurus due
to an absence of overlapping elements, this specimen was
included in the Therizinosaurus OTU.
Oviraptorosaurians. Although North American ovirap-
torosaurians are known from only fragmentary specimens,
their unique morphology relative to Asian oviraptorosauri-
ans renders them a critical component of the clade. North
American oviraptorosaurian taxonomy is contentious, but
is represented by unique taxa, including several poten-
tially synonymous species (e.g. Caenagnathus collinsi,
Caenagnathus sternbergi,Chirostenotes pergracilis and
Elmisaurus elegans).
No record could be located of the specimens representing
the Chirostenotes OTU in various iterations of the TWiG;
however, mandibular characters are coded for this taxon in
TWiG matrices, leading to the likelihood that Chirostenotes
includes specimens described as the genus Caenagnathus.
Preliminarily, a conservative approach was taken by remov-
ing the mandibular characters from the OTU Chirostenotes,
and coding the two species of Caenagnathus (C.collinsi
and C.sternbergi) as independent OTUs. However, C.
sternbergi and C.collinsi differ in only a single char-
acter state (#70) and add a large percentage of missing
data to the analysis (C.collinsi 7% complete, C.stern-
bergi 6%). Thus both ‘species’ were combined as Caenag-
nathus sp. and character 70 was noted as polymorphic for
this OTU. Several authors argue for the synonymization of
Chirostenotes and Caenagnathus (Currie & Russell 1988;
Sues 1997); however, there is currently no more compelling
evidence for their synonymization than for their distinction.
When coded as a distinct OTU, Caenagnathus is comprised
of a unique set of character combinations; as such, its inclu-
sion is warranted either as a distinct taxon or as part of the
Chirostenotes hypodigm. Iterations of the analysis under
both conditions were conducted here to examine the differ-
ence in topology (see Results).
Senter et al. (2004) considered Incisivosaurus (IVPP
V13326, lower Yixian Formation; Xu et al. 2002b) a
subjective junior synonym of Protarchaeopteryx (NGMC
2125; Chaomidianzi Formation; Ji & Ji 1997; Ji et al. 1998)
based on the possession of incisiform first premaxillary
teeth, peg-like second premaxillary teeth, lanceolate
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510 L. E. Zanno
maxillary teeth, and an edentulous rostral dentary in both
taxa, and suggested maintaining taxonomic distinction
only at the species level. The potential congeneric status of
these species is irrelevant for the purposes of phylogenetic
analysis, and these taxa are maintained as distinct OTUs in
this study.
Other coelurosaurians. Turner et al. (2007a) follow
Makovicky et al. (2005) in considering Neuquenraptor
(MCF PVPH 77) a junior synonym of Unenlagia (MCF
PVPH 78); this coding is maintained here. Following Turner
et al. (2007a), Microraptor gui is not considered part
of the Microraptor zhaoianus hypodigm (these two taxa
plus Crypotovolans pauli were considered junior subjec-
tive synonyms in Senter [2007]). The generic distinction
of Gorgosaurus and Albertosaurus (as per Holtz 2004) is
maintained; thus the OTU formerly listed as Albertosaurus
libratus in the TWiG is listed as Gorgosaurus libratus,as
in Senter (2007).
A partial dentary has been referred to the type spec-
imen of Coelurus agilis (YPM 2010), which has been
synonymized with Coelurus fragilis (Ostrom 1980; Carpen-
ter et al. 2005). Carpenter et al. (2005) questioned the asso-
ciation of the dentary because of its slender morphology.
Due to its ambiguous association, a conservative approach
was taken and the dentary was not coded within the Coelu-
rus OTU.
Outgroups. Following the general practice of the TWiG,
Allosaurus fragilis is maintained as the outgroup taxon
in this analysis. Despite its consistent use as an outgroup
taxon for phylogenetic studies of coelurosaurian theropods,
Allosaurus fragilis is poorly figured in the current litera-
ture. First-hand observation of specimens accessioned at
the UMNH necessitated the revision of certain charac-
ter codes and led to repolarization of several characters
(see Appendix 2: Annotated List of Added and Modified
Characters).
Characters. The base character set of 251 characters was
adopted from a recent TWiG analysis (Turner et al. 2007a).
Base TWiG character 110 (number of sacral vertebrae) was
revised according to Turner et al. (2007b). Base TWiG char-
acters 39, 40, 84, 138 and 142 were revised based on Senter
(2007). Different base versions of the TWiG data matrix
are used in both studies, hence character numbers vary
between Senter (2007) and this analysis. Likewise, Turner
et al. (2007b) and this paper use different revised versions
of the base characters 136, 148 and 151; therefore derived
states for these characters may represent different charac-
ter conditions. Explanations for changes made to the base
TWiG in this analysis are given in Appendix 2 (Annotated
List of Added and Modified Characters).
In total 97 characters were added to the base TWiG,
resulting in a 348 character database (complete character
list and data matrix provided in Appendix 1). These addi-
tions (50% novel, 50% adapted from cited sources) repre-
sent a 39% increase in character information. The database
used in this study is nearly identical to the base TWiG in
data per skeletal region (see Table 2), although the new char-
acter database contains a higher percentage of postcranial
(75% versus 69%) characters overall, as is necessary given
the relative over-abundance of therizinosaurian postcranial
materials. Commentary on newly added characters is also
provided in Appendix 2.
Phylogenetic methods
The dataset was constructed in MacClade v. 4.0 (Maddison
& Maddison 2000) and analysed using TNT (Tree analy-
sis using New Technology) v. 1.1 (Goloboff et al. 2008).
Most parsimonious trees (MPTs) were obtained via heuris-
tic search methods on 1000 replicates of Wagner trees
with random addition sequences and then subject to TBR
(tree bisection-reconnection) swapping methods holding 10
trees per replicate. Subsequent rounds of TBR swapping
were performed to capture additional MPTs. All charac-
ters were equally weighted. Base TWiG characters 165
and 215 were excluded from this analysis because they
are redundant, with revisions to characters 230 and 66
respectively.
A conservative approach was taken in the ordering of
characters. In an attempt to accommodate missing data in
the study without inflating tree length and reduce apriori
assumptions regarding character homology, multistate char-
acters were designated additive (ordered) only if they repre-
sent nested states of change within a transformation series
for which the relationship and polarity of the characters are
understood. Characters 27, 37, 40, 68, 76, 78, 97, 106, 113,
157, 163, 168, 253, 303, 308, 309, 310, 334, 342 and 345
were designated additive in this study.
Ambiguous nodes were collapsed following Rule 1
of Coddington & Sharff (1994). Maximum agreement
subtrees (Finden & Gordon 1986) calculated using TNT
were used to identify labile taxa and common topology
among all MPTs. Resampling methods were also conducted
in TNT and included standard bootstrapping (1000 repli-
cates; Felsenstein 1985) and symmetrical resampling (1000
replicates; Goloboff et al. 2003). Mesquite (Maddison &
Maddison 2008) and MacClade v. 4.0 (Maddison & Maddi-
son 2000) were used for ad hoc data collection includ-
ing examining character optimization and distribution, and
generating tree statistics.
Systematic palaeontology
Dinosauria Owen, 1842
Saurischia Seeley, 1887
Theropoda Marsh, 1881
Coelurosauria von Huene, 1914
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Re-evaluation of Therizinosauria 511
Tab l e 2 . Character support by skeletal region. Data from this study is compared to the base TWiG (Turner et al. 2007a). The percent increase in characters is calculated by dividing
the number of new characters per skeletal region by the total number of characters per skeletal region in this dataset, not as compared to those listed in Turner et al. 2007a. New
characters refer to both novel characters and characters added to the base TWiG from cited sources.
Cranial Mandibular Dentition Axial Pectoral Forelimb Pelvic Hind limb
Dataset %total %incr. %total %incr. %total %incr. %total %incr. %total %incr. %total %incr. %total %incr. %total %incr.
Rank 15526433
65343132
This analysis 25 10 7 13 7 29 16 24 4 29 11 53 15 29 15 44
Rank 1 — 5 — 7 — 2— 8 — 6— 3— 4—
Turn e r et al. 2007a 31 — 8 — 6 — 16 — 4 — 7 — 15 — 12 —
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512 L. E. Zanno
Therizinosauria Russell, 1997 nomen cladi conversum
1980 Segnosauria[R] Barsbold & Perle
1992 Segnosaurischia[R] Dong
1998 Therizinosauridae Sereno
1999a Therizinosauridae Sereno
2001 Therizinosauroidea Zhang et al.
2002a Therizinosauroidea Xu et al.
Definition. The most inclusive clade containing Ther-
izinosaurus cheloniformis but not Tyrannosaurus rex,
Ornithomimus edmontonicus,Mononykus olecranus,
Oviraptor philoceratops or Troodon formosus.
Heterodefinitional junior synonyms. Therizinosauroidea
(Zhang et al. 2001); Therizinosauroidea (Xu et al.
2002a); Therizinosauridae (Sereno, 1998); Therizinosauri-
dae (Sereno, 1999a).
Subjective junior synonyms. Segnosaurischia[R] (Dong,
1992); Segnosauria[R] (Barsbold & Perle 1980).
Remarks. Russell (1997) coined the name Therizinosauria
and provided a branch-based definition and diagnosis for
the clade. Russell’s (1997) definition is given as Alxas-
aurus,Enigmosaurus,Erlikosaurus,Nanshiungosaurus,
Segnosaurus,Therizinosaurus, and all taxa more closely
related to them than to oviraptorosaurians, ornithomimids
and troodontids. The inclusion of six therizinosaurian
species as internal specifiers is now superfluous and renders
the definition unstable if one of these genera should be
reclassified or considered a junior synonym (Padian &
May 1993; Padian et al. 1999; Sereno 1999b). In an
attempt to promote stability, the redefinition provided
here uses a single internal qualifier: the type species,
Therizinosaurus cheloniformis. The utility of multiple
external specifiers is also contentious (Padian et al. 1999)
and carries the same potential problems as multiple inter-
nal specifiers; however, as Sereno (1998, 1999b) noted,
this problem can be minimized by using nested rather than
basal taxa (or clades, e.g. Russell 1997) to guard against
potential discrepancies in taxonomic content even under
circumstances of varying topology. Furthermore, multiple
external specifiers are currently necessary as outgroup rela-
tionships of Therizinosauria remain somewhat unstable.
Recent phylogenetic studies have suggested close evolu-
tionary relationships between therizinosaurians and ovirap-
torosaurians, alvarezsaurids, ornithomimosaurians and/or
troodontids (see Table 3); thus the use of multiple external
Tab l e 3 . Previous hypotheses of therizinosaur outgroup relationships. Shown are selected recent phylogenetic analyses of
Coelurosauria. Abbreviations: AL, Alvarezsauridae; AV, Avialae; D, Dromaeosauridae; OR, Ornithomimosauria; OV,
Oviraptorosauria; TH, Therizinosauria; TR, Troodontidae; TY, Tyrannosauridae.
(Therizinosauria/Oviraptorosauria) Monophyly
TWiG base Other Partial skeletal analyses
Novas et al. 2008 Xu et al. 2006 Frankfurt & Chiappe 1999
Xu et al. 2007 Rauhut & Xu 2005 Zanno, 2006
Turn e r et al. 2007a,b Xu & Wang 2004
G¨
ohlich & Chiappe 2006 Holtz et al. 2004
Norell et al. 2001, 2006 Holtz & Osm´
olska 2004
Xu & Zhang 2005 Rauhut, 2003
Novas & Pol 2005 Novas & Pol 2002
Kirkland et al. 2005 Holtz, 2001
Mayr et al. 2005 Makovicky & Sues 1998
Calvo et al. 2004 Holtz, 1998
Xu & Norell 2004 Sues, 1997
Makovicky et al. 2003, 2005
Hwang et al. 2002, 2004
Clark et al. 2002
Xu et al. 1999b, 2002b, c
(Therizinosauria(Alvarezsauridae(Oviraptorosauria(Paraves))))
This paper (and Zanno et al. 2009)
Senter, 2007
Other topologies
Russell & Dong 1993 ((O/TR)TH)OR))
Sereno 1997, 1998 ((OR/TH)(TY(OV(AV(TR/DR)))))
Sereno, 1999a (((AL/OR)TH)(TY(OV(AV(TR/D)))))
Paul, 1984 ((Ornithischia)(TH)(Saurischia))
Barsbold & Marya´
nska 1990 ((Sauropodamorpha)(TH)(Theropoda))
Marya´
nska et al. 2002 ((AV/OV)TH)
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Re-evaluation of Therizinosauria 513
qualifiers for the clade is apropos (Phylocode 2007, Recom-
mendation 11E). Here Mononykus is chosen because of
its derived status within Alvarezsauridae. Emendment of
Therizinosauria is conducted under the guidelines of the
PhyloCode (2007), which permit unrestricted emendations
when the definitional type is maintained and the intent is to
preserve conceptualization of the clade as intended by the
original author (Art. 15.1, 15.8, 15.11). Both conditions are
met here.
Both Therizinosauridae and Therizinosauroidea were
given phylogenetic definitions postdating Russell (1997)
that rendered them junior heterodefinitional synonyms
to Therizinosauria. Therizinosauroidea[R] was coined
by Russell & Dong (1993) as a rank elevation for
Therizinosauridae[R] (Maleev, 1954). Russell & Dong
(1993, p. 2121) provide a taxon-based definition for Ther-
izinosauroidea[R] as the clade encompassing Alxasaurus
and ‘therizinosaurids’, and added an extensive diagnosis.
In their description of the primitive therizinosauroid
Beipiaosaurus,Xuet al. (1999b) assigned this taxon to
Therizinosauroidea[R] although a phylogenetic definition
expanding Therizinosauroidea beyond that given by
Russell & Dong (1993) was not provided. Xu et al. (2001)
further assigned the enigmatic taxon Eshanosaurus to the
clade Therizinosauroidea[R]. Thus it is clear that Xu et al.
(1999b) intended the taxon Therizinosauroidea to be more
inclusive than might be interpreted from Russell & Dong
(1993).
Accordingly, Zhang et al. (2001) provided the first
formal phylogenetic definition for Therizinosauroidea:
all coelurosaurians closer to Therizinosaurus than to
Ornithomimus,Oviraptor,Velociraptor or Neornithes.
However, in doing so they rendered Therizinosauroidea a
junior synonym of Therizinosauria (sensu Russell, 1997),
leaving Therizinosauroidea open for redefinition (Art.
15.5). Clark et al. (2004) corrected this mistake by emend-
ing the definition of Therizinosauroidea as the least inclu-
sive clade containing Therizinosaurus and Beipiaosaurus.
Having composed their manuscript prior to the publi-
cation of Clark et al. (2004), Kirkland et al. (2005)
assigned the basal-most therizinosaurian Falcarius to Ther-
izinosauroidea sensu Zhang et al. (2001) and this assign-
ment is maintained in Zanno (2006, 2010). Here the node-
based definition provided by Clark et al. (2004) is followed,
which serves to exclude Falcarius from Therizinosauroidea
on the basis of its relatively primitive morphology. Further
support for this reassignment is provided in Zanno (2010)
and in the synapomorphy list provided in Appendix 3.
Falcarius utahensis Kirkland et al., 2005
Holotype. UMNH VP 15000, partial juvenile braincase.
Referred specimens. Over 3000 complete or partial
elements have been collected from the holotype locality and
referred to Falcarius utahensis. Currently only a small frac-
tion of these materials have been prepared and are available
for study. Although the majority of the skeleton is repre-
sented in the collection, portions of the axial column and a
large portion of the skull are currently unknown. Accession
numbers and identifications for catalogued referred speci-
mens can be found in Kirkland et al. (2005), Zanno (2006,
in 2010) and Smith et al. (in press). More recent materials
are on file at the UMNH.
Occurrence. Crystal Geyser Quarry (CGQ) approxi-
mately 12 miles SE of Green River, UT; lower Yellow Cat
Member of the Early Cretaceous Cedar Mountain Forma-
tion; minimally dated at 124.5 ma (Greenhalgh et al. 2006).
Diagnosis. A therizinosaurian bearing the following
autapomorphies: antorbital fenestra extends onto lateral
margin of nasal; facet for postorbital on frontal rostral to
postorbital process; expansive, deeply depressed, and highly
pneumatic subcondylar and subotic recesses each possess-
ing multiple pneumatic fossae; inflated basisphenoid with
hypertrophied basisphenoidal recess; rostral dentary teeth
conical, extending to, at minimum, the fifth dentary tooth
position; cupped, dorsoventrally elongate rostral teeth lack-
ing denticles; median ridge dividing ventral sulcus of
cervical vertebrae; infraprezygapophyseal fossa on cranial
dorsal vertebrae divided into three accessory fossae; acces-
sory caudal centrodiapophyseal lamina on cranial dorsal
vertebra; hypertrophied, obliquely oriented humeral entepi-
condyle, with concave caudal margin, and well-defined
groove extending proximally up humeral shaft; flexor tuber-
cle on manual PI-II with deep, distally positioned collat-
eral ligament pits; proximal pubic tubercle (sensu Hutchin-
son 2001) well developed and caudolaterally oriented,
occurring at cranialmost margin of acetabulum; midseries
chevrons possessing markedly distinct cranial tubercles
(Zanno 2010).
Remarks. Falcarius utahensis is the most basal and most
complete therizinosaurian yet recovered and is known from
a mass death assemblage estimated to hold hundreds of indi-
viduals of various growth stages. The anatomy of this taxon
is the best documented among therizinosaurians, having
been extensively reviewed in three separate manuscripts
(Zanno 2006, 2010; Smith et al. in press) following its
initial description (Kirkland et al. 2005). Due to the compre-
hensive literary treatment of this taxon, a re-evaluation of
the genus Falcarius is unnecessary. The original diagnosis
presented in Kirkland et al. (2005) was emended in Zanno
(2010).
Therizinosauroidea Russell & Dong, 1993 (sensu Clark
et al., 2004)
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514 L. E. Zanno
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Re-evaluation of Therizinosauria 515
Definition. The least inclusive clade containing Beip-
iaosaurus inexpectus and Therizinosaurus cheloniformis
(Clark et al. 2004).
Remarks. Under the ICZN, Maleev (1954), author of
Therizinosauridae, is credited with establishing Ther-
izinosauroidea (Russell & Dong 1993; Clark et al. 1994;
Zhang et al. 2001; Xu et al. 2001, 2002a; Kirkland et al.
2005; Zanno 2006, 2010; Li et al. 2007). However, under
the Phylocode (2007, Note 9.8A.2), Therizinosauroidea is
attributable to those who coined it (i.e. Russell & Dong
1993), as referenced in Xu et al. (1999b).
Since its establishment as a higher rank taxon for
Therizinosauridae[R] (Russell & Dong 1993), Ther-
izinosauroidea[R] has become the taxon of choice for
the most inclusive grouping of therizinosaurians. As
discussed above, the lack of a formal phylogenetic defi-
nition upon establishment (Russell & Dong 1993) and the
obscure publication of both the branch-based, phylogenet-
ically defined Therizinosauria by Russell (1997) and the
heterodefinitional taxon Therizinosauridae by Sereno (in
a table, Sereno 1998, p. 65; and as a footnote, Sereno
1999c, p. 2147) resulted in referral of several recent basal
therizinosaurian discoveries to Therizinosauroidea (sensu
Zhang et al. 2001) rather than Therizinosauria (sensu
Russell, 1997) or Therizinosauridae (sensu Sereno 1999a)
(see Xu et al. 1999b; 2002a; Zhang et al. 2001; Kirk-
land et al. 2005). Whereas it might seem appropriate to
conserve one of these more widely used taxa for the group,
maintaining Therizinosauria on the basis of precedence
follows the general intent of the Phylocode (2007, Article
14.2). Furthermore, the high degree of morphological diver-
sity encompassed by known therizinosaurian taxa, together
with the morphological disparity between Falcarius and
more derived therizinosaurians, renders the maintenance
of all three taxa of taxonomic and practical utility. This
can only be accomplished if Therizinosauria is used for
the most inclusive branch-based clade. Finally, the node-
based phylogenetic definition provided here encompasses
the historical taxonomic content of Therizinosauroidea as
given by most authors, including those conducting explicit
phylogenetic studies (Xu et al. 1999b; Clark et al. 2004),
those recently adding to the diversity of the clade (Russell
& Dong 1993; Xu et al. 1999b; 2002a; Kirkland & Wolfe
2001; Zhang et al. 2001), and those using a historical
conceptualization of the clade (e.g. Perle 1979, 1981; Bars-
bold & Perle 1980; Barsbold 1983; Barsbold & Marya´
nska
1990; Clark et al. 1994; 2004; Marya´
nska, 1997; Dong &
Yu 1997). The only exceptions are the most recent concep-
tualizations of the clade (Kirkland et al. 2005; Zanno 2006,
2010; Li et al. 2007; Senter 2007).
Beipiaosaurus inexpectus Xu et al., 1999b
Holotype. IVPP V11559, immature individual (Xu et al.
1999b) represented by cranial material, rostral portion of
mandible, and nearly complete postcranial skeleton. Addi-
tional materials of the holotype were discovered subse-
quently and published in a separate manuscript (Xu et al.
2003). These include the right ilium missing the postacetab-
ular portion, both ischia, two sacral vertebrae, 30 caudal
vertebrae including pygostyle.
Occurrence. Sihetun locality near Beipiao, Liaoning,
China. Lower Yixian Formation, Early Cretaceous
(Barremian, 125 Ma; Swisher III et al. 1999, 2002; Zhu
et al. 2007).
Revised diagnosis. (Fig. 2A–E). A therizinosaurian bear-
ing the following autapomorphies: elongate lateral articular
surface on manual phalanx I-I (Xu et al. 1999b); obtura-
tor process of ischium sinusoidal, with ventrally deflected
distal portion; ischial boot approximately twice craniocau-
dal depth of distal shaft; low ridge on cranial femoral
shaft extending proximally from medial condyle. Posses-
sion of four fused caudal dorsals and a pygostyle incorpo-
rating up to seven caudal vertebrae may also be autapo-
morphic for this species, as well as the presence of a
prominent triangular flange extending from ventrolateral
surface of metacarpal I, which although found on other
theropod taxa is likely of autapomorphic morphology in
Beipiaosaurus and may be homologous with rectangular
buttress on advanced taxa.
Remarks. Beipiaosaurus is consistently recovered as one
of the most basal therizinosaurians (Xu et al. 1999b; Clark
et al. 2004; Kirkland et al. 2005; Zanno 2006, 2010; Li
et al. 2007; Senter 2007). In the initial and still most exten-
sive description of the taxon, Xu et al. (1999b) provided a
←−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
Figure 2. Diagnostic features of the holotype of Beipiaosaurus (IVPP V11559) (A-F)andAlxasaurus (IVPP 88402) (G). A, ventral view
of dorsal vertebrae, showing fusion; B, left lateral view of caudal vertebrae with pygostyle; C, ventral view of right metacarpal I, showing
triangular flange; D, ventral view of right manual phalanx I-I, showing enlarged lateral condyle; E, lateral view of right ilium and ischium,
showing ratio of ischial boot to distal shaft and sinusoidal obturator process; F, cranial view of right femur, showing craniomedial ridge
on shaft; G, cranial and left lateral views of chevron. Abbreviations: af, articular facet on chevron; cv, caudal vertebrae; dvf, fused dorsal
vertebrae; ib, ischiadic boot; il, ilium; lc, lateral condyle; lfb, lateral flare of cranial chevron blade; mc, medial condyle, MCI, metacarpal
one; mfr, ridge on cranial femoral shaft; op, obturator process; PI-I, first phalanx of digit one; py, pygostyle; rf, right femur; ris, right
ischium; and tf, ventrolateral triangular flange. Photographs not to scale.
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516 L. E. Zanno
differential diagnosis focusing on its primitive morphology
relative to other therizinosaurians (e.g. relatively large skull,
functionally tridactyl pes, bearing proximally compressed
first metatarsal, unexpanded preacetabular process of ilium,
elongate manus; tibia elongate relative to femur; and prox-
imally compressed metatarsus). Although apropos at the
time of its composition, the original diagnosis has been
rendered insufficiently differential by the discovery of the
more basal taxon Falcarius, necessitating re-evaluation
of Beipiaosaurus. Although none of the character states
provided in this rediagnosis are recognized on other ther-
izinosaurians, some may represent an early stage in a contin-
uum rather than discrete morphologies (e.g. ischial boot
twice width of distal shaft; triangular flange on metacarpal
I), whereas the absence of others may be preservational
(e.g. presence of a pygostyle). Additional features are not
autapomorphic among coelurosaurians, yet are included
because they are presently apomorphic (and thus differ-
ential) among therizinosaurians (e.g. cranial ridge on distal
portion of femur, also present on several oviraptorosaurians,
but not on more basal therizinosaurians).
Alxasaurus elesitaiensis Russell & Dong, 1993
Holotype. IVPP 88402 (larger individual), right dentary
and most of postcranial skeleton including five disartic-
ulated cervical vertebrae, two cervical ribs, seven caudal
disarticulated vertebrae, three right and three left dorsal
ribs, sacrum, four sacral ribs, 21 caudal vertebrae, 15
chevrons, scapula, left and right coracoids, left and right
humeri, radius, left and right ulnae, left and right distal
carpal 1, distal carpal 2, ulnare?, radiale, metacarpals I-III,
manual PI-I-II, II-I-III, III-I, III-II, left and right ilia, left
and right ischia, left and right femora (Russell & Dong
1993).
Referred specimens. IVPP 88402 (small individual),
three articulated caudal cervicals and/or cranial dorsals,
four articulated dorsal vertebrae; IVPP 88501 (immature
individual), five dorsal vertebrae, first sacral, second sacral,
right metacarpal I, left metacarpal III, manual PI-I-I-II, II-I-
II-III, III-II, and III-IV, left postacetabular portion of ilium,
left and right femora, left and right tibiae, left and right
fibulae, MTI-IV, pedal PII-I-III, III-I-IV, IV-I-V, IVPP 88301
(scattered vertebrae, ribs, and appendicular material), IVPP
88510, appendicular material.
Occurrence. Bayin Gobi Formation (lacustrine facies),
Inner Mongolia, Albian (Jerzykiewicz & Russell 1991).
Remarks. Russell & Dong (1993, p. 2108) provided the
original diagnosis for Alxasaurus: “dentary teeth number
approximately 40; symphyseal region of dentary bears
teeth; ribs not fused to cervical centra; ligament pits often
well developed in manual phalanges; ilium not greatly
shortened craniocaudally; preacetabular ala moderately
expanded, ungual shorter than or subequal to first phalanx
in pedal digits II-IV”. The original diagnosis suffers from
the same problems as that given for Beipiaosaurus in that
it is composed of plesiomorphies which, in light of recent
discoveries, are no longer of use as differentiae (see also
Clark et al. 2004). Although clearly necessary, the rediag-
nosis of Alxasaurus must await the availability of the type
materials (see Materials and Methods). A single autapo-
morphic feature can be recognized on the type materials
available for this study: cranial blade of proximal chevrons
with profound lateral flare at region of bifurcation into artic-
ular facet (Fig. 2F–G).
Neimongosaurus yangi Zhang et al., 2001
Holotype. LH V0008: partial braincase; cranial end of
right dentary; majority of axial column except atlas; mid
and caudal dorsal vertebrae; distal-most caudal vertebrae;
pectoral girdle and forelimb elements, including the left and
partial right scapulocoracoids, furcula, both humeri, and
left radius; pelvic girdle and hind limb elements including
partial left and right ilia, both femora and tibiae, left distal
tarsals three and four, and partial left pes. The axial column
and pes were discovered in articulation.
Referred specimens. LH V 0008, sacrum with six coossi-
fied vertebrae articulated with both ilia.
Occurrence. Sanhangobi, Sunitezuoqi, Nei Mongol
Autonomous Region (20 km south-west of Erlian city);
Iren Dabasu Formation, Late Cretaceous (Campanian-
Maastrichtian; von Itterbeeck et al. 2005).
Remarks. Zhang et al. (2001, p. 39) provided the follow-
ing diagnosis for Neimongosaurus: “cranial caudal verte-
brae with circular fossa under transverse process; radius
with prominent biceps tuberosity; proximal pedal phalanges
with well developed heels; tibia with extremely long fibular
crest exceeding half total tibia length; and lateral surface of
preacetabular process twisted to face dorsally; and caudal
vertebrae are characterized by widely divergent prezy-
gapophyses (distribution poorly known among other ther-
izinosauroids)”.
Of these characters, cranial caudal vertebrae with a circu-
lar fossa under the transverse process are also found in
other therizinosaurians (e.g. No. mckinleyi and No.graf-
fami) as well as in oviraptorosaurians (e.g. Nomingia,
IGM 100/119), and proximal pedal phalanges with well-
developed heels occur in No.mckinleyi and No.graf-
fami. Possession of an elongate fibular crest exceeding
half the total length of the tibia (approximately 57% in
Neimongosaurus) also does not significantly differ from
the condition in Nothronychus (approximately 55% in No.
graffami). The presence of a preacetabular process in
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Re-evaluation of Therizinosauria 517
which the lateral surface is twisted so as to face dorsally
is a derived therizinosaurian feature (e.g. Nothronychus,
Segnosaurus,Nanshiungosaurus brevispinus), probably the
result of lateral deflection of the preacetabular portion of
the ilium. Zhang et al. (2001) note that widely divergent
caudal prezygapophyses are potentially diagnostic, yet the
prezygapophyses of Neimongosaurus are not unusual with
respect to the condition in other therizinosaurians (Zanno
in press). The radius of Neimongosaurus does possess a
more proximally located and pronounced biceps tubercle
than is known for other therizinosaurians (e.g. Segnosaurus,
Falcarius), so at present this taxonomic distinction appears
to be valid. Neimongosaurus is not rediagnosed here (see
Erliansaurus below).
Erliansaurus bellamanus Xu et al., 2002a
Holotype. LH V 0002: five vertebrae, left forelimb lacking
carpus, partial right ilium and fragments of the ischium and
pubis, right femur, both tibiae, right fibula, several partial
metatarsals.
Occurrence. Sanhangobi, Sunitenzuoqi, Nei Mongol
Autonomous Region (20 km south-west of Erlian city);
Iren Dabasu Formation, Late Cretaceous (Campanian-
Maastrichtian; von Itterbeeck et al. 2005).
Remarks. Xu et al. (2002a, p. 230) provided the following
diagnosis for Erliansaurus: “enlarged nutritional foramen
on cranial caudals; crest-like caudal trochanter on humerus
bordered medially by oval depression; rugose swelling
located dorsal to ischiadic peduncle on lateral surface of
ilium; fibular proximal end with caudal margin significantly
higher than cranial margin; and distally located hypertro-
phied cranial trochanter on fibula”.
The humerus of Erliansaurus was not examined first-
hand; however, an oval depression in the region of the
caudal trochanter is present on other therizinosaurians
(e.g. Falcarius,Neimongosaurus) and other coelurosauri-
ans (e.g. Coelurus,Ornitholestes), although not coupled
with a caudal trochanter as in Erliansaurus. The crest-
like morphology of the caudal trochanter in Erliansaurus
is unknown in other taxa. The ilium of Erliansaurus is
severely crushed and badly preserved. Thus the rugose
swelling located dorsal to the ischiadic peduncle is most
likely to be the craniolateral edge of the cubic tuberosity
present on other derived therizinosaurians (Segnosaurus,
Enigmosaurus and Nothronychus), rather than an autapo-
morphy as suggested by Xu et al. (2002a). Enlarged nutri-
ent foramen on cranial caudal vertebrae and proximal fibula
with the caudal margin significantly higher than the cranial
margin are autapomorphic features, whereas a distally
located hypertrophied cranial trochanter on fibula is also
known in Nothronychus.
The Erliansaurus bellamanus holotype comes from
the same quarry as the holotype and referred material
of Neimongosaurus yangi (Xu et al. 2002a). Research
cast material obtained for both Erliansaurus and Neimon-
gosaurus (see Specimen Availability above) indicate the
presence of more than two individuals, although this cannot
be confirmed in the literature. Xu et al. (2002a) note the
presence of a cranial caudal vertebra among the holo-
type materials for Erliansaurus bellamanus possessing an
incompletely fused neural arch. This feature, coupled with
the larger size of several Erliansaurus elements, indicates
that this taxon achieved significantly larger adult body sizes
than Neimongosaurus. The argument of Xu et al. (2002a)
for the presence of two species is compelling if the materials
belong to only two individuals and the elements associated
with each taxon have been properly assigned. Quarry maps
showing the distribution of elements referred to these two
taxa could not be obtained. Thus these taxa could not be
re-evaluated with any confidence.
Enigmosaurus mongoliensis Barsbold, 1983
Holotype. IGM 100/84; pelvic girdle lacking dorsal aspect
of ilia and distal ischia.
Occurrence. Bayan Shire Formation (Baynshirenskaya),
Khara Khutul, south-east Mongolia, Cenomanian-Turonian
(Shuvalov 2000).
Revised diagnosis. A therizinosaurian bearing the follow-
ing autapomorphies: dorsoventrally shallow pubic boot
with pronounced, subequal cranial and caudal processes;
pubic boot fused, enlarged medially, with medial expansion
forming V-shaped trough (adopted from Barsbold 1983);
medial fusion of obturator process and pubic apron form-
ing tetraradiate process (Fig. 3).
Remarks. The original diagnosis for Enigmosaurus (Bars-
bold, 1983) contains two characters: pubis and ischium
narrow; and cranial presymphyseal region of distal pubis
with elongated indentation. Possession of a [craniocau-
dally] narrow pubis and ischium is plesiomorphic for
the group. The second feature (elongated indentation of
the pubic boot) is incorporated in the revised diagnosis
presented here.
A key point not previously noted in the literature is
that the holotype pelvis of Enigmosaurus mongoliensis
is clearly abnormal, possessing numerous resorption pits
and areas of bone remodelling. As these features appear
to be restricted to the dorsal iliac blade, the ilium has not
been referenced in this diagnosis. However, if the abnor-
mal condition of the ilium is the result of advanced onto-
genetic stage in this individual, this finding could have
implications for the unusually extreme medial fusion of the
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518 L. E. Zanno
Figure 3. Diagnostic features of Enigmosaurus mongoliensis holotype pelvis (IGM 100/84). A, cranial view; B, dorsal view; C, oblique
caudal/left lateral view; and D, right lateral view, showing shallow pubic boot with subequal cranial and caudal processes and abnormalities
in the dorsal ilium; E, inset of tetraradiate obturator process in dorsal view; and F, pubic boot in oblique cranial/right lateral view, showing
fused v-shaped trough. Abbreviations: a1–4, abnormal holes 1–4; at, antitrochanter; cbt, cubic tuberosity of postacetabular process; cp,
caudal process of ischium; cpb, caudal process of pubic boot; crpb, cranial process of pubic boot; dmi, dorsal margin of ilium; ilp, iliac
peduncle of pubis; lis, left ischium; lp, left pubis; nsr, neural spine ridge, sacrum; op, obturator process; ris, right ischium; rp, right pubis;
sv, sacral vertebra; tr, medial trough on pubic boot. Scale bar 10 mm (D only).
obturator process and pubic boot, both of which are consid-
ered autapomorphic in this diagnosis. Further study of the
holotype is necessary to determine the nature and extent of
these influences.
A large left femur (estimated length 105 cm) is labelled
IGM 100/84, yet is clearly not associated with the pelvis
due to its massive size, which more closely approximates
that of Segnosaurus.
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Re-evaluation of Therizinosauria 519
Suzhousaurus megatherioides Li et al., 2007
Holotype. FRDC-GSJB-99; an associated partial postcra-
nial skeleton, including 10 partial dorsal vertebrae,
dorsal rib fragments, right scapulocoracoid, right humerus,
isolated pubic peduncle of left ilium, nearly complete left
and fragmentary right pubes.
Paratypes. FRDC-GSJB-2004–001; partial postcranial
skeleton, including three dorsal vertebrae, sacrum, six
caudal vertebrae, dorsal ribs, chevrons, nearly complete left
ilium (missing ventral-most preacetabular process), nearly
complete left pubis (missing boot), complete left ischium,
left femur, distal right femur.
Occurrence. Yujingzi Basin, north-western Gansu
Province, China. Xinminpu (=Xinminbao) Group, Early
Cretaceous (Aptian-Albian, Tang et al. 2001).
Revised diagnosis. Suzhousaurus can be differentiated
from all other therizinosaurians by the following autapo-
morphies: gently rounded craniodorsal margin of preac-
etabular process of ilium (Li et al. 2008); obturator fora-
men, craniocaudal width greater than dorsoventral height;
obturator notch on pubis deep (approximately half the maxi-
mum dorsoventral length of ischiadic peduncle of pubis)
and wide (approximating craniocaudal dimension of proxi-
mal pubic shaft); pubis short, maximum length nearly sube-
qual with maximum dorsoventral height of ilium; ischiadic
shaft distal to ventral caudal process deflected caudally.
Remarks. The original diagnosis for Suzhousaurus given
by Li et al. (2007) consists of two features: shallow, poorly
demarcated glenoid fossa with “prominent rounded and
striated tumescence” on dorsomedial surface of its scapular
portion; and pubis with strongly concave cranial margin (Li
et al. 2007, p. 541). Based upon newly described paratype
materials, Li et al. (2008) added the following features to
their diagnosis: transverse expansion of the distal aspect
of dorsal, sacral and cranial caudal neural spines; “laterally
deflected, thin, and flat” preacetabular process of ilium; and
“smoothly curved” craniodorsal margin of preacetabular
process of ilium (Li et al. 2008, pp. 769–770).
Transverse expansion of the caudal aspect of the distal
neural spine is present on the dorsal vertebrae of Falcarius
(also concomitant with bifurcation in therizinosaurians).
Both cranial and caudal expansion is present in the small-
bodied taxon Neimongosaurus, although the feature is more
subtle in this species. At first inspection this feature appears
to be related to ossified reinforcement along the dorsal
axial series, which may be correlated with increased body
size in some clades. This characteristic is found in large-
bodied theropod taxa such as Allosaurus fragilis and Tyran-
nosaurus rex. However, as the dorsal neural spines are
also caudally bifurcate in Falcarius and Mononychus,this
feature also appears to carry phylogenetic significance.
The circumglenoid region in the holotype of
Suzhousaurus megatherioides (FRDC-GSJB-99) is
damaged; thus although it can be ascertained that the
glenoid faces ventrally in this taxon, poor demarcation of
the articular surface of the glenoid cannot be confirmed.
Even if this character can be demonstrated on additional
specimens of Suzhousaurus,No. mckinleyi also exhibits
poor differentiation of the glenoid, therefore this feature is
not autapomorphic. Additionally, transverse thickening of
the scapulocoracoid at the contact between these elements
is common in derived therizinosaurians and noted to
be extreme in Therizinosaurus (Barsbold 1976, 1983),
indicating it may be related to large adult body-size.
Although Barsbold’s (1976) description of this feature in
Therizinosaurus suggests that it mirrors the condition of
Suzhousaurus, the scapulocoracoid of Therizinosaurus
could not be examined first hand and has never been
figured in medial view. Thus the contention that the degree
of thickening in Suzhousaurus is sufficiently distinct to be
considered autapomorphic cannot presently be verified or
invalidated.
A laterally deflected, [transversely] thin preacetabu-
lar process is probably a characteristic of derived ther-
izinosaurians and is documented in several taxa, includ-
ing Segnosaurus,Nanshiungosaurus brevispinus and No.
graffami. The body of this process is ‘flat’ (i.e. not medi-
ally or laterally curved) in at least N. brevispinus, and may
also be in No. graffami and Beipiaosaurus although crush-
ing in these specimens renders this feature ambiguous. The
smoothly curved craniodorsal margin of the preacetabu-
lar process noted by Li et al. (2008) remains a potential
autapomorphy of this taxon. The craniodorsal aspect of the
preacetabulum forms a more acute angle in No. graffami
and N. brevispinus, the only taxa in which it is completely
preserved. However, the condition is unknown in many taxa
including Falcarius,Neimongosaurus,Enigmosaurus and
Segnosaurus, making comparison difficult.
Li et al. (2007, 2008) noted that the pubis of
Suzhousaurus exhibits a greater degree of concavity than is
present in all other therizinosaurians for which this element
is known. While the pubis of Suzhousaurus is indeed more
concave than Falcarius,Beipiaosaurus and Enigmosaurus,
examination of the pubes of Segnosaurus (IGM 100/80)
and No. graffami (UMNH VP 16420) reveal an almost
identical degree of curvature; thus this condition cannot be
considered autapomorphic in Suzhousaurus. In sum, prior
diagnoses provided for Suzhousaurus based on holo- and
paratype materials contains only a single valid autapomor-
phy, therefore a rediagnosis is presented here.
Therizinosauridae Maleev, 1954
1979 Segnosauridae Perle
1983 Enigmosauridae Barsbold
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520 L. E. Zanno
1997 Nanshiungosauridae Dong & Yu
2001 Erlikosauridae Ryan & Russell
2001 Therizinosauridae Zhang et al.
2002a Therizinosauridae Xu et al.
2004 Therizinosauridae Clark et al.
Definition. The least inclusive clade containing Nothrony-
chus (UMNH VP 16420), Segnosaurus galbinensis (IGM
100/80), Erlikosaurus andrewsi (IGM 100/111) and Ther-
izinosaurus cheloniformis (PIN 551–483) (Zanno et al.
2009).
Subjective junior synonyms. Enigmosauridae[R] (Bars-
bold, 1983), Nanshiungosauridae[R] (Dong & Yu 1997),
Erlikosauridae[R] (Ryan & Russell 2001); Therizinosauri-
dae (Zhang et al. 2001); Therizinosauridae (Xu et al.
2002a); Therizinosauridae (Clark et al. 2004).
Remarks. The taxon Therizinosauridae has undergone
several proposed modifications in recent years. Zanno
et al. (2009) provide an extensive review of the definitional
history of the taxon and provide a slightly modified defini-
tion for the clade. Their definition, based on a comprehen-
sive phylogenetic analysis derived from this study, includes
Nothronychus (UMNH VP 16420) – the most complete
derived therizinosaurian known to date – as an internal
specifier and removes Nanshiungosaurus on the basis of the
current inaccessibility of specimens. Here, Suzhousaurus
and Enigmosaurus are considered outside Therizinosauri-
dae based on the recovery of these species as sister-taxon to
the clade in the agreement subtree of all MPTs, or when the
labile taxon Therizinosaurus is pruned from the analysis
(see Results).
As in Zanno et al. (2009), the clades Enigmosauri-
dae[R] (Barsbold, 1983), Nanshiungosauridae[R] (Dong &
Yu 1997) and Erlikosauridae[R] (Ryan & Russell 2001) are
considered junior synonyms of Therizinosauridae. Alxas-
auridae[R] (Russell & Dong 1993), although currently
consisting of only a single species and of dubious future util-
ity, cannot be rendered a junior synonym of Therizinosauri-
dae and remains valid.
Nothronychus Kirkland & Wolfe, 2001
Type species. No. mckinleyi Kirkland & Wolfe, 2001.
Occurrence. Moreno Hill and Tropic Shale formations,
Late Cretaceous, early-to-middle Turonian (Wolfe & Kirk-
land 1998).
Diagnosis. A therizinosaurian bearing the following
autapomorphies: scapula with dorsoventrally tapering
blade; distinctly subcircular obturator process; ovoid,
dorsoventrally elongate obturator foramen (maximum
length greater than 200% of maximum width); contact
between pubis and ischium restricted to proximal half of
obturator process; and presence of a deep notch between
ventral aspect of obturator process and cranial ischial shaft
(Zanno et al. 2009).
Remarks. The genus Nothronychus is currently repre-
sented by two species, No. mckinleyi and No.graffami,from
the upper Cretaceous of New Mexico and Utah respectively.
The type species, No. mckinleyi was the first definitive ther-
izinosaurian to be discovered outside of Asia. Discovery
of a second species referable to the genus – No. graffami
from slightly older sediments – necessitated revising the
diagnosis of No. mckinleyi into genus- and species-level
characters. Zanno et al. (2009) provided a thorough discus-
sion of the methodology used in creating the diagnoses for
the genus Nothronychus, the species No. mckinleyi and No.
graffami. Their diagnoses are followed here.
Nothronychus mckinleyi Kirkland & Wolfe, 2001
Holotype. MSM P-2117, partially disarticulated skeleton
including isolated teeth, partial braincase, two potential
cranial fragments, fragments of eight cervical vertebrae,
one cranial dorsal vertebra, one dorsal centrum, caudal
vertebrae 6 or 7, cervical and dorsal ribs, fused gastralia,
chevron fragment, left scapula, right humerus, complete
right ulna, unidentified antebrachial shaft, distal metacarpal
II, manual PI-I and PII-I, both ischia, both tibiae, right
fibula, partial left metatarsal, pedal PI-I, PIII-III, PIV-II,
pedal unguals I-IV.
Occurrence. Moreno Hill Formation, southern Zuni
Basin, Catron Co., New Mexico, Late Cretaceous, Middle
Turonian (Wolfe & Kirkland 1998).
Diagnosis. No. mckinleyi can be differentiated from all
other therizinosaurians by the following autapomorphies:
ventral notch between obturator process and iliac shaft cran-
iocaudally narrow; and from No.graffami by the follow-
ing combination of features: platycoelous cranial caudal
centra; heel on caudoventral surface of caudal centra hyper-
trophied (approximately one third the dorsoventral length
of the caudal centrum); glenoid lacking caudal buttress;
caudal process of ischium poorly defined and elongate,
extending to shaft opposite obturator process (Zanno et al.
2009).
Remarks. Based on comparisons with the new species
No. graffami, Zanno et al. (2009) revised the holotype of
No.mckinleyi to include previously unidentified elements
(left pedal PI-I, dorsal centrum, and chevron fragment),
refinement of previously identified elements (metacarpals,
manual phalanges, and a caudal vertebra), and misidentified
elements (pedal PIV-III and PIV-IV, reidentified as PIII-III
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Re-evaluation of Therizinosauria 521
and PIV-II and two manual unguals now recognized to be
pedal).
Nothronychus graffami Zanno et al., 2009
Holotype. UMNH VP 16420, partial skeleton missing
skull, most cervical vertebrae, several dorsals, several
caudals.
Occurrence. Tropic Shale, Late Cretaceous (Early Turo-
nian) approximately 4.5 metres below the Lowest Strati-
graphic Datum of the Mammites nodosoides ammonite
zone (Albright et al. 2007a, b),southern margin of
Kaiparowits Basin, Kane County, Utah.
Diagnosis. No. graffami can be differentiated from all
other therizinosaurians by the following autapomorphies:
neural spines of sacrum ankylosed into dorsoventrally
low continuous ridge with individual spines unidentifiable;
pubic boot with small yet pronounced knob-like caudal
process; ventral margin of pubic boot dorsally convex
between cranial and caudal process; caudal process of
ischium dorsoventrally restricted (less than 5% of total
length of ischium), subtriangular, and dorsally positioned;
caudal process of ischium located proximal to obtura-
tor foramen. No. graffami can be differentiated from
No.mckinleyi by the following four features: strongly
amphicoelus cranial caudal centra; heel on caudoventral
surface of caudal centra poorly developed; glenoid with
pronounced caudal buttress; ventral notch between obtura-
tor process and iliac shaft craniocaudally wide (Zanno et al.
2009).
Remarks. No. graffami is the most complete derived ther-
izinosaurian known to date and serves as an important
reference point for comparing the anatomy of more poorly
preserved species. Thus its inclusion as an internal specifier
for the taxon Therizinosauridae bolsters character support
and stability of this node.
Nanshiungosaurus brevispinus Dong, 1979
Holotype. IVPP V 4731, ilium missing ventral preacetab-
ular process, proximal portions of the pubis and ischium,
mostly complete vertebral series from axis to first caudal
vertebrae.
Occurrence. Yuanpu (Nanxiong) Formation, Dapingcun,
Shuikou Commune, (Dong 1979, English translation by
Will Downs), Guandong, China, Upper Cretaceous (Dong
1992).
Remarks. Lacking access to the holotype vertebrae and
without preservation of the distal pubis and ischium,
Nanshiungosaurus brevispinus is difficult to diagnose. The
diagnosis provided by Dong (1979) was based on the assign-
ment of N.brevispinus to Titanosaurinae. This misiden-
tification combined with the discovery of other derived
therizinosaurian taxa renders Dong’s (1979, p. 343) differ-
entiae undiagnostic: “short neck with platycoelous cranial
cervicals, pleurocoels undeveloped, neural spines low, and
caudal series not distinctly bifid; 12 cervical vertebrae with
centrum length 2.5 times that of dorsal centra; ten dorsal
vertebrae with platycoelous centra of equivalent height and
length and shallow pleurocoels; neural spines low and trans-
versely broadened with a broad apex; five fused sacral
centra with short unified neural spines, inflated apices, and
saddle-shaped depressions; ilium low with extremely well-
developed narrow and elongated preacetabular process;
pubic peduncle of ilium straight and robust; pubis linear
with thick lateral margin and closed obturator foramen;
ischium thinly plate-shaped with expansive and fused distal
ends; acetabulum large and circular”. A single feature from
the original diagnosis – opisthocoelous caudal cervical
centra – is retained here as a potential autapomorphy, as
it is undocumented in other therizinosaurians.
The reconstruction of the pubis and ischium provided
by Dong (1979, p. 345) is likely to be incorrect in that it
portrays the ischium terminating at the obturator process
rather than extending beyond, as in all other therizinosauri-
ans.
Erlikosaurus andrewsi Barsbold & Perle, 1980
Holotype. IGM 100/111, nearly complete skull with
mandible, several disarticulated fragmentary cervical verte-
brae, left humerus, articulated right pes missing proximal
ends of two metatarsals (Clark et al. 1994). (Barsbold &
Perle [1980] note a left and right pes with the holotype;
Perle [1981] notes only a pes).
Occurrence. Baysheen Tsav locality (=Baynshin,
Bayshin, or Bainshin, Tsav; Clark et al. 1994), Bayan
Shireh Svita (Barsbold & Perle 1980; Perle 1981) (=Bayn-
shirenskaya Svita; Tsybin & Kurzanov 1979; Shuvalov &
Chkikvadze 1979), south Gobi ( ¨
Omnog¨
ov) Aimak, Mongo-
lia (Clark et al. 1994). Upper Cretaceous, Cenomanian to
Turonian (Shuvalov 2000).
Remarks. The original diagnosis for Erlikosaurus by
Barsbold & Perle (1980, p. 190): “medium-sized [ther-
izinosaurian] with laterally compressed pedal unguals”
does not adequately diagnose this taxon. Barsbold (1983,
p. 105) added two characters to the diagnosis: “mandibular
teeth small and closely positioned; and rostral portion of
lower jaw edentulous”. However, these characters are more
widespread within the clade. Clark et al. (1994) redescribed
the skull of Erlikosaurus and provided a comprehen-
sive diagnosis on the basis of this study. However, as
noted by Clark et al. (1994), the near complete absence
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522 L. E. Zanno
of comparative cranial materials prevented assessment
of these features as autapomorphic, synapomorphic or
plesiomorphic. The recent discovery of therizinosaurian
cranial materials pertaining to Falcarius (Zanno 2010) and
No. mckinleyi (Kirkland & Wolfe 2001) is shedding light on
the utility of these features in differentiating Erlikosaurus.
The following features noted as possible autapomorphies
by Clark et al. (1994, pp. 36–37) are now known to have
a broader distribution among therizinosaurians: edentulous
premaxilla with sharp, vertical ventrolateral edge; vomer
extremely elongate and extending caudally to meet cultri-
form process; parabasisphenoid with extremely large pneu-
matic spaces; external auditory meatus restricted ventrally
by lateral expansion of braincase; medial wall of antorbital
fossa extensive; and maxilla with medially inset dentition
and few nutrient foramina on caudal part of facial process.
The taxonomic distribution of the following features (Clark
et al. 1994, pp. 36–37) remains unknown: extremely elon-
gate nares due to regression of the maxilla; antorbital fossa
with well-developed over-hanging lip; medial wall of antor-
bital fossa imperforate; caudal process of jugal covering
cranial surface of quadratojugal; and a homodont maxil-
lary dentition of numerous (23) small, lanceolate, coarsely
serrated, unrecurved, transversely flattened teeth constricted
at the base. Unfortunately, the skull of Erlikosaurus could
not be accessed during this study and the holotype postcra-
nial materials cannot be located (see Specimen Availabil-
ity), preventing a re-evaluation of Erlikosaurus.
Segnosaurus galbinensis Perle, 1979
Holotype. IGM 100/80 (incorrectly noted as 100/90 in
Barsbold [1983, English translation by C. Siskron & S. P.
Welles]); mandible, incomplete humerus, complete radius
and ulna, manual phalanges including one ungual, nearly
complete pelvis, incomplete right femur, 10 cranial caudals,
15 caudal caudals, dorsal and ventral rib fragments (Perle
1979). Barsbold (1983, English translation by C. Siskron
& S. P. Welles) also lists carpal elements with the holotype,
although their presence could not be confirmed.
Paratypes. IGM 100/81, left tibia and fibula (noted as
right in Barsbold & Perle 1980); IGM 100/82, right femur,
tibia, fibula, tarsals III and IV, metatarsals I–V, five pedal
phalanges including one pedal ungual, rib fragments, ilium,
proximal ischium and distal pubis; IGM 100/83 left scapu-
locoracoid, humerus, radius, ulna, manual unguals, cervical
fragment (Perle 1979; Barsbold & Perle 1980). Barsbold
(1983) noted the presence of two additional referred spec-
imens, IGM 100/87 and IGM 100/88, consisting of frag-
ments of a postcranial skeleton including a pelvis. Given the
presence of several apparent typographical errors regard-
ing specimen numbers in the manuscript, these may be the
known paratypes IGM 100/82 and IGM 100/83.
Holotype occurrence. Bayan Shireh Svita (=Baynshiren-
skaya [Barsbold 1983], Bayan Shiren Svita [Marya´
nska
1997], and Bayaanshiree [Shuvalov 2000]) Upper Creta-
ceous (Cenomanian-Turonian; Shuvalov 2000) locality
Amtgay (Amtgai), Mongolia, and Khara-Khutul, Bayshin-
Tsav and Urilbe-Khuduk, Mongolia (Barsbold & Perle
1980; Barsbold 1983 [referred]).
Referred occurrence. Khara-Khutul, Bayshin-Tsav and
Urilbe-Khuduk, SE Mongolia (Barsbold & Perle 1980;
Barsbold 1983).
Revised diagnosis. A therizinosaurian bearing the follow-
ing autapomorphies: well-developed coronoid process on
mandible (not figured); straight-shafted humerus lack-
ing cranially deflected distal aspect; dorsal margin of
ilium with pronounced ventral overhang; caudal process
of ischium caudally extensive, nearly 50% of craniocau-
dal depth of obturator process; hypertrophied epiphyses
on distal metatarsals (noted by Perle, 1979, yet could not
be confirmed first hand; not figured). Segnosaurus can
be differentiated from Nothronychus by the following two
characters (ischium with subrectangular obturator process;
subcircular obturator foramen); from Enigmosaurus by the
following three characters (dorsoventrally deep obturator
process that does not fuse medially with its counterpart;
unfused pubic boot; craniocaudally wide distal pubic shaft);
and from both Nothronychus and Enigmosaurus in possess-
ing a deep brevis fossa and lacking a well developed caudal
projection on pubic boot (Fig. 4).
Remarks. Perle (1979), Barsbold & Perle (1980) and
Barsbold (1983) all provided diagnoses for Segnosaurus
galbinensis. The following diagnosis was given by Bars-
bold & Perle (1980, p. 188): “large segnosaurid with non-
compressed pedal unguals”. Barsbold (1983, p. 104) added
several characters: “mandibular teeth small, compactly
positioned, slightly bent cranially and straight caudally;
cranial jaw with small diastema; and pedal unguals slightly
laterally compressed”. Both diagnoses are insufficient to
differentiate this taxon from other therizinosaurians as
they focus primarily on perceived variations in the degree
of pedal ungual compression between Segnosaurus and
Erlikosaurus. Whereas the degree of compression of the
pedal unguals may currently distinguish Erlikosaurus,the
lack of severe pedal ungual compression in Segnosaurus
closely resembles the condition in other therizinosaurians.
Perle (1979, pp. 46–47) provided the following diagno-
sis for Segnosaurus: mandible elongate and dorsoventrally
narrow; teeth compact; cranial dentary teeth reduced in
size, straight; scapulocoracoid massive, fused; deltopec-
toral crest hypertrophied, manual phalanges dorsoventrally
compressed, lacking collateral ligament pits; PI-II sharply
recurved and laterally compressed; preacetabular portion of
ilium broad; postacetabular portion of ilium reduced, with
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Re-evaluation of Therizinosauria 523
Figure 4. Diagnostic features of Segnosaurus galbinensis.A, cranial view of referred humerus (IGM 100/80); and B, medial view
of humerus showing straight profile; C, right lateral view of holotype ilium with sacrum (IGM 100/80) with inset, showing ventral
overhang of dorsal margin and D, dorsal view of ilium and sacrum; E, lateral view of composite left pelvis, referred ilium (IGM 100/82),
holotype ischium and pubis (IGM 100/80), showing caudally extensive caudal process of ischium, subrectangular, dorsoventrally deep
obturator process, subcircular obturator foramen, craniocaudally wide distal pubic shaft; F, cranial view of referred left proximal femur
(IGM 100/82); and G, cranial view of referred right distal tibia with astragalus (IGM 100/82). Abbreviations: apa, ascending process of
astragalus; as, astragalus; at, antitrochanter; cbt, cubic tuberosity on postacetabular process; crpb, cranial process of pubic boot; ct, caudal
tuberosity of humerus; dmi, dorsal margin of ilium; dpc, deltopectoral crest; ec, ectepicondyle; en, entepicondyle; fh, femoral head; fifi,
M. iliofemoralis insertion on femur; flt, lesser trochanter; gt, greater trochanter; hh, humeral head; ib, ischiadic boot; it, internal tuberosity;
mtt, medial tuberosity on cranial distal tibia; ns, neural spine, of, obturator foramen; op, obturator process; pil, pubic peduncle of ilium;
pra, preacetabular process; rc, radial condyle, and uc, ulnar condyle. Scale bar 7 mm (A-B), 10 mm (E-G), C-D not to scale.
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524 L. E. Zanno
cubic tuberosity; iliac peduncle of pubis elongate; ischiadic
shaft flattened, with distally positioned obturator process
contacting pubis; ascending process of astragalus extends
lateral to tibia, overlapping fibula; and functionally tetrapo-
dal pes.
Although most of these features are too widespread
among therizinosaurians to be useful differentiae, Perle’s
(1979) diagnosis represents a summary of the distinguish-
ing features characterizing derived species and may be
useful in the creation of an apomorphy-based diagnosis
for Therizinosauridae. Dorsoventral compression of the
manual phalanges and the morphology of the teeth may
represent autapomorphies of Segnosaurus;howeverthey
are not included in the revised diagnosis until they can be
confirmed first hand.
Therizinosaurus cheloniformis Maleev, 1954
Holotype. PIN 551–483, three manual unguals (two
incomplete), metacarpal fragment, several rib fragments
(see Remarks) (Maleev 1954).
Referred specimens. Three additional specimens were
referred to Therizinosaurus by Barsbold (1976): IGM
100/15, right and left scapulocoracoid (left poorly
preserved), right and left humeri, right antebrachium,
left ulna, two right distal carpals, right metacarpus with
phalanges of the second digit including ungual, fragmentary
dorsal ribs, gastralia; IGM 100/17, manual ungual missing
distal portion; and IGM 100/16, manual ungual missing
proximal portion. Perle (1982) also referred a hind limb
(IGM 100/45) from the same beds in Khermin Tzav to Ther-
izinosaurus. The specimen consists of femoral fragments,
tibia, astragalus, calcaneum, a distal tarsal, four metatarsals,
nearly complete digit one, complete digits two and four
missing unguals, and presumed PIII-II from third digit
of pes.
Holotype occurrence. Nemegtskaya Svita; Nemegtu
region, Nemegt Formation, south-west Mongolia (Maas-
trichtian; Barsbold 1983).
Referred occurrence. Khermin Tzav, Nemegt Formation
Omnogov Aimak, SW Mongolia.
Revised diagnosis. A therizinosaurian bearing the follow-
ing autapomorphies: deltopectoral crest on humerus hyper-
trophied, exceeding 60% length of humerus; metacarpal I
greater than two-thirds length of metacarpal III; metacarpal
I with enlarged medial crest connecting medial distal
condyle and proximal medial lobe; manual unguals trans-
versely compressed (transverse dimension less than 20%
dorsoventral height in cross-section), poorly recurved, and
hypertrophied, proximodistal length to dorsoventral height
greater than 4:1; and manual unguals lack ventrally project-
ing flexor tubercle in lateral view (Fig. 5).
Remarks. Since its original description in 1954, Ther-
izinosaurus has been rediagnosed twice by Barsbold (1976,
1983). The diagnosis presented here encompasses more
quantitative versions of two characters retained from itera-
tions of earlier diagnoses: ungual of giant size, compressed
laterally, narrowing distally, with a slight bend (Barsbold
1976, p. 78) (laterally compressed unguals sensu Bars-
bold [1983, p. 101]); and humerus with strongly enlarged
deltopectoral crest (Barsbold 1976, p. 78) (deltopectoral
crest highly developed sensu Barsbold [1983, p. 101]).
The remaining features noted by Barsbold (1976, p.
78, 1983, p. 101) (listed below) are not diagnostic for
Therizinosaurus: scapulocoracoid with shortened scapu-
lar blade and enlarged acromion; humeral epiphyses trans-
versely wide; humerus distally expanded and oriented in
same plane as proximal humerus; ulna with semilunar
proximal articular surface; sigmoidal radius; semilunate
carpus; metacarpal bones of unequal transverse width (i.e.
metacarpal I short and dorsoventrally flattened, metacarpal
II elongate and metacarpal III reduced, thin); and phalanges
of the second manual digit shortened.
Perle (1982) referred a fragmentary hind limb (IGM
100/45) to Therizinosaurus cheloniformis. IGM 100/45
exhibits multiple therizinsaurian synapomorphies includ-
ing: the apparent reduction of the astragalar body, develop-
ment of a craniomedial tuberosity on the distal tibia, prox-
imodistally short and laterally deflected ascending process
of the astragalus, and functionally tetradacyl pes. Based
on its morphology, geographic proximity to and strati-
graphic continuity with the holotype specimen of Ther-
izinosaurus cheloniformis (Perle 1982), IGM 100/45 is
reasonably referred to this taxon. However, this material
is excluded from the diagnosis presented here in order
to prevent future complications if the hind limb is ulti-
mately found to represent another taxon. The rib material
originally considered part of the holotype (Maleev 1954)
was identified by Rozhdestvensky (1970) as pertaining to
Sauropodomorpha and is not considered here.
Therizinosauria incertae sedis
‘Nanshiungosaurus’ bohlini Dong & Yu, 1997
Holotype. IVPP V11116, articulated series of 11 cervicals
(including atlas), five dorsals, fragmentary ribs.
Occurrence. Gongpoquan Basin, Mazongshan Area,
Gansu Province, uppermost Xinminbao Group (late Early
or early Late Cretaceous, Tang et al. 2001).
Remarks. The features originally used to diagnose ‘N.’
bohlini (Dong & Yu 1997) are now known to represent
other therizinosaurians (e.g. Falcarius,Neimongosaurus,
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Re-evaluation of Therizinosauria 525
Figure 5. Diagnostic features of Therizinosaurus cheloniformis. Casts are a composite of referred materials IGM 100/15, 100/16, and/or
100/17. A, right lateral view of right manual unguals I-III, showing poor recurvature and hyperelongation; B, dorsal view of right manus,
showing ratio of first to third metacarpals; C, dorsal view of right PI-II, showing extreme transverse compression; and D, lateral view of
right MCI, showing enlarged medial crest. Abbreviations: dit, dorsal intercondylar tuberosity; ft, flexor tubercle; MCI-MCIII, metacarpals
I-III; mc, medial condyle; mcr, medial crest; PI-II, first manual ungual; PII-III, second manual ungual; PIII-IV, third manual ungual. Scale
bar 10 mm (A-B), C-D not to scale.
Nothronychus) including possession of platycoelous verte-
bral centra; low and craniocaudally long cervical neural
spines; large zygapophyses situated dorsal to the neural
canal; and neural spines of dorsals reduced. Dong & Yu
(1997) recognize the presence of ventral keels on the
first four dorsals in ‘N.’ bohlini. To date the only ther-
izinosaurian for which these elements are preserved is
Neimongosaurus, in which the cranial dorsal centra are
reported as lacking a ventral keel. However, the primitive
taxon Falcarius has a minimum of two ventrally keeled
cranial dorsals and may possess up to four (Zanno 2010),
so the apomorphic status of this feature appears unlikely.
As noted by Li et al. (2007), the taxonomic status of ‘N.’
bohlini is uncertain. Whereas the authors offer an argu-
ment for excluding ‘N.’ bohlini from the genus Nanshiun-
gosaurus, they also note that a lack of available overlapping
elements currently precludes assignment to its potential
subjective synonym Suzhousaurus. Additional specimens
of Suzhousaurus may resolve these issues. As noted by Li
et al. (2007), two options are available for IVPP V11116: (1)
new specimens of Suzhousaurus will reveal that this taxon
is indistinguishable from IVPP V11116 and the resultant
name will be Suzhousaurus bohlini; or (2) IVPP V11116
is unique, in which case a nomen novum will need to be
erected to form a new combination for bohlini.
Phylogenetic analysis
Results
Analysis of the new phylogenetic dataset generated by this
study resulted in 756 MPTs (TL =1225), reached after a
single subsequent round of TBR swapping. Strict consen-
sus of the recovered MPTs (Fig. 6) illustrates the rela-
tively high degree of resolution recovered here, includ-
ing lower-level relationships. In contrast to nearly all
recent studies, none of the MPTs recovered a mono-
phyletic Therizinosauria/Oviraptorosauria. Here Ovirap-
torosauria is posited as the sister taxon to Paraves
under strict consensus. Forcing a monophyletic Ther-
izinosauria/Oviraptorosauria requires an additional eight
steps in this analysis. Forcing a monophyletic relationship
between Therizinosauria and Alvarezsauridae requires an
additional six steps, and forcing various topologies produc-
ing an exclusive common ancestor for all three clades (inde-
pendent of Paraves) requires a minimum of 11 and up to 18
additional steps.
Resolution of derived therizinosaurians and ornithomi-
mosaurs is poor under strict consensus, with all
therizinosaurians more derived than Neimongosaurus unre-
solved (except for a monophyletic Nothronychus), and a
large basal polytomy within Ornithomimosauria. A maxi-
mum agreement subtree (MAST) was used to identify
causes of ambiguity, which include the labile therizinosauri-
ans ‘Nanshiungosaurus’bohlini,Segnosaurus galbinen-
sis,Erlikosaurus andrewsi,Therizinosaurus cheloniformis
(Fig. 7) and the basal ornithomimosaur Pelecanimimus
polydon, and identify common topology among all MPTs.
Upon subsequent investigation it was determined that ‘N.’
bohlini could be excluded from subsequent analyses a priori
(i.e. ‘safe taxonomic reduction’, Wilkinson 1995). Lability
of ‘N.’ bohlini results from a high percentage of missing
data (97%) rather than character conflict as indicated by
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526 L. E. Zanno
Figure 6. Strict consensus of 756 most parsimonious trees (tree length 1225). Numbers above lines represent Bremer values; numbers
below line represent symmetrical resampling GC values (left) and standard bootstrap frequencies (right, in bold). Node numbers represent
taxa as follows: (1) Coelurosauria; (2) Maniraptora; (3) Paraves; (4) Tyrannosauroidea; (5) Compsognathidae; (6) Ornithomimosauria;
(7) Therizinosauria; (8) Alvarezsauridae; (9) Oviraptorosauria; (10) Aves; (11) Troodontidae; (12) Dromaeosauridae; (13) Unenlagiinae.
Unambiguous synapomorphies shown in Appendix 3.
tree length stability after its exclusion from the dataset.
However, Segnosaurus,Erlikosaurus,Therizinosaurus and
Pelecanimimus contain unique character combinations
and cannot be pruned in the total evidence approach used
here.
Ingroup lability of the taxa Erlikosaurus and Ther-
izinosaurus is not surprising. Due to the unknown where-
abouts of associated postcranial materials, few of the
newly added postcranial characters could be coded for
Erlikosaurus. Thus, in this study, the taxon is represented
predominantly from cranial material, which is unknown
for the majority of other therizinosaurians. Highly trans-
formed forelimb materials and a morphologically conser-
vative hind limb represent Therizinosaurus, and forelimb
elements display a suite of autapomorphic features repre-
senting extreme stages of observable phylogenetic trends
within the clade. Thus although the derived position of
Therizinosaurus is evident and may be anchored in future
studies through the use of these features, the discrete and
additive quantification of these trends to produce such a
result can only be accomplished by arbitrary state divi-
sion, a method not employed here. The hind limb materials
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Re-evaluation of Therizinosauria 527
Figure 7. Time-calibrated phylogeny of Therizinosauria based on relationships recovered in the maximum agreement subtree of all
MPTs. Shown temporal ranges represent uncertainties in chronostratigraphy, not actual species ranges. Segnosaurus,Erlikosaurus and
Therizinosaurus are shown as unresolved therizinosaurids due to their lability in this study. Minimum temporal hypothesized faunal
interchange events between North America and Asia in the Late Cretaceous shown.
referred to Therizinosaurus lack a unique suite of character
states and thus are of little use in discerning its taxonomic
affinities. In this study Therizinosaurus is the only labile
taxon to be posited outside Therizinosauridae (between
Neimongosaurus and Enigmosaurus) among MPTs. Thus
the taxon is the sole culprit for the irresolution recovered for
derived therizinosauroids under strict consensus of MPTs.
Pruning Therizinosaurus from the matrix a priori anchors
Suzhousaurus and Enigmosaurus as successive outgroups
to Therizinosauridae (this topology is also recovered in the
MAST, see below), but does not resolve ambiguity within
the derived subclade. However, pruning Therizinosaurus
does dramatically reduce the number of MPTs recovered
from 756 to 108.
Segnosaurus possesses a highly transformed humerus
that compares poorly to other therizinosaurians in that it
possesses a straight rather than sigmoid shaft, little expan-
sion of the distal end relative to the shaft, no expansion
of the medial aspect of the humerus, and a poorly devel-
oped entepicondyle. In the absence of these features the
humerus of Segnosaurus is more akin to ornithomimosaurs
(and in some respects troodontids) than to other ther-
izinosaurians. Although Segnosaurus can clearly be iden-
tified as a therizinosaurid, character conflict with other
members of the subclade renders its position unstable.
In several respects, the pelvis of Segnosaurus is simi-
lar to Nothronychus (particularly the ischia); however, it
is not yet clear if these similarities reflect synapomor-
phies or a morphological grade early in the evolution of
therizinosaurids, as these features are unknown in other
derived taxa. The MAST (from which Therizinosaurus,
Erlikosaurus and Segnosaurus are removed a posteriori
after MPT generation; Fig. 7) also supports a more basal
position of Suzhousaurus and Enigmosaurus (successive
sister taxa to Therizinosauridae) and a more derived posi-
tion for Nothronychus and Nanshiungosaurus brevispi-
nus. The poor condition, fragmentary nature and unknown
whereabouts of Asian therizinosaurid materials are major
impediments to phylogenetic resolution among the subclade
Therizinosauridae, and are unlikely to be overcome
without the addition of new discoveries, especially for the
taxa Therizinosaurus,Segnosaurus and Erlikosaurus.
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528 L. E. Zanno
Despite the increase in therizinosaurian resolution gained
from pruning ‘N.’ bohlini, ornithomimids remain poorly
resolved in this study. Maximum agreement subtrees iden-
tified Pelecanimimus as a primary source of ambiguity. A
priori pruning of Pelecanimimus results in nearly complete
resolution of Ornithomimosauria under strict consensus
(also recovered by the MAST), another nine-step drop in
tree length, and reduction of MPTs (from 756 to 378).
Iterations of the control analysis were run coding
Chirostenotes and Caenagnathus collinsi as independent
OTUs in order to investigate the impact of these alterna-
tives on tree topology. Coding Caenagnathus as a separate
OTU resulted in a shorter tree score yet caused reduc-
tion in resolution among oviraptorosaurians and signifi-
cantly reduced frequency support among internal ovirap-
torosaurian subclades. Under majority rule consensus,
Chirostenotes remains within a derived subclade of Ovirap-
torosauria whereas Caenagnathus is recovered at the base
of the clade. Senter (2007) also reported this discrep-
ancy when the taxa were coded as separate OTUs. The
contention put forth that wide topological distance casts
doubt on the synonymy of these taxa (Senter 2007, p. 1) is
unwarranted. Many of the features lacking on the dentary
of Caenagnathus that, in turn, support a more primitive
position among oviraptorosaurians (e.g. downturned rostral
dentary, shortened dentary ramus, rostral displacement of
external mandibular fenestra; see Senter 2007) can just as
easily be reversals attributed to secondary elongation of the
mandible. Furthermore, the dentary of Caenagnathus does
not resemble that of Incisivosaurus, widely regarded as the
most basal member of the clade. The topological discrep-
ancy in these taxa when coded separately is interesting and
probably points to mosaic evolution in the skeletons of
North American oviraptorosaurians relative to their Asian
counterparts, specifically with regard to skull morphology.
The dromaeosaurid topology recovered here differs
significantly from that of most recent version of the base
TWiG (Turner et al. 2007b). Turner et al. (2007b) included
four additional paravian taxa as well as several character
states not incorporated in this analysis; thus, with regard
to paravian relationships, their analysis is based on more
comprehensive evidence.
The matrix was tested using standard bootstrapping
(Felsenstein 1985) and symmetrical resampling (Goloboff
et al. 2003) methods to investigate clade support and
robusticity (Fig. 6). The clades Therizinosauria (bootstrap
frequency [BF] =53), Therizinosauroidea (BF =97),
therizinosauroids more derived than Beipiaosaurus (BF =
96) and therizinosauroids more derived than Alxasaurus
(BF =77) are among a handful recovered among all of
Coelurosauria with greater than 50% frequency in this
study. Symmetrical resampling using GC values illus-
trates a similar pattern. Here, Therizinosauroidea (GC
=97) and all therizinosaurians more derived than Beip-
iaosaurus (GC =99) have greater support values than
any other coelurosaurian subclade except the mono-
phyletic Mononykinae (GC =100) and Tyrannosauridae
(GC =100). Thus the current matrix establishes strong
character support for the monophyly of Therizinosauria,
Therizinosauroidea and basal therizinosaurian tree
topology.
Comparisons with previous analyses
Ingroup relationships of Therizinosauria. To date, only
four phylogenetic studies have attempted to resolve ingroup
relationships among more than three therizinosaurian
species, and all were conducted within the past five
years (Table 4). Although the analysis of Zanno (2006)
contains 10 therizinosaurian OTUs, that study was intended
to describe the morphological evolution of the ther-
izinosaurian forelimb and is therefore considered only in
a marginal context here. Among the four comprehensive
analyses, Kirkland et al. (2005) is too poorly resolved to
be considered further. Clark et al. (2004), Li et al. (2007)
and Senter (2007) include the most resolved hypotheses of
therizinosaurian ingroup relationships and form the basis
of comparison with this work.
The therizinosaurian topology recovered in this anal-
ysis differs from previous hypotheses in several regards
(Fig. 8) – most notably in the positions of Neimon-
gosaurus,Erliansaurus,Nothronychus,Suzhousaurus and
Nanshiungosaurus brevispinus, and the taxonomic compo-
sition of Therizinosauridae. In this study, Neimongosaurus,
Erliansaurus and Enigmosaurus represent intermediate-
grade therizinosaurians restricted from Therizinosauridae
– following both the phylogenetic definition proposed by
Zhang et al. (2001) and the modified definition proposed in
Zanno et al. (2009) – whereas Nothronychus is posited as
a therizinosaurid and Suzhousaurus is posited as the sister-
taxon to Therizinosauridae (Fig. 7). Thus, as noted by Zanno
et al. (2009), this study provides the first systematic support
for the proposal of Xu et al. (2002a) regarding the interme-
diate status of Neimongosaurus and Erliansaurus and the
proposal by Kirkland & Wolfe (2001) regarding the derived
status of Nothronychus. By comparison, Therizinosauridae
is maximally inclusive in Clark et al. (2004) and Senter
(2007), including all therizinosaurians other than Falcarius
(Senter 2007 only), Beipiaosaurus,Alxasaurus and Enig-
mosaurus (Clark et al. 2004 only), and containing equiva-
lent taxonomic content under both definitions.
In Clark et al. (2004), Li et al. (2007) and Senter
(2007), Neimongosaurus is considered a therizinosaurid;
the latter two studies also include Erliansaurus within
Therizinosauridae. As Zanno (2006) demonstrated, the
humerus of Neimongosaurus is among the most derived of
any therizinosaurian, exhibiting strongly expanded proxi-
mal and distal aspects, a hypertrophied internal tuberos-
ity delineated by a pronounced notch, and a tubercle on
the cranial face of the distal humerus, yet lacking a well
developed caudal tubercle on the humeral shaft. However,
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Re-evaluation of Therizinosauria 529
Tab l e 4 . Statistical summary of previously hypothesized therizinosaur ingroup relationships. Phylogenetic analysis including at least two therizinosaur OTUs (i.e. ingroup or
partial ingroup analyses) shown. This list does not include TWiG based analyses other than Kirkland et al. (2005) and Senter (2007).
Reference Character list
Therizinosaur OTUs
(bold taxa pruned from final tree) Taxa/characters
#MPTs/tree
score Tree stats
This paper 28% new (=50% novel; 50% adapted from cited sources) Falcarius Neimongosaurus 76/348 756/1225 CI =0.36
72% TWiG (=Tur ner et al. 2007a; 10% modified) Beipiaosaurus Enigmosaurus Ratio: 4.6 RI =0.70
Alxasaurus N.brevispinus RC =0.25
No.mckinleyi N.bohlini
No.graffami Erlikosaurus
Suzhousaurus Segnosaurus
Erliansaurus Therizinosaurus
Li et al. 2007 100% (Clark et al. 2004) Falcarius Neimongosaurus 17/40 6092/70 CI =0.67
Beipiaosaurus Enigmosaurus Ratio: 2.4 HI =0.33
Alxasaurus N. brevispinus RI =0.70
No. mckinleyi Erlikosaurus RC =0.47
Suzhousaurus Segnosaurus
Erliansaurus Therizinosaurus
Senter 2007 35% new characters (=15% novel; 21% from cited sources) Falcarius Neimongosaurus 85/360 7290/1224 CI =0.36
64% TWiG (Kirkland et al. 2005; 18% modified) Beipiaosaurus N. brevispinus Ratio: 4.2 HI =0.64
Alxasaurus Erlikosaurus RI =0.76
No. mckinleyi Segnosaurus RC =0.27
Erliansaurus Therizinosaurus
Zanno 2006 47% novel Falcarius Neimongosaurus 13/32 35/46 CI =0.72
53% adapted from cited sources Beipiaosaurus Erlikosaurus Ratio: 2.5 RI =0.78
Alxasaurus Segnosaurus RC =0.56
No. mckinleyi Therizinosaurus
Erliansaurus “Alectrosaurus”
Kirkland et al. 2005 4% novel Falcarius No. mckinleyi 58/231 5000/679 CI =0.41
96% TWiG (Hwang et al. 2004) Beipiaosaurus Erlikosaurus RI =0.74
Alxasaurus Segnosaurus RC =0.31
Clark et al. 2004 Xu et al. 1999b Beipiaosaurus Neimongosaurus 15/40 31815/63 CI =0.75
Alxasaurus Erlikosaurus Ratio: 2.7 RI =0.75
No. mckinleyi Segnosaurus RC =0.56
Enigmosaurus Therizinosaurus
N. brevispinus Eshanosaurus
Xu et al. 1999b 14% novel Beipiaosaurus Therizinosauridae 8/84 CI =0.71
86% from cited sources Alxasaurus Ratio: 10.5 1/133 RI =0.65
RC =0.46
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530 L. E. Zanno
Figure 8. Previously hypothesized ingroup relationships of Therizinosauria. A,Xuet al. (1999b); B, Xu & Wang (2004); C,Clarket al.
(2004), strict consensus; D,Clarket al. (2004) Adams consensus; E, Zanno (2006); F,Liet al. (2007); G, Senter (2007); H, Kirkland
et al. (2005). Therizinosauria (as defined here) shown in grey.
the pelvic and hind limb morphology is intermediate in
Neimongosaurus, suggesting a more basal position as was
reported by Xu et al. (2002a). Neimongosaurus lacks the
following features present in more derived therizinosauri-
ans: dorsal surface of postacetabular process of ilium
hyper-rugose, with hypertrophied caudal tuberosity; pubic
peduncle of ilium severely compressed craniocaudally, with
transverse dimension more than twice craniocaudal depth;
ischiadic peduncle of ilium forms hypertrophied, spherical
boss; cranial tuberosity on exposed distal tibia; proximal
metatarsal IV lacking caudal flange buttressing metatarsal
III, and metatarsus less than 39% length of tibia.
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Re-evaluation of Therizinosauria 531
Relative over-representation of pectoral and forelimb
characters can result in derived placement for this taxon.
This appears to be the case in Senter’s (2007) analysis,
where the therizinosaurid placement of Neimongosaurus
is supported only by unambiguous synapomorphies of the
pectoral girdle and forelimb – fused scapulocoracoid; and
width of distal humerus relative to length. In Clark et al.
(2004) and Li et al. (2007) the derived position of Neimon-
gosaurus is supported by a rostrally edentulous dentary and
laterally facing glenoid fossa. Rostral edentulism remains
a synapomorphy of Neimongosaurus and therizinosaurids
under the topology presented here. However, the glenoid
is primitive in Neimongosaurus, facing ventrally rather
than laterally. A laterally facing glenoid characterizes only
the therizinosaurids Nothronychus,Suzhousaurus and Ther-
izinosaurus.
Neimongosaurus and Erliansaurus are relatively small-
bodied taxa from the Iren Dabasu Formation, for which
dates have not yet been firmly established (micropalaeonto-
logical studies [Ma 1994; von Itterbeeck et al. 2005] support
a Campanian-Maastrichtian age, whereas macrovertebrates
support a Cenomanian-early Campanian age [Jerzykiewicz
& Russell 1991; Currie & Eberth 1993; Godefroit et al.
1998]). Although numerous hypotheses can be put forth
to explain the discrepant morphology of Neimongosaurus,
two are regarded as most plausible here: either (1) the
pectoral girdle and forelimb were a centre of rapid adap-
tation occurring early in the evolution of therizinosauri-
ans (preceding the development of the characteristic
therizinosaurian pelvis and hind limb), indicating that
Neimongosaurus and/or Erliansaurus represent true
intermediate-grade taxa; or (2) the more primitive pelvis
and hind limb of these species is attributable to secondary
reduction of body size, which would require adaptive
coupling of body size and several of the pelvic and hind limb
characters used in this analysis. In the latter case Neimon-
gosaurus and/or Erliansaurus would be expected to nest
within Therizinosauridae in future studies, pending discov-
ery of transitional species that demonstrate secondary rever-
sal of pelvic and hind limb characters.
The phylogenetic hypothesis put forth by Li et al. (2007;
Fig. 8F) is somewhat anomalous in that Nothronychus and
Suzhousaurus are recovered as a monophyletic clade more
basal than Alxasaurus. The monophyly of Nothronychus
and Suzhousaurus in Li et al. (2007) is supported by a single
purported synapomorphy (length of deltopectoral crest less
than one-third total length of humerus). The deltopectoral
crest is 42–46% the total length of the humerus in the two
species of Nothronychus; therefore, this character is incor-
rectly coded. No evidence of a monophyletic relationship
between Nothronychus and Suzhousaurus was recovered in
this study.
The evolutionary relationships of Nothronychus have
been fairly contentious, yet only recently subject to empir-
ical study. Kirkland & Wolfe (2001) suggest an inter-
mediate morphology for N. mckinleyi yet included this
species within Therizinosauridae. Xu et al. (2002a, p. 3)
excluded No. mckinleyi from Therizinosauridae based
on the presumed absence of an expanded and laterally
deflected preacetabular process of ilium, straight dorsal
margin of ilium, and blade-like pedal unguals. However,
No. graffami demonstrates that the North American genus
possesses numerous therizinosaurid features, including the
altiliac condition. Li et al. (2007) consider the Early
Cretaceous taxon Suzhousaurus to represent a basal ther-
izinosaurian based on the absence of a caudal tubercle on the
humerus (also lacking in Neimongosaurus and Nothrony-
chus). Here, Suzhousaurus is supported as sister-taxon to
Therizinosauridae (MAST and/or Therizinosaurus pruned;
see Results). The derived placement of Nothronychus and
Suzhousaurus in this study is based on possession of the
following derived synapomorphies: dorsal flange on dorsal
scapular blade (present only for Suzhousaurus); glenoid
fossa extends onto external surface of scapulocoracoid;
ulnar shaft straight (unknown in Suzhousaurus); pubic boot
cranially oriented with little to no caudal process; ischiadic
peduncle and antitrochanter form hypertrophied, spheri-
cal boss (not in Suzhousaurus); pubic shaft mediolater-
ally flattened; distal pubic shaft enlarged, more than twice
craniocaudal depth of proximal shaft; mediocranial distal
tibia exposed, expressed as cranial tuberosity (unknown for
Suzhousaurus); and metatarsus less than 36% length of tibia
(unknown for Suzhousaurus).
In contrast to this study, Senter (2007) recovered Nanshi-
ungosaurus as the basalmost therizinosaurid (Fig. 8G);
however, no synapomorphies are presented for the most
inclusive clade of therizinosaurians lacking Nanshiun-
gosaurus, so it is unclear what characters exclude this taxon
from occupying a more nested position. Here Nanshiun-
gosaurus is recovered as a derived therizinosaurian based
on pelvic morphology (see above). Although Enigmosaurus
was found to be an unstable taxon in Clark et al. (2004), it is
posited outside of Therizinosauridae in this study, primarily
due to the absence of a flattened pubic shaft (also absent
in Erliansaurus) and lack of a cranially oriented pubic
boot. Fusion of the pubis and ischium via the obturator
process indicates a close evolutionary relationship to Ther-
izinosauridae; however, this feature is unknown in other
therizinosauroids (e.g. Neimongosaurus,Erliansaurus and
Alxasaurus) and may have a broader distribution than
currently recognized.
Outgroup relationships of Therizinosauria. Perhaps the
most discrepant finding of this study is that it chal-
lenges the widely regarded hypothesis of a monophyletic
Therizinosauria/Oviraptorosauria clade (see Table 3). The
anatomy of the basalmost therizinosaurian Falcarius
directly contradicts many of the characters previously
purported to be synapomorphic for this grouping and
demonstrates that basal therizinosaurians possessed many
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532 L. E. Zanno
primitive coelurosaurian features not currently known in
basal oviraptorosaurians. Pruning Falcarius from the anal-
ysis results in the re-establishment of a monophyletic rela-
tionship between therizinosaurians and oviraptorosauri-
ans; thus this taxon is critical to the recovery of the
topology presented here. The absence of a monophyletic
Therizinosauria/Oviraptorosauria clade was also recovered
by Senter (2007) and, although apparently not tested, was
probably due to the inclusion of Falcarius in the study.
Unambiguous derived synapomorphies reported in
previous analyses as supporting a monophyletic Ther-
izinosauria/Oviraptorosauria clade include: edentulous
premaxilla (Makovicky & Sues 1998); caudal margin of
naris nearly reaching or overlapping rostral border of antor-
bital fossa (Xu et al. 2002b [i.e. TWiG]); rim around antor-
bital fossa well developed (Sues 1997); reduced basiptery-
goid processes (Sues 1997; Makovicky & Sues 1998;
Rauhut 2003; Xu et al. 2002b [i.e. TWiG]); suborbital
fenestra reduced or absent (Xu et al. 2002b [i.e. TWiG]);
ectopterygoid lateral to palatine (Sues 1997); palatine not
tetraradiate (Rauhut 2003); symphyseal region of dentary
inflected medially toward midline [U-shaped rostral symph-
ysis] (Makovicky & Sues 1998; Xu et al. 2002b [i.e.
TWiG]); cranial part of dentary flexed medially [in dorsal
view] (Rauhut 2003); moderate number of small teeth
(25–30) in dentary (Xu et al. 2002b [i.e. TWiG]); two pairs
of pleurocoels in cervical vertebrae (Rauhut 2003); prezy-
gapophyses of distal caudal vertebrae not elongate (Rauhut
2003); neural spines on distal caudals form low ridge (rever-
sal) (Xu et al. 2002b [i.e. TWiG]); distal caudal centra short
(Sues 1997; Makovicky & Sues 1998; Rauhut 2003); prox-
imal end of chevrons of proximal caudals craniocaudally
short with cylindrical shaft (reversal) (Xu et al. 2002b [i.e.
TWiG]); pronounced lip on manual unguals (Sues 1997;
Makovicky & Sues 1998; Xu et al. 2002b [i.e. TWiG]);
preacetabular portion of ilium deep relative to postacetabu-
lar portion (Makovicky & Sues 1998; Rauhut 2003). Several
of these features are absent in basal therizinosaurians and/or
oviraptorosaurians thus cannot represent synapomorphies.
Zanno et al. (2009) reviewed these features and presented
a dozen additional convergences among derived members
of the clades that are absent in basal members.
Although appearing morphologically primitive in several
respects relative to their Asian counterparts, North Amer-
ican (NA) oviraptorosaurians are recovered in this study
as more derived than Incisivosaurus,Protarchaeopteryx
and Caudipteryx (Fig. 6). Therefore, although NA ovirap-
torosaurians share a number of features exclusively with
therizinosaurians, these characters are refuted as synapo-
morphies by the nested topology of the former clade
in this analysis. Such features include cervical centra
with two pairs of pneumatic foramina (also in Avim-
imus); absence of accessory antorbital fenestra; coracoid
subcircular, caudoventral process poorly developed (also
in Avimimus); subcondylar recess absent; and ligament
groove on caudal femoral head reduced (also in Avimimus).
In addition, two oviraptorosaurian genera, Avimimus and
Ingenia, share features exclusively with therizinosaurians.
Features shared between therizinosaurians and Avimimus
alone include hypertrophied diapophyseal facets on cranial
dorsal vertebrae (not in Falcarius); and transverse processes
of cranial dorsal vertebrae pendent. Features shared exclu-
sively with Ingenia include distal scapula blade tapered;
deltopectoral crest more than 60% total length of humerus
(also in Mononykus); and ratio of metacarpal II/(phalanx
II-I +phalanx II-II) greater than 100%. The unusually
high degree of convergence between derived therizinosauri-
ans and oviraptorosaurians suggests that similar adaptive
drivers were propelling morphological evolution within
these clades. The convergence exhibited between all ther-
izinosaurians and several species of oviraptorosaurians (e.g.
Avimimus) is particularly intriguing. Evolutionary studies
from a functional and/or palaeobiological perspective are
needed to add broader significance to these findings and
establish causal factors that may be responsible for the
patterns identified here.
This study also recovered several homoplastic charac-
ters present on therizinosaurians and alvarezsaurids that
are refuted as homologies by the anatomy of the basalmost
members of these clades, including: opisthocoelous cervi-
cal vertebrae; femoral head confluent with neck, lack-
ing raised ventral rim; transverse width of distal humerus
significantly expanded relative to transverse width of shaft;
medial aspect of distal humerus expanded, subtriangular;
entepicondyle displaced medially relative to ulnar condyle;
and lateral fossa of proximal tibia shallow.
The discovery of primitive, Neocomian members of
the clades Therizinosauria and Oviraptorosauria is adding
critical information to studies of character transformation
within these clades and helping to resolve their unusual
degree of convergence. However, the current rarity of
alvarezsaurid remains may be hindering our understand-
ing of the evolutionary history of this group. Although
this study did not recover exclusive common ances-
try among therizinosaurians, oviraptorosaurians and/or
alvarezsaurids, as was recovered in Xu & Wang (2004), a
high degree of morphological similarity between several
members of these clades may present evidence for a
close evolutionary relationship. Several characters that
cannot currently be refuted by ancestral state reconstruc-
tion remain possible synapomorphies linking at least two
of these clades; these include: basipterygoid processes
reduced; premaxillary symphysis rounded; suborbital
fenestra reduced; increased number of cervical vertebrae;
caudolateral margins of ventral cervical centra with promi-
nent crests; neural spines on dorsal vertebrae caudally bifur-
cated; cranial portion of proximal fibula dorsally elevated;
and proximal fibula narrows caudally in dorsal view.
However, as was noted in this study (see Results), forc-
ing a monophyletic relationship between any combination
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Re-evaluation of Therizinosauria 533
of Therizinosauria, Alvarezsauridae and Oviraptorosauria
requires a significant number of additional steps, suggest-
ing that the basal topology recovered here is well supported
by the present data. Alternatively these features may repre-
sent basal maniraptoran symplesiomorphies subsequently
lost in paravians.
Persistent problems
Although the phylogenetic analysis presented here is one of
the most comprehensive yet conducted for Therizinosauria,
species-level relationships within the derived subclade
Therizinosauridae are not adequately resolved and a high
degree of homoplasy remains a compounding factor in the
support of basal tree topology.
Several factors are likely to account for much of
our inability to resolve evolutionary relationships among
derived therizinosaurians. The most significant of these
include: (1) inaccessibility, damage and potential loss
of holotype materials; (2) paucity of therizinosaurian
cranial remains; and (3) fragmentary specimens with
non-overlapping elements. The discovery of additional,
well-preserved specimens and/or the rediscovery of
currently missing elements of the derived therizinosauri-
ans Segnosaurus,Nanshiungosaurus,Therizinosaurus and
Erlikosaurus will be necessary to increase resolution and
support at internal nodes within Therizinosauridae.
Primary tree topology among non-paravian
coelurosaurian clades remains tenuously supported.
This ambiguity is due to the lack of transitional taxa at
basal nodes and may be resolved with the addition of
characters and taxa, whereas character conflict at the base
of Ornithomimosauria as documented by this study (due
primarily to the mosaic morphology of Pelecanimimus)
will probably remain until the discovery of additional basal
ornithomimosaurs.
The high degree of adaptive convergence (possibly
related to palaeobiological factors) between nonparavian
coelurosaurians obscures our ability to identify phyloge-
netic signals amongst members of these clades. This is espe-
cially true for derived therizinosaurians, oviraptorosaurians
and alvarezsaurids, which, as demonstrated by this study,
possess an unusually high degree of morphological conver-
gence. The exceedingly poor fossil record of North Amer-
ican oviraptorosaurians, lack of a postcranial skeleton for
the basalmost oviraptorosaurian Incisivosaurus, and poor
condition of the holotype of Protarchaeopteryx robustus
are all factors precluding morphological comparisons with
the anatomy of therizinosaurians in particular. Similarly
the sparse fossil record of alvarezsaurids (including a lack
of pre-Turonian species) is currently hindering our under-
standing of the phylogenetic relationships of this clade.
Future study of nonparavian coelurosaurian clades with
an eye to identifying homoplastic morphology as related
to palaeobiological factors is needed to advance our under-
standing of the evolution and morphological transforma-
tion of these clades. In addition, character correlation tech-
niques will probably prove useful in helping to tease out the
functional and/or dietary significance of convergent char-
acteristics amongst coelurosaurian lineages and identify-
ing homoplastic transformations (see Zanno et al. 2009).
Finally, the re-evaluation and addition of several known
Late Jurassic and Early Cretaceous coelurosaurians may
yield additional insight into morphological trends within
coelurosaurian subclades and their early diversification as
well as provide increased understanding of character evolu-
tion within basal coelurosaurian nodes.
Discussion
Morphological trends
Zanno (2010) reviewed several morphological trends
evident within Therizinosauria. However, these interpreta-
tions are general and based on prior definitions of the major
subclades Therizinosauroidea and Therizinosauridae rather
than species as the phylogenetic study presented here was
not yet completed. Here the discussion of the trends iden-
tified in Zanno (in press) is augmented with specific refer-
ence to character development based on the phylogenetic
relationships recovered in this study.
Zanno (2010) noted the presence of unserrated, incisi-
form and elongate rostral teeth in basal therizinosaurians
and oviraptorosaurians and hypothesized that this condition
may have been present in a shared common ancestor based
on prior phylogenetic studies suggesting a monophyletic
relationship between these clades. The broad phyloge-
netic relationships recovered here (i.e. therizinosaurians and
oviraptorosaurians representing successive basal manirap-
toran clades) suggest that unserrated and enlarged incisi-
form rostral teeth (and marked heterodonty) appeared early
in the evolution of maniraptorans and do not represent
an apomorphy restricted to these clades. If correctly opti-
mized as a basal maniraptoran condition, this feature is
predicted to be found in basal members of other manirap-
toran subclades (e.g. alvarezsaurids, paravians).
Regarding basal therizinosaurians, a high degree of
morphological change is evident between Beipiaosaurus
and Falcarius, although these taxa are approximately
coeval. The basalmost therizinosauroid Beipiaosaurus
possesses several derived features not present in Falcarius,
including: coarsely serrated cheek teeth; expanded preac-
etabular portion of ilium; reduced postacetabular portion
of ilium; subequal pubic and ischiadic peduncles of ilium;
elongate ischium with distal boot; lateral ridge on ischiadic
shaft; tibia/femur ratio approximately 1; and shortening of
the distal caudal vertebrae. These features indicate a high
degree of adaptive change occurring in the pelvis between
basal therizinosaurians and basal therizinosauroids.
Although the fragmentary condition of most derived
therizinosaurian specimens makes identifying the timing
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534 L. E. Zanno
of character development within the clade to the level of
species difficult, several patterns regarding the sequence
of character acquisition can be hypothesized based on
the ingroup topology recovered in this study. Follow-
ing a high degree of development in the pelvis of basal
therizinosauroids (e.g. Beipiaosaurus), adaptive change
appears to be concentrated in the forelimbs, ankle and
pes, which are highly transformed in the intermediate ther-
izinosauroids Neimongosaurus and Erliansaurus; whereas
pelvic morphology in these taxa is relatively conservative.
Derived therizinosauroids and basal therizinosaurids (e.g.
Nothronychus,Suzhousaurus and Enigmosaurus) exhibit
a second concentration of character development in the
pelvis, with continued transformation of the forelimb and
hind limb.
In addition to general trends identified here, specific
morphological features can be identified as characterizing
increasingly derived therizinosaurian subclades. These are
listed along with the basalmost therizinosaurian for which
the trait is known; however, the fragmentary condition of
most derived therizinosaurian specimens creates minimal
estimates on the most inclusive clade for which the charac-
ter can be attributed. Cranial characteristics are especially
problematic given the near lack of cranial materials known
for therizinosaurians.
Features present in Alxasaurus and more derived ther-
izinosauroids include: increase in crown symmetry; elon-
gation of neural spines on dorsal vertebrae; markedly short-
ened distal caudal vertebrae; loss of fused semilunate carpal
(DCI and DCII); and metatarsals reduced in length.
Features present in Neimongosaurus and/or Erliansaurus
and more derived therizinosauroids include: development
of broad, U-shaped rostral dentary; internal tuberosity
of humerus hypertrophied and delineated from humeral
head by notch, cranial tubercle on distal humerus; caudal
tuberosity on humeral shaft; medial aspect of distal
humerus expanded, entepicondyle located well medial to
ulnar condyle; distal metacarpal III ginglymoid in dorsal
view, with straight shaft; manual phalanges reduced in
length; caudodorsal rugosity on postacetabular portion
of ilium; femoral head dorsally inclined, region bridg-
ing head and greater trochanter depressed and cranio-
caudally constricted; lesser trochanter of femur cylindri-
cal, terminates ventral to greater trochanter; fibular crest
on tibia elongate; lateral condyle of astragalus reduced;
cranial trochanter on fibula distally located at midshaft;
and first metatarsal contacts tarsus, pes functionally
tetradactyl.
Features present in Enigmosaurus and therizinosaurids
include: six sacral vertebrae; hyperrugose cubic tuberos-
ity on postacetabular process of ilium; pubic peduncle of
ilium severely flattened craniocaudally, caudally recurved,
and more than triple the length of the ischiadic pedun-
cle; obturator process of ischium contacts pubis; contact
between ischium and pubis sinusoidal; loss of lateral tuber-
cle on pubis proximal to acetabulum; pubic shaft cranially
concave; and development of a ventrocaudal tuberosity on
ischium.
Features present in Nothronychus and more derived ther-
izinosaurids include: foreshortening of basicranial region;
increasing basicranial pneumaticity; enclosure of basisphe-
noidal recess by peripheral intramembraneous ossification
(Smith et al. in press); lateral reorientation of glenoid;
manual unguals enlarged (more than twice the length of
penultimate phalanx) and transversely flattened; hypere-
longate preacetabular portion of ilium; antitrochanter and
ischiadic peduncle of ilium indistinguishable, forming a
spherical boss; loss of caudal component of pubic boot;
and pubic shaft flattened.
Features present in derived therizinosaurids other than
Nothronychus include: caudal extension of the vomers;
development of a coronoid process on the dentary; down-
turned and edentulous rostral dentary; secondary reduction
in number of dentary teeth; hyperelongate manual unguals;
and ascending process of astragalus braces cranial fibular
shaft.
Geographic distribution of Therizinosauria
North America. Three species and two genera of ther-
izinosaurians are known from the western USA: Falcar-
ius utahensis from the Lower Cretaceous Cedar Mountain
Formation of Utah, No. mckinleyi from Upper Cretaceous
Moreno Hill Formation of New Mexico and No. graffami
from the Upper Cretaceous Tropic Shale of Utah.
Several isolated elements from the Late Cetaceous of
Alberta, Canada have been referred to the clade (Russell
1984; Currie 1992; Ryan & Russell 2001). These include
two isolated frontals. One of these specimens, CMN 12355,
was regarded as ‘deinonychosaurian’ gen. et. sp. indet. by
Sues (1978), and as cf. Erlikosaurus by both Currie (1987;
see also Currie 1992 in which the specimen is apparently
listed mistakenly as CMN 12349), and Ryan & Russell
(2001). The second specimen is TMP 81.16.231, regarded
as cf. Erlikosaurus by Currie (1992). Other possible ther-
izinosaurian elements from Alberta include an ungual
(TMP 79.15.1, cf. therizinosaurid, Currie 1992) from the
Dinosaur Park Formation, an astragalus (therizinosaurid
indet., Russell 1984) from the Hell Creek Formation and
a cervical vertebra (TMP 86.207.17, therizinosaurid indet.,
Ryan & Russell 2001) from the Scollard Formation. Refer-
ral of these finds has been questioned by several authors
(Barsbold & Marya ´
nska 1990; Clark et al. 1994; Marya ´
nska
1997), although no anatomical evidence has been put forth
to refute these assignments. Moreover it is unclear whether
it is referral of these elements to Therizinosauria indet. or
to cf. Erlikosaurus that is in doubt.
Asia. China and Mongolia are the only Asian coun-
tries from which therizinosaurians have been named.
However, several putative therizinosaurian elements have
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Re-evaluation of Therizinosauria 535
been reported from Russia, Uzbekistan, Kazakhstan and
Japan.
Since 1979 eight species of therizinosaurian have been
described from China, six of these within the past decade
alone. China currently has the highest known diversity
of therizinosaurians and the longest documented temporal
span for the clade (with or without the alleged Early Jurassic
therizinosaurian, Eshanosaurus). Chinese therizinosauri-
ans include: Beipiaosaurus inexpectus from the Yixian
Formation (Neocomian); Alxasaurus elesitaiensis from
the Bayin Gobi Formation (Aptian/Albian); Erliansaurus
bellamanus and Neimongosaurus yangi from the Iren
Dabasu Formation (Campanian/Maastrichtian); Nanshiun-
gosaurus brevispinus from the Yuanpu (Nanxiong) Forma-
tion (Campanian/Maastrichtian); the problematic taxa
‘Nanshiungosaurus’ bohlini from the uppermost Xinmin-
bao Group (late Early Cretaceous) and Eshanosaurus
deguchiianus from the lower Lufeng Formation (Early
Jurassic, Hettangian; Luo & Wu 1994).
Eshanosaurus deguchiianus was described by Xu et al.
(2001) from an isolated, incomplete left mandible with
teeth (IVPP V11579). If referral of Eshanosaurus to
Therizinosauria is correct and the specimen is correctly
dated as Early Jurassic, its existence extends the phylo-
genetically estimated divergence time of the nine major
coelurosaurian clades by at least 50 million years, produc-
ing lengthy ghost lineages. As Kirkland et al. (2005) noted,
Eshanosaurus possesses derived therizinosaurian features
(e.g. lateral shelf on the dentary; downturned mandibular
symphysis) absent in the most primitive North American
therizinosaurian Falcarius, known from approximately 80
million years later. Although the combination of morpho-
logical and stratigraphical incongruence casts doubt on
the assignment of Eshanosaurus to Therizinosauria, this
evidence is not definitive. The existence of the Late Juras-
sic paravian Archaeopteryx produces 15–20 million year
ghost lineages across major coelurosaurian clades, yet its
taxonomic affinities are unquestioned. Furthermore, it is
conceivable that Falcarius is a late-surviving member of
a basal clade of therizinosaurians that persisted in North
America until the Early Cretaceous, a conclusion consis-
tent with the observation that Beipiaosaurus, an Asian ther-
izinosaurian of comparable age to Falcarius, is morpholog-
ically advanced relative to the Utah taxon (Zanno 2010).
Finally, a recent study (Barrett 2009) disputed the refer-
ral of the dentary to Sauropodomorpha, and Eshanosaurus
possesses many features unknown in ornithischians (Xu
et al. 2001) and thus cannot confidently be referred to these
taxa. Overall, the identity of Eshanosaurus remains enig-
matic and unlikely to be satisfactorily resolved until addi-
tional materials are recovered.
Putative therizinosaurian materials from China include
portions of the Alectrosaurus olseni hypodigm from the Iren
Dabasu Formation (Mader & Bradley 1989), several speci-
mens from the Iren Dabasu Formation housed at the Erenhot
Dinosaur Museum (Currie & Eberth 1993) and ‘Chilan-
taisaurus’zheziangensis from the Tangshang Formation
(Dong 1979; Barsbold & Marya´
nska 1990).
Mader & Bradley (1989) assigned one of the syntypes of
Alectrosaurus olseni (AMNH 6368) to Therizinosauridae
(Fig. 9). AMNH 6368 includes a proximally and distally
damaged right humerus, a manual phalanx and a manual
ungual. First-hand examination of the material suggests it is
referable to Therizinosauria and appears to be intermediate
in morphology. Currie & Eberth (1993) also refer red several
additional isolated elements from the Iren Dabasu to Ther-
izinosauria (including a dentary with teeth). They referred
these elements specifically to Erlikosaurus,Segnosaurus
and possibly Enigmosaurus based on a hypothesis of
biostratigraphical correlation between the Iren Dabasu
and the Bayan Shire Formation of Mongolia (see also
Jerzykiewicz & Russell 1991). However, von Itterbeeck
et al. (2005) suggested a Campanian-Maastrichtian age for
the Iren Dabasu Formation based on charophyte and ostra-
cod fossils, which would indicate closer temporal correla-
tion with the Nemegt Formation.
AMNH 6368 is not referable to the Iren Dabasu ther-
izinosauroid Neimongosaurus, based on the relatively slen-
der proportions, prominent caudal trochanter and poorly
defined internal tuberosity of the humerus. Presence of
the latter character also precludes its referral to the Bayan
Shire taxon Erlikosaurus and the Nemegt therizinosauroid,
Therizinosaurus, as does its more moderately developed
deltopectoral crest. Segnosaurus possesses a remarkably
straight humeral shaft, more so than AMNH 6368, with
the latter exhibiting moderate deflection of the proximal
and distal aspects. Furthermore, although the distal aspect
of the humerus of AMNH 6368 is crushed, this element
appears notably narrow craniocaudally relative to that of
Segnosaurus. The humerus is more gracile than that of
Erliansaurus and possesses a different caudal trochanter
morphology, yet the holotype of Erliansaurus bellamanus
is a juvenile (Xu et al. 2002a) and AMNH 6368 is approx-
imately 50% larger; therefore ontogenetic variation cannot
be excluded as an explanation for these differences. Refer-
ral to the Bayan Shire taxon Enigmosaurus (for which the
humerus is unknown) is also possible. The material identi-
fied by Currie & Eberth (1993) as therizinosaurian has not
yet been examined first hand by the author, so no obser-
vations regarding the referral of these elements are made
here.
‘Chilantaisaurus’ zheziangensis is known from a prox-
imal portion of a right tibia, incomplete metatarsals II,
III and IV and pedal unguals (Dong 1979, translated by
Will Downs) (although Barsbold & Marya´
nska [1990] and
Marya´
nska [1997] only noted the presence of pedal digits
II and III) and was originally referred to ‘Megalosauri-
dae’ (Dong 1979). ‘Chilantaisaurus’ zheziangensis is ques-
tionably referred to this genus, which also contains the
species ‘C.’ tashuikouensis and ‘C.’ maortuensis (Barsbold
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536 L. E. Zanno
Figure 9. Therizinosaurian humerus (AMNH 6368) (A-C) and manual unguals (AMNH 6554) (D) formerly part of Alectrosaurus olseni
hypodigm. A, cranial view showing poorly defined internal tuberosity; B, medial view showing moderately developed deltopectoral crest
and cranial deflection and gracile morphology of distal humerus and morphology of caudal trochanter; and C, caudal view; D, lateral view.
Abbreviations: ct, caudal tuberosity of humerus; dpc, deltopectoral crest; hh, humeral head; it, internal tuberosity; and uc, ulnar condyle.
Scale bars: 5 mm (A-C); 3 mm (D).
& Marya ´
nska 1990). Extreme lateral compression and large
size of the pedal unguals are indicative of Therizinosauria;
however, examination of the proximal portion of the tibia
is needed for a confident referral to the clade.
To date the Cretaceous deposits of Mongolia have
produced four therizinosaurian species: Erlikosaurus
andrewsi (Perle 1981), Enigmosaurus mongoliensis (Bars-
bold 1983) and Segnosaurus galbinensis (Perle 1979)
from the Late Cretaceous Bayan Shire Formation,
and Therizinosaurus cheloniformis (Maleev 1954), the
first therizinosaurian recovered, from the Upper Creta-
ceous Nemegt Formation (Maastrichtian). Several isolated
therizinosaurian elements from Mongolia are known,
including an exceptionally preserved left pubis from
Baishin Tsav locality I-IV lacking a specimen number
(listed as Field # 93) discovered in the IGM collections and
referable to cf. Segnosaurus (pers. obs.), as well as a fibula
from the same locality (Field # 08) which may also be refer-
able to this taxon. Finally, a considerable amount of as yet
undescribed therizinosaurian material was collected during
the Japan–Mongolian Joint Palaeontological Expeditions
in 1993–1994, including cranial, axial and appendicular
elements (Watabe & Suzuki 2000a, b) and a well preserved
pelvis (M. Watabe pers. comm., December 2006).
A partial braincase, several teeth and a humeral shaft of
a therizinosaurian were reported from the Mifune Group
(Upper Cretaceous) on Japan’s Kyushu Island (Ikegami
& Tomida 2005). Reported basisphenoidal pneumaticity,
as well as the presence of a posterior trochanter on the
humeral shaft, is strong evidence supporting assignment of
this material to Therizinosauria, as suggested by Ikegami
& Tomida (2005).
Putative therizinosaurian materials are also known from
Uzbekistan, Karakalpakstan, Tajikistan, Kazakhstan and
Russia. Although much of this material is represented by
large, isolated unguals of questionable affinity, additional
postcranial and cranial elements potentially referable to
Therizinosauria have been described.
Averianov (2007) referred several elements from the
Santonian Bostobe Formation of Kazakhstan to Ther-
izinosauria; these include a right femur (CCMGE
601/12457), tooth (ZIN PH 35/49), two proximal portions
of left femora (ZIN PH 24/49 and 37/49), and pedal
phalanx (ZIN PH 25/49). Averianov (2007) also cited ther-
izinosaurian material from the Turonian Khirkindek Forma-
tion, including a juvenile sacral centrum (ZIN PH 32/49,
partial left metacarpal III (ZIN PH 1/49), and two proximal
portions of left femora (ZIN PH 38/49, and 39/49).
Nessov (1995) referred the right femur noted above
from the Bostobe Formation (CCMGE 601/12457) to
Tarbosaurus. Averianov (2007) assigned it to cf. Neimon-
gosaurus sp. based on presumed similarity to this taxon.
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Re-evaluation of Therizinosauria 537
However, the femur of Neimongosaurus does not possess
autapomorphic features and, other than its unusually small
size, there is no basis for a genus-level assignment. Further-
more, the femora of Neimongosaurus possess three signa-
ture characters of derived therizinosaurians, none of which
are evident on CCMGE 601/12457: depressed bridge
between femoral head and greater trochanter in cranial
view; dorsally inclined femoral head; and bridge between
femoral head and greater trochanter constricted, hourglass
shaped in proximal view. Averianov (2007) noted the pres-
ence of two therizinosaurian taxa in the Bostobe Forma-
tion based on the differing morphologies of CCMGE
601/12457 versus ZIN PH 24/49 and 37/49. Although
CCMGE 601/12457 does not possess the three ther-
izinosaurian synapomorphies mentioned above, the remain-
ing two specimens (ZIN PH 24/49 and 37/49) do possess all
these features and may be referable to Therizinosauroidea.
Femora morphologically similar to ZIN PH 24/49 and ZIN
PH 37/49 are also known from the Khirkindek Forma-
tion, suggesting that therizinosauroids are present in both
formations.
Rozhdestvensky (1964, 1969, 1970, fig. 2b),
Rozhdestvensky & Khozatsky (1967, =Alectrosaurus)
and Suslov (1982) all considered several large, transversely
compressed unguals (IZK 2/1, 2/2 and 2/3, and PIN
2229/19) from the Bostobe Formation (Shakh-Shakh
locality) to be therizinosaurian (Nessov 1995; Averianov
2007).
Isolated elements from the Mogoito locality of the Murtoi
Formation, south-eastern Russia (upper Barremian-middle
Aptian, Averianov et al. 2003) have been referred to Ther-
izinosauria. These include the first dinosaur fossil found
from Mogoito: a distal pedal phalanx referred to Theropoda
by Riabinin (1937), later referred to Sauropoda by Nessov
(1995), and most recently considered therizinosaurian by
Averianov et al. (2003). Dmitriev (1960) described a
large ungual from Mogoito, considered ‘carnosaurian’ by
Rozhdestvensky (1970, fig. 2a) yet regarded as a possible
therizinosaurian by Nessov (1995). Averianov et al. (2003)
cited a partial pedal phalanx from Mogoito (ZIN PH 2/13)
as referable to Therizinosauria, as well as a phalangeal frag-
ment described by Riabinin (1937). The phalanx is referred
to Therizinosauria by Averianov et al. (2003) based on
unequal expression of the collateral ligament pits on the
distal aspect. However, this character has a broader distri-
bution; in therizinosaurians, extremely poor expression of
collateral ligament pits constitutes a characteristic feature.
Averianov et al. (2003) noted that Riabinin (1937) described
deep collateral ligament pits on this phalanx.
Nessov (1995), Averianov (2007) and Alifanov & Averi-
anov (2006) reported additional putative therizinosaurian
materials from Uzbekistan, Karakalpakstan and Tajikistan.
Potential therizinosaurian materials from Uzbekistan have
been recovered from the late Cretaceous Bissekty Forma-
tion (Dzharakuduk locality). Nessov (1995) reported three
ungual phalanges (Nos. 453, 454, 455) and a tooth (No.
3/111822) from upper Turonian strata and a vertebra (No.
456) from Coniacian strata, all referred to Therizinosauria
indet. Other potential therizinosaurian elements noted by
Nessov (1995) include preungual phalanges (669–702),
proximal (721–722) and distal portions (715–716) of
humeri, cervical vertebrae (704–710), presacral vertebrae
(711–712), dorsal vertebra (713–714), possible basisphe-
noid (719) and frontal. The presence of two therizinosaurian
taxa based on postcranial elements from the Bissekty
Formation at Dzharakuduk, Uzbekistan is noted by Averi-
anov (2007), as is a therizinosaurian femur from the
lower Santonian Yalovach Formation, Tajikistan. An ungual
phalanx (457) from the lower Cenomanian, Khodzhakul
Formation, Sheikhdzhelili locality, Karakalpakstan was
also reported by Nessov (1995). Alifanov & Averianov
(2006) referred those specimens from the Kansai local-
ity of the Yalovach Formation, identified by Nessov (1995)
as cf. Oviraptoridae, to Therizinosauridae.
Europe. A cervical vertebra from the Barremian Wessex
Formation of the Isle of Wight, England, described by
Seeley (1888) and ultimately named Thecocoelurus daviesi
(von Huene, 1923), was considered a possible ther-
izinosaurian by Kirkland et al. (2005). The same specimen
(BMNH R181) was earlier referred to Oviraptorosauria by
Naish & Martill (2002). Two characters present on this
ventral centrum – a ventral sulcus and lateral ridges –
characterize both oviraptorosaurians and therizinosauri-
ans. Naish & Martill (2002) suggest that round pneumatic
foramina and a more gracile neural spine refines placement
of the specimen to Oviraptorosauria. However, cervical
vertebrae of Falcarius (e.g. UMNH VP 14657) exhibit both
features, so the taxonomic affinities of BMNH R181 remain
unclear. If BMNH R 181 is a primitive therizinosaurian its
presence in Europe would indicate that therizinosaurians
were widespread across Asia, Europe and North America
in the Barremian. If the specimen represents an ovirap-
torosaurian, it would be the oldest evidence of this clade
outside of Asia.
Africa. A large incomplete ungual phalanx from the Early
Cretaceous Sundays River Formation of South Africa
(Mateer 1987, fig. 3a-b) was identified by Nessov (1995) as
a possible therizinosaurian, as was an ungual from Lower
Cretaceous beds in Niger (Rozhdestvensky 1970). Thus
far, however, therizinosaurians are not definitively known
from Gondwana, although their hypothesized presence in
the Early Cretaceous of Europe, together with a hypoth-
esized land bridge linking Europe and Northern Africa
during this time (Galton & Taquet 1982; Buffetaut 1989;
Sereno et al. 1994, 1996, 1998; Bosellini 2002) raises the
possibility of dispersal to this southern landmass. Never-
theless the presence of large theropods in Africa during
this interval (e.g. Sereno et al. 1994, 1996, 1998; Sereno
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538 L. E. Zanno
& Brusatte 2008) suggests caution regarding the referral of
isolated large unguals to Therizinosauria, especially given
that the above-noted specimens do not exhibit the extreme
transverse compression typical of the enlarged claws of
therizinosaurids (e.g. No. graffami,Therizinosaurus).
Palaeobiogeographical implications
Two possible biogeographical scenarios account for the
pan-Laurasian distribution of basal therizinosaurians in the
Barremian (Early Cretaceous; Fig. 7): (1) vicariance, i.e.
basal therizinosaurians were present on Asia and North
America prior to development of major north Atlantic
rifting in the Late Triassic (Smith et al. 1994); and (2)
dispersal, i.e. basal therizinosaurians dispersed between
Asia and North America via Europe after the rifting
event yet prior to 125 million years ago. The presence of
derived coelurosaurians (e.g. Archaeopteryx) in the Juras-
sic, coupled with the phylogeny presented here, is sugges-
tive of at least a Middle Jurassic origin for at least some
coelurosaurian lineages including therizinosaurians, which
supports the former hypothesis; however, to date no defini-
tive record of Therizinosauria is known prior to the Early
Cretaceous.
If a dispersal event is solely responsible for the
Early Cretaceous distribution of therizinosaurians, fossil
evidence indicates that it must have occurred prior
to the middle Barremian. Formation of a land bridge
between North America and Europe during the Berriasian-
Valanginian (138–132 Ma), with a reversion to isolation by
the Hauterivian-Barremian, was proposed by Smith et al.
(1994) based primarily on the ocean floor magnetic anomaly
record. Haq et al. (1988) argued for a period of global
sea-level regression between 135 and 125 million years
ago (Valanginian-Barremian), supporting this hypothesis.
If correct, the timing of emplacement of a Eurasian land
bridge could account for the Early Cretaceous distribution
of basal therizinosaurians and the significant morpholog-
ical discrepancy noted between the relatively coeval taxa
Falcarius from the Barremian of North America and Beip-
iaosaurus from the Barremian of China.
Due to the absence of data regarding the geograph-
ical distribution of therizinosaurians prior to their pan-
Laurasian appearance in the Early Cretaceous, we lack
enough fossil evidence to refute either hypothesis. However,
it is important to note that these hypotheses are not mutu-
ally exclusive and an Early Cretaceous dispersal event could
have occurred even if therizinosaurians had already estab-
lished a pan-Laurasian distribution in the Jurassic as a result
of vicariance.
The assignment of Nothronychus to Therizinosauridae
indicates that therizinosaurids were present in North Amer-
ica and Asia by the Turonian (early Late Cretaceous)
(Fig. 7). Given that a monophyletic relationship between
North American therizinosaurians is not recovered here, at
least one dispersal event between Asia and North America
prior to the Turonian is necessary to account for this finding.
Hypothesized faunal interchange between North Amer-
ica and Asia via a late Early Cretaceous Beringian land-
bridge is well established in the literature (e.g. Barsbold
1983; Kirkland 1996, 1998; Kirkland et al. 1997, 1998).
The timing of the bridge can be refined based on biostrati-
graphical evidence to the Aptian/Albian, when dinosaur
clades present in Asia begin appearing in the fossil record of
western North America and sea level reached its maximum
regression for the next 20 million years (Haq et al. 1988;
Fig. 7). Indeed, faunal interchange between the larger land-
mass of Asia and the smaller landmass of North America
has been proposed as a primary cause in the regional extinc-
tion of endemic North American dinosaur faunas during
this time (Russell 1993; Kirkland et al. 1997). The pres-
ence of the derived therizinosaurid Nothronychus in North
America during the early Late Cretaceous provides strong
independent support for previous hypotheses of a faunal
interchange event between North America and Asia in the
late Early Cretaceous.
Conclusions
This study provides an extensive and comprehensive
re-evaluation of the taxonomy and phylogeny of Ther-
izinosauria based on new information concerning the
anatomy of Falcarius utahensis (Zanno 2010) and No. graf-
fami (the most complete therizinosauri) (Zanno et al. 2009),
as well as recently discovered therizinosauroid taxa (e.g.
Zhang et al. 2001; Xu et al. 2002a; Li et al. 2007, 2008).
Several previously described therizinosaurians – Beip-
iaosaurus,Enigmosaurus,Suzhousaurus,Segnosaurus and
Therizinosaurus – are rediagnosed. Inaccessibility of rele-
vant specimens prevented assessment of several important
therizinosaurian taxa for which the original diagnoses are,
at least in part, no longer applicable, including Nanshi-
ungosaurus brevispinus,‘N.’ bohlini,Alxasaurus and
Erlikosaurus. Phylogenetic definitions for the clades Ther-
izinosauria and Therizinosauroidea are examined based on
the most inclusive and character-rich phylogeny of Ther-
izinosauria to date. Therizinosauria is redefined, and justifi-
cation for maintaining the definition of Therizinosauroidea
given by Clark et al. (2004) is provided (the redefinition of
Therizinosauridae based on this study is maintained as per
Zanno et al. [2009]).
In contrast to all previous studies, ingroup relationships
of Therizinosauria suggest a more primitive placement for
Neimongosaurus, based largely on the absence of derived
pelvic morphology. In this study (as was reported in Zanno
et al. 2009) Nothronychus is posited as a therizinosaurid,
whereas Suzhousaurus and Enigmosaurus are recovered as
successive sister taxa to Therizinosauridae, respectively.
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Re-evaluation of Therizinosauria 539
Therizinosaurians are known to have inhabited North
America and Asia from the Early to early Late Creta-
ceous (and may have been present in the Jurassic of China).
However, the clade appears to have been much more preva-
lent in the Late Cretaceous of Asia. The assignment of
therizinosaurian materials from Kazakhstan to cf. Neimon-
gosaurus (Averianov 2007) is unsupported by this study.
The presence of derived therizinosaurids in North Amer-
ica and Asia by the early Late Cretaceous requires either
a cosmopolitan presence of basal therizinosaurians prior
to the break up of Lauasia, or at least one dispersal event
between the two landmasses prior to the Turonian. Although
the presence of Asian dinosaur clades in the late Early
Cretaceous of North America is well documented, the
appearance of Therizinosauridae in the Turonian of Utah
offers independent evidence of a late Early Cretaceous inter-
change event between the two landmasses.
Acknowledgements
Special thanks go to P. Makovicky and A. Turner for soft-
ware assistance, data files and specimen photos, J. Harris
for discussions on phylogenetic taxonomy, T. Gates for
assistance with figures, H. You and X. Xu for access to
unpublished specimens, S. Sampson, X. Xu, J. Kirkland,
A. Ekdale, F. Brown and T. Gates for commentary on
the manuscript, and A. Turner and J. Clark for thoughtful
reviews. Thanks go to J. Kirkland, D. DeBlieux, J. Cavin,
S. Masters, M. Hayden, E. Lund, M. Getty, and the UGS,
UMNH, UFOP and Eccles Dinosaur Park volunteers for
preparation and field assistance. Access to specimens was
provided by K. Tsogtbaatar, C. Tsogtbaatar and R. Bars-
bold, Y. Hailu, X. Xu, P. Sereno, M. Norell and I. Rutzky,
P. Makovicky, R. Barrick, J. Bartlett, J. Bird, P. Currie, J.
Gardner, K. Seymour, D. Brinkman, W. Joyce and J. Horner.
This project represents dissertation research supported by a
University of Utah Graduate Research Fellowship and two
NSF GK-12 Fellowships, and the Utah Museum of Natural
History. Additional grants were provided by the Jurassic
Foundation, The Palaeontological Association, The Pale-
ontological Society, The Discovery Channel and National
Science Foundation. Free online versions of TNT and
Mesquite were made available by the Willi Hennig Soci-
ety and the Free Software Foundation Inc.
Note
Appendices 1–3 can be viewed online.
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