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A NORIAN LAGERPETID
DINOSAUROMORPH FROM THE
QUEBRADA DEL BARRO FORMATION,
NORTHWESTERN ARGENTINA
1
Instituto y Museo de Ciencias Naturales, Universidad Nacional de San Juan, Avenida España 400 Norte, 5400 San Juan, Argentina.
2
Museo Paleontológico “Egidio Feruglio”, Avenida Fontana 140, U9100GYO Trelew, Chubut, Argentina.
3
CONICET, Consejo Nacional de Investigaciones Científicas y Técnicas, Ciudad Autónoma de Buenos Aires, Buenos Aires, Argentina.
RICARDO N. MARTÍNEZ
1
CECILIA APALDETTI
1,2,3
GUSTAVO A. CORREA
1,3
DIEGO ABELÍN
1
Submitted: February 13
th
, 2015 - Accepted: June 21
st
, 2015
To cite this article: Ricardo N. Martínez, Cecilia Apaldetti, Gustavo A. Correa, Diego Abelín (2015). A Norian
lagerpetid dinosauromorph from the Quebrada del Barro Formation, northwestern Argentina. Ameghiniana
53: 1–13.
To link to this article: http://dx.doi.org/10.5710/AMGH.21.06.2015.2894
PLEASE SCROLL DOWN FOR ARTICLE
Also appearing in this issue:
A new large-bodied dinosauriform
from the Late Triassic of
northwestern Argentina.
Stable isotope data show
gomphotheriids had more flexible
diets than mammoths.
Chironomids and diatoms show
environmental changes in the
recent past of Patagonia.
1
AMGHB2-0002-7014/12$00.00+.50
A NORIAN LAGERPETID DINOSAUROMORPH FROM THE
QUEBRADA DEL BARRO FORMATION, NORTHWESTERN
ARGENTINA
RICARDO N. MARTÍNEZ
1
, CECILIA APALDETTI
1
,2,3
, GUSTAVO A. CORREA
1
,3
, AND DIEGO ABELÍN
1
1
Instituto y Museo de Ciencias Naturales, Universidad Nacional de San Juan, Avenida España 400 Norte, 5400 San Juan, Argentina. martinez@unsj.edu.ar;
capaldetti@unsj.edu.ar; gustavoalejandrocorrea@yahoo.com.ar; dabelin@unsj.edu.ar
2
Museo Paleontológico “Egidio Feruglio”, Avenida Fontana 140, U9100GYO Trelew, Chubut, Argentina.
3
C
ONICET, Consejo Nacional de Investigaciones Científicas y Técnicas, Ciudad Autónoma de Buenos Aires, Buenos Aires, Argentina.
Abstract. The early evolution of Ornithodira –the clade including pterosaurs and dinosaurs– is poorly known. Until a decade ago, the basal
radiation of Dinosauromorpha, the clade including dinosaurs and birds, was poorly understood because of the poor fossil record restricted to
specimens known from the Ladinian Chañares Formation in Argentina. Over the last years the discovery of several non-dinosaurian dinosauro-
morphs dramatically expanded this record and also demonstrated that this group –previously restricted to de Middle Triassic– survived at
least well into the Norian. Although Norian non-dinosaurian dinosauromorphs have been reported from several regions around the world, the
only known Norian non-dinosauriform dinosauromorphs –Dromomeron romeri Irmis, Nesbitt, Padian, Smith, Turner, Woody, and Downs and
Dromomeron gregorii Nesbitt, Irmis, Parker, Smith, Turner, and Rowe– come from North America. We report here the first record from the
Southern Hemisphere of a non-dinosauriform dinosauromorph, Dromomeron gigas sp. nov., from the Norian Quebrada del Barro Formation,
northwestern Argentina. A phylogenetic analysis recovers Dromomeron gigas nested into the monophyletic group Lagerpetidae, and as the
sister taxon to Dromomeron romeri. The inclusion of D. gigas within Lagerpetidae suggests that body size increased in this lineage over time, as
was previously demonstrated for Dinosauriformes as a whole, and that lagerpetids reached a larger size than previously thought. Finally, the
new finding provides new information on the basal radiation of Dinosauromorpha constituting the first record of a Norian association of di-
nosaurs with non-dinosauriform dinosauromorphs outside North America.
Key words. Dinosauromorpha. Lagerpetidae. Dromomeron. Norian. Marayes.
Resumen. UN DINOSAUROMORPHA LAGERPETIDO NORIANO DE LA FORMACIÓN QUEBRADA DEL BARRO, NOROESTE DE ARGENTINA.
La evolución temprana de Ornithodira, el clado que incluye pterosaurios y dinosaurios, es escasamente conocido. Hasta hace una década atrás,
el conocimiento de la radiación basal de Dinosauromorpha, clado al que pertenecen dinosaurios y aves, estaba limitado a los especímenes
provenientes del Ladiniano de la Formación Chañares de Argentina. En los últimos años, el descubrimiento de varios nuevos especímenes de
dinosauromorfos no-dinosaurios expandió notablemente este registro demostrando que este grupo –previamente restringido al Triásico
Medio– persistió bien avanzado el Noriano. Aunque los dinosauriformes no-dinosaurios provienen de diversos sitios de alrededor del mundo,
los únicos registros de dinosauromorfos no-dinosauriformes norianos –Dromomeron romeri Irmis, Nesbitt, Padian, Smith, Turner, Woody, and
Downs y Dromomeron gregorii Nesbitt, Irmis, Parker, Smith, Turner, and Rowe– han sido hallados en Norteamérica. En este trabajo reportamos
el primer registro de un dinosauromorfo no-dinosauriforme del Hemisferio Sur, Dromomeron gigas sp. nov., proveniente de afloramientos
norianos de la Formación Quebrada del Barro, al Noroeste de Argentina. El resultado del análisis filogenético recuperó a Dromomeron gigas
como parte del grupo monofilético Lagerpetidae, y taxón-hermano de Dromomeron romeri. La inclusión de D. gigas entre los Lagerpetidae
sugiere que el tamaño corporal de este linaje también se incrementó a través del tiempo, tal como fue previamente demostrado para Dino-
sauriformes; y que los lagerpetidos alcanzaron un tamaño mayor del actualmente conocido. El nuevo hallazgo provee novedosa información
al conocimiento de la radiación basal de Dinosauromorpha constituyendo el primer registro de una asociación faunística noriana de dinosau-
rios con dinosauromorfos no-dinosaurios fuera de Norteamérica.
Palabras clave. Dinosauromorpha. Lagerpetidae. Dromomeron. Noriano. Marayes.
AMEGHINIANA - 2016 - Volume 53 (1): 1 – 13 ARTICLES
ISSN 0002-7014
THE early evolution of Ornithodira (Gauthier and Padian,
1985), the clade including pterosaurs and dinosaurs, is
poorly known (Langer et al., 2009; Brusatte et al., 2010a;
Benton et al., 2014). Within this group, knowledge of the
basal radiation of Dinosauromorpha (dinosaurs and their
closest extinct relatives) was long obscured because fossils
of these animals were so scarce. Until a decade ago, all
known non-dinosaurian dinosauromorphs were from the
Middle Triassic (Ladinian) Chañares Formation in Argentina.
This record includes the non-dinosauriform dinosauro-
morph Lagerpeton chanarensis Romer, 1971 (Arcucci, 1986;
Sereno and Arcucci, 1994a), and the non-dinosaurian di-
nosauriforms Lewisuchus admixtus Romer, 1972; Pseudolago-
suchus major Arcucci, 1987 (a junior synonym of L. admixtus,
sensu Arcucci, 1997, 1998; Nesbitt et al., 2010), and Mara-
suchus lilloensis Sereno and Arcucci, 1994b. On the other
hand, the oldest early dinosaurs were reported from Late
Triassic rocks in South America (e.g., Ischigualasto Forma-
tion in Argentina; Santa Maria Formation in Brazil). The afore-
mentioned record exhibited a scenario of non-dinosaurian
dinosauromorphs restricted to the Middle Triassic and di-
nosaurs restricted to the Late Triassic (and younger ages).
This has dramatically changed as from 2003, with the re-
port of several Middle and Late Triassic non-dinosaurian di-
nosauriforms such as Silesaurus opolensis Dzik, 2003, from
the Carnian Krasiejów deposits of Poland; Sacisaurus agu-
doensis Ferigolo and Langer, 2007, from the Norian Caturrita
Formation of Brazil; Asilisaurus kongwe Nesbitt, Sidor, Irmis,
Angielcczyk, Smith, and Tsuji, 2010, from the late Anisian
Lifua Member of Tanzania; Diodorus scytobrachion Kam-
merer, Nesbitt, and Shubin, 2011, from the ?Carnian-Norian
Timezgadiouine Formation of Morocco; Ignotosaurus fragilis
Martínez, Apaldetti, Colombi, Alcober, Sereno, Fernández,
Santi Malnis, Correa, and Abelín, 2012, from the Carnian-
Norian Ischigualasto Formation of Argentina; Lutungutali
sitwensis Peecook, Sidor, Nesbitt, Smith, Steyer, and Angiel-
czyk, 2013, from the Anisian Ntawere Formation from Zam-
bia; and an unnamed silesaurid from the Norian Zvaszinek
Beds from Southern Poland (Niedzwiedzki et al., 2014). The
new records also include the non-dinosauriform dinosauro-
morphs Dromomeron romeri Irmis, Nesbitt, Padian, Smith,
Turner, Woody, and Downs, 2007, and Dromomeron gregorii
Nesbitt, Irmis, Parker, Smith, Turner, and Rowe, 2009, from
the Norian Chinle and Colorado City formations of North
America; and a putative unnamed lagerpetid from the
Carnian beds of the Carnian-Norian Ischigualasto Formation
in Argentina (PVSJ 883, Martínez et al., 2012). Such new
findings reveal that non-dinosaurian dinosauromorphs per-
sisted and coexisted with dinosaurs at least well into the
Norian in USA (Ezcurra, 2006; Irmis et al., 2007) and Europe
(Niedzwiedzki et al., 2014). Nevertheless, until now the
record of Norian non-dinosauriform dinosauromorphs was
restricted to North America (Irmis et al., 2007; Nesbitt et al.,
2009).
Here we report a basal dinosauromorph from the Que-
brada del Barro Formation in northwestern Argentina. This
unit has been recently interpreted as late Norian (Martínez
et al., 2015) and therefore represents the first Norian
record of a non-dinosauriform dinosauromorph in the
Southern Hemisphere and forms part of probably the
youngest association of dinosaurs and non-dinosauriform
dinosauromorphs known worldwide.
GEOLOGICAL SETTINGS
The holotype of the new taxon (PVSJ 898) was found in
the Quebrada del Barro Formation during the fieldwork
carried out by a team from the Instituto y Museo de Ciencias
Naturales of the Universidad Nacional de San Juan. The
Quebrada del Barro Formation crops out in northwestern
Argentina and forms part of the continental Triassic
Marayes-El Carrizal Basin (Fig. 1).
The Marayes-El Carrizal Basin, like the neighboring Is-
chigualasto-Villa Union Basin, is one of a series of exten-
sional basins developed during the early Mesozoic along
the southwestern edge of Pangaea (Spalletti, 1999). The
stratigraphy of the Basin was described by Bossi (1976),
who defined the Marayes Group as composed by –from
base to top– the Esquina Colorada (Middle Triassic), Carrizal
(late Carnian–early Norian) and Quebrada del Barro (late
Norian–Rhaetian) formations (Fig.1). Recent studies showed
that the Marayes Group also includes another still unnamed
lithostratigraphic unit resting conformably over the Que-
brada del Barro Formation (Colombi et al., 2013, 2014)
which has been interpreted as Early Jurassic in age given the
presence of the massospondylid Leyesaurus; see Martínez
et al. (2015) for a recent summary of previous work and new
evidence from the upper part of this sequence.
The remains described here were found in the Quebrada
del Barro Formation, a 600–1400 m-thick unit including
coarse sandstones and conglomerates interbedded with
sandy-claystone with sabulitic clasts (Fig. 1). The deposi-
tional environment was recently reinterpreted as a dis-
tributive fluvial system, in which fluvial channels with a
large range of sinuosity form a complex deposit with mud-
flow dominated floodplains, and terminal splays formed
by heterolithic sandstone and mudstone accumulations
(Colombi et al., 2014). Based on a single specimen of an in-
complete basal sauropodomorph dinosaur Bossi and Bona-
AMEGHINIANA - 2016 - Volume 53 (1): 1 – 13
2
parte (1978) initially proposed a Norian age for the Que-
brada del Barro Formation based on a biostratigraphic cor-
relation with the Los Colorados Formation in the
neighboring Ischigualasto-Villa Unión Basin. A new and di-
verse vertebrate fauna recently discovered suggested a late
Norian–Raethian (?) age for this unit (Martínez et al., 2015).
MATERIALS AND METHODS
Terminology
We use the term ‘‘basal’’ for the earliest diverging
branches of a lineage with respect to the later diverging
branches, which are often more derived. We employ tradi-
tional, or “Romerian” anatomical and directional terms
over veterinarian alternatives (Wilson, 2006). “Anterior” and
“posterior”, for example, are used as directional terms
rather than the veterinarian alternatives “rostral” or “cra-
nial” and “caudal”.
Femur length estimation
In order to estimate the femur length of the new speci-
men we used the multiple linear regression obtained by
Barrett et al. (2015) because it recovered a statistically sig-
nificant relationship between femoral length and width of
the distal and proximal ends.
In that formula the femoral length (l), proximal femoral
width (w1), and distal femoral width (w2) are related by the
MARTÍNEZ ET AL.: NORIAN LAGERPETID DINOSAUROMORPH
3
Figure 1. Location and geology of the Marayes-El Carrizal Basin; 1, geological map of the Marayes-El Carrizal Basin; 2, stratigraphic section of
the upper levels of the Quebrada del Barro Formation at the “Quebrada del Puma” locality, type locality of Dromomeron gigas sp. nov. PVSJ 898.
Black circle indicates the finding site, located 160 m below the top of the Quebrada del Barro Formation.
equation: l= -5.37 + 2.94(w1) + 3.17(w2).
Phylogenetic Analysis
In order to determine the phylogenetic position of PVSJ
898 within Dinosauromorpha, we added the new taxon to
the data matrix for avian-line archosaurs published by Nes-
bitt et al. (2010) including the taxa added by Martínez et al.
(2012). In addition, one of the states of one of the original
characters was modified here (character 211), and three
extra characters were added to the data set, which refer
to the distal femur anatomy. Two of the new characters
(presence of an anteromedial ridge and a lateral tuberosity)
were previously considered autapomorphies of D. romeri
but now are also observed in the specimen PVSJ 898 des-
cribed here (See Supplementary Online Information).
We maintained the original character states for all the
previous taxa, and we follow the same criteria used by the
authors in the original analysis (i.e., Erythrosuchus was con-
strained as outgroup; all characters were equally weighted;
characters: 21, 78, 89, 98, 116, 142, 159, 169, 175, 177,
195, 200, 227, 250, 281 were ordered; and Pseudolago-
suchus and Lewisuchus were combined).
The new dataset of 36 taxa and 293 characters was
analyzed in the parsimony-based phylogenetic program
TNT 1.1 (Goloboff et al., 2008a,b). The heuristic search of
1000 replicates of Wagner trees followed by TBR branch
swapping found 9 most parsimonious trees (MPTs) of 744
steps (CI=0.469, RI=0.708) in 100% of the replicates. A
second round of TBR branch swapping on these trees did
not find any additional MPTs.
Institutional Abbreviations. NHMUK, Natural History Mu-
seum, London, UK. PVSJ, Instituto y Museo de Ciencias
Naturales, San Juan, Argentina.
SYSTEMATIC PALEONTOLOGY
ARCHOSAURIA Cope, 1869 sensu Gauthier and Padian, 1985
D
INOSAUROMORPHA Benton, 1985 sensu Sereno, 1991
L
AGERPETIDAE Arcucci, 1986 sensu Nesbitt, Irmis, Parker,
Smith, Turner, and Rowe, 2009
Genus Dromomeron Irmis, Nesbitt, Padian, Smith, Turner,
Woody, and Downs, 2007
Type species. Dromomeron romeri Irmis, Nesbitt, Padian, Smith,
Turner, Woody, and Downs, 2007.
Dromomeron gigas sp. nov.
Figures 2–3.4
Etymology. The specific name “giant” is derived from the
Greek word gigas (giant), in allusion to the large size of Dro-
momeron gigas sp. nov. compared with the other lagerpetid
dinosauromorphs.
Holotype. PVSJ 898, a partial left femur represented by its
proximal and distal portions, but lacking the mid-shaft and
most of the medial condyle of the distal end.
Type Locality and Horizon. “Quebrada del Puma” Locality,
Caucete Department, San Juan Province, Argentina (Fig. 1).
The fossil locality belongs to the southern outcrops of the
Norian Quebrada del Barro Formation included in the
Marayes-El Carrizal Basin (Bossi, 1976). A late Norian age
was recently proposed for this formation based on bios-
tratigraphic correlations with the Los Colorados Forma-
tion from the neighboring Ischigualasto-Villa Unión Basin
(Martínez et al., 2015). The reddish muddy-sandstones of
the horizon including PVSJ 898 are located 160 meters
below the top of the unit (Fig. 1).
Diagnosis. Large Dromomeron diagnosed by the following
autapomorphies: blunt and rugose anterior trochanter
reaching the level of the posterolateral edge of the proxi-
mal portion of the femur; pronounced muscle scar on the
posteromedial surface of the proximal portion of the femur.
Description. Only two fragments corresponding to the
proximal and distal portions of the left femur of PVSJ 898
are preserved (Fig. 2). Most of the distal medial condyle is
lost; otherwise, the fragments are well preserved.
The shaft is hollow and filled in by calcite. The section is
elliptical with thin walls. The thin cortex of the mid-shaft
measures 1.4 mm in thickness, which represents a cortex
thickness/minimum shaft diameter ratio of near 0.14. This
ratio is smaller than that of other non-dinosauriform di-
nosauromorphs [e.g., 0.2 in Dromomeron romeri and the
unnamed lagerpetid from the Ischigualasto Formation
(Martínez et al., 2012)], or than that of the basal theropod
Eodromaeus murphi Martínez, Sereno, Alcober, Colombi,
Renne, Montañez, and Currie, 2011, in which this ratio is
also 0.2.
AMEGHINIANA - 2016 - Volume 53 (1): 1 – 13
4
MARTÍNEZ ET AL.: NORIAN LAGERPETID DINOSAUROMORPH
5
The loss of the fragment corresponding to the mid-shaft
of the femur precludes knowing the exact orientation of the
head; nevertheless we presume it is anteromedially directed
as in other archosauriforms. In anterolateral and postero-
medial views the head is hook-shaped and presents a ven-
tral emargination on the anterolateral side (Fig. 2.1–2) as in
the lagerpetids Lagerpeton (Sereno and Arcucci, 1994a)
and both species of Dromomeron (Nesbitt et al., 2009), but
contrasting with the rounded head with a ventral notch
presents in the dinosauriforms Asilisaurus, Silesaurus and
Sacisaurus. The anterolateral surface of the proximal end is
slightly convex as in D. romeri and D. gregorii, but different
from the flat anterolateral surface of Lagerpeton.
The proximal surface is roughly sub-triangular in proxi-
mal view as in other basal dinosauromorphs, although it is
delimited by rounded borders (Fig. 2.4), thus resembling the
condition of lagerpetids and the dinosauriform Asilisaurus,
instead the straight borders in the dinosauriforms Silesaurus
and Sacisaurus. As in other lagerpetids, the proximal surface
is smooth and rounded, lacking the groove characteristic
of dinosauriforms.
The anterolateral surface of the proximal portion is
slightly convex as in both species of Dromomeron and lacks
the anterolateral tuber as in all lagerpetids. The small and
rounded anteromedial tuber is separated from the slightly
more expanded posteromedial tuber by a shallow cleft or
Figure 2. 1–5, Dromomeron gigas sp. nov., PVSJ 898, left femur; 1, anterolateral; 2, posteromedial; 3, anterior; 4, proximal view; 5, distal view.
Abbreviations: ads, anterodistal scar for attachment of M. femorotibialis externus; amt, anteromedial tuber; at, anterior trochanter; ctf, crista
tibiofibularis; faa, facies articularis antitrochanterica; fl, flange; gr, groove; lc, lateral condyle; mc, medial condyle; pms, posteromedial scar; pmt,
posteromedial tuber; samcl, scar for attachment of M. caudifemoralis longus; t, tuberosity, ve, ventral emargination. Scale bar= 10 mm.
groove (Fig. 2.2, 4). Among basal dinosauromorphs, the pos-
teromedial tuber is slightly more prominent than the medial
one in Lagerpeton, D. romeri, D. gregorii, and Sacisaurus; ab-
sent in Silesaurus and Diodorus; and pronounced, small and
rounded in Marasuchus, Pseudolagosuchus, and Asilisaurus.
In basal dinosaurs the anteromedial tuber is better developed
and hook-shaped in proximal view (e.g., Eoraptor, Eodromaeus,
Chromogisaurus; Sereno et al., 2012; Ezcurra, 2010). The
ventrally descending facies articularis antitrochanterica
extends onto the posteromedial side of the femoral head as
in other dinosauromorphs (Fig. 2.2). On the posteromedial
surface of the proximal portion of the femur, PVSJ 898 bears
a blunt and rugose posteromedial scar extending from the
posteromedial edge to the anteromedial region (Fig. 2.2).
The proximal border of this scar, not present in other di-
nosauromorph, contacts the shaft by a sinuous step (Fig. 2.2).
On the anterolateral surface of the proximal portion of
the femur, the anterior trochanter is a rugose and blunt
tuberosity that tappers dorsally (Fig. 2.1, 3). This rugose
tuberosity extends posteroventrally ending at the level of
the posterolateral edge of the proximal portion of the femur.
Distally, the tuberosity tapers towards the shaft, but proxi-
mally contacts the shaft abruptly although not forming a
step. Among lagerpetids, in Lagerpeton, D. romeri and smaller
specimens of D. gregorii the anterior trochanter is absent, but
it is present in larger specimens of the latter form suggesting
it is an ontogenetic feature (Nesbitt et al., 2009). Neverthe-
less, as the same authors pointed out, the largest speci-
men of D. romeri lacks the anterior trochanter as well as
the trochanteric shelf, suggesting that D. romeri never de-
veloped an anterior trochanter or, conversely, this individual
might have reached maturity at a larger size compared to D.
gregorii. Whether the character is ontogenetic or not, when
the anterior trochanter and trochanteric shelf are present
in other dinosauromorphs, the shape is quite different from
the blunt tuberosity of PVSJ 898. For this reason, this trait
is considered as autapomorphic and distinguishes the new
taxon from other dinosauromorphs.
PVSJ 898 lacks a fourth trochanter; instead it has a ru-
gose muscle scar for attachment of M. caudifemoralis longus
extending on the posteromedial surface of the femur, from
the area below the neck of the femoral head (Fig. 2.2). In this
trait PVSJ 898 is similar to D. romeri, but differing from the
slightly pronounced trochanter present in D. gregorii (Nes-
bitt et al., 2009), or the better developed one of Lagerpeton
and dinosauriforms. This scar extends ventrolaterally al-
though the lack of the mid-shaft portion of the femur pre-
cludes measuring its total length.
In PVSJ 898 the distal portion of the anteromedial edge
of the femur is a ridge which forms an acute angle between
the anterior and medial surfaces. This angle is close to or
smaller than 90°, similar to the lagerpetid PVSJ 883 and the
other Dromomeron species (Fig. 3). The most distal portion of
this edge ends in a sharp ridge or flange (Fig. 2.1–3) similar
to D. romeri, although in the latter specimen this ridge is
smaller and less developed. The anterior surface of the dis-
tal end of the femur has a distinct mediolaterally oriented
scar for attachment of M. femorotibialis externus as in D. gre-
gorii and D. romeri (Nesbitt et al., 2009). PVSJ 898 bears a
lateral tuberosity on the anterolateral edge of the distal end
of the femur (Fig. 2.1, 3); this trait is present in D. romeri, but
absent in D. gregorii.
AMEGHINIANA - 2016 - Volume 53 (1): 1 – 13
6
Figure 3. Femur comparison of lagerpetid dinosauromorphs in distal
view; 1, Lagerpeton chanarensis. 2, Dromomeron gregorii. 3, Dromo-
meron romeri. 4, Dromomeron gigas sp. nov PVSJ 898 (1, 2, and 3 from
Nesbitt et al., 2009). Abbreviations: ctf, crista tibiofibularis; fl, flange;
gr, groove; lc, lateral condyle; mc, medial condyle. Arrow indicates
anterior direction. Scale bars= 10 mm.
MARTÍNEZ ET AL.: NORIAN LAGERPETID DINOSAUROMORPH
7
In distal view, the femur presents the characteristic
polygonal contour of the other non-dinosauriform dinosauro-
morphs (e.g., Lagerpeton, PVSJ 883, D. romeri, and D. gregorii;
Figs. 2.5, 3), different from the rounded contour of di-
nosauriforms (e.g., Silesaurus, Diodorus, Eoraptor). The crista
tibiofibularis is greatly enlarged as in other lagerpetids (Fig.
3). It is robust as in D. gregorii and D. romeri, and different
from the less stout crista tibiofibularis of Lagerpeton. The
lateromedial width of the base of the crista tibiofibularis of
PVSJ 898 represents 50% of the width of the distal end of
the femur, similar than that of both species of Dromomeron
and PVSJ 883, but contrasting with Lagerpeton in which it is
narrower (38%). The crista tibiofibularis has a wide con-
cavity on the posterolateral surface (Figs. 2.5, 3.4), similar
–although wider– to that present on both species of Dro-
momeron (Fig. 3.2–4), and different from the convex surface
of Lagerpeton (Fig. 3.1). Anterolaterally and posteromedially,
the crista tibiofibularis of PVSJ 898 is separated from the
lateral and medial condyles by deep grooves running along
the distal surface of the femur (Figs. 2.1–3, 5, 3.4). In pos-
terior view, the groove that separates the crista tibiofibu-
laris from the lateral condyle tapers proximally ending
abruptly as in other dinosauromorphs, but different from
the progressive ending present in basal dinosauriforms (e.g.,
Silesaurus, Eoraptor). As in the other Dromomeron, the pos-
terolateral groove that separates the lateral condyle from
the crista tibiofibularis is very deep, even deeper than that
present in D. romeri (Figs. 2.5, 3.3–4) and that of the unnamed
lagerpetid PVSJ 883.
Most of the medial condyle is lost (Figs. 2.2, 5, 3.4), but
clearly it is much smaller than the lateral condyle as in other
non-dinosauriform dinosauromorphs.
Phylogenetic Analysis
The phylogenetic analysis is based on a modified version
matrix of Nesbitt et al. (2010; see Materials and Methods
section and Supplementary Online Information) and the tree
search resulted in 9 MPTs of 744 steps. The strict consen-
sus of the MPTs (Fig. 4.1) depicts Dromomeron gigas as the
sister taxon of D. romeri, and both taxa forming a sister-
clade with D. gregorii (Fig. 4.1). This result was also obtained
using the matrix of Nesbit et al. (2010; modified by Martínez
et al., 2012) without including the three new characters
added in this contribution. Additionally, we tested the new
taxon through another recently published dataset for basal
dinosauromorphs (Bittencourt et al., 2015), which placed
D. gigas in an unresolved polytomy with the two previously
known species of the genus Dromomeron (Fig. 4.2).
In the results obtained from the modified version of the
dataset of Nesbitt et al. (2010), the clade formed by D. gigas
and D. romeri is supported by three unambiguous synapo-
morphies: absence of fourth trochanter, or indistinct ridge
for the attachment of the M. caudifemoralis (character
217.2); presence of a sharp ridge on the anteromedial edge
of the distal femur (character 291.1); and presence of a
lateral tuberosity on the anterolateral edge of the distal
femur (character 292.1). This clade is supported by a Bre-
mer support value of 3 and bootstrap frequencies of 82%.
The clade of D. gigas and D. romeri is placed as the sister
clade of Dromomeron gregorii, sharing a single unambigu-
ous synapomorphy, the presence of a distinct scar medio-
laterally oriented on the anterior surface of the distal end of
the femur (character 224.1). This clade is supported by Bre-
mer value of 1 and a bootstrap frequency of 59%.
The Dromomeron clade together with Lagerpeton
chanarensis and the unnamed lagerpetid of the Carnian Is-
chigualasto Formation (PVSJ 883) form the clade Lager-
petidae. All lagerpetids share six unambiguous synapo-
morphies: absence of anterolateral tuber on the proximal
portion of the femur (character 205.1), hook-shaped
femoral head in medial and lateral views (character 209.1),
presence of a ventral emargination on the anterolateral side
of the femoral head (character 213.1), crista tibiofibularis
larger than the medial condyle of the distal end of the femur
(character 225.1), presence of a dorsally expanded process
on the posterolateral portion of the tibia facet of the astra-
galus (character 249.1, unknown in D. gigas), and calcaneum
with a concave articular surface for the tibia (character
266.2, unknown in D. gigas). This clade is supported by a
Bremer support value of 2 and bootstrap frequencies of
54%.
DISCUSSION
PVSJ 898 as a new taxon
The femoral morphology of PVSJ 898 preserves enough
features to precisely locate it within basal Dinosauromor-
pha. PVSJ 898 has all the unambiguous femoral synapo-
morphies that diagnose the family Lagerpetidae: a) absence
of the anterolateral tuber of the proximal portion of the
femur, b) hook-shaped femoral head, c) ventral emargina-
tion on anterolateral side of the femoral head, d) crista
tibiofibularis larger than the medial condyle. These same
features were also recovered for Lagerpetidae by other re-
cent studies (e.g., Nesbitt, 2011; Bittencourt et al., 2015).
PVSJ 898 also shares the three femoral unambiguous
synampomorphies that characterized the clade D. romeri +
D. gregorii recovered by Nesbitt (2011): a) deep groove be-
tween the lateral condyle and crista tibiofibularis on the
distal surface, b) distinct scar on the anterior surface of the
distal end of the femur, and c) anteromedial corner of the
distal end of the femur forming an angle near or smaller
than 90°. The phylogenetic result obtained here recovered
the three species of Dromomeron sharing just one of these
unambiguous synapomorphies: distinct scar oriented medi-
olaterally on the anterior surface of the distal end of the
femur (character 224.1). The character optimization in our
phylogenetic analysis revealed that the other two features
shared for all Dromomeron specimens sensu Nesbitt (2011)
–but not recovered here for the Dromomeron clade– are not
optimized here as Dromomeron synapomorphies due to the
close phylogenetic affinities of the unnamed lagerpetid PVSJ
883. The latter taxon also shares the presence of a deep
groove between lateral condyle and crista tibio-fibularis
(character 221.1), and an acute anteromedial edge of the
distal femur (character 226.1). Therefore these features are
not unambiguous synapomorphies of the genus Dromo-
meron in the context of our phylogenetic result. On the other
hand, the presence of a wide concavity on the posterolateral
surface of the crista tibiofibularis is a feature also shared
by the three Dromomeron species (character 293.1), but
given this feature is unknown (?) in PVSJ 883 the phyloge-
netic analysis optimizes it as an ambiguous synapomorphy
of the Dromomeron clade (being at the same time an am-
biguous synapomorphy of the clade formed by PVSJ 883 and
the genus Dromomeron). Therefore, we recovered here the
Dromomeron clade diagnosed by the presence of the follow-
ing two anatomical features on the distal end of the femur:
distinctive scar on the anterior surface (unambiguous
synapomorphy), and concave posterolateral surface of the
crista tibiofibularis. Future findings in the Carnian Ischigua-
AMEGHINIANA - 2016 - Volume 53 (1): 1 – 13
8
Figure 4. Phylogenetic relationship of Dromomeron gigas sp. nov PVSJ 898 in the context of two different analyses; 1, using the modified ver-
sion of matrix published by Nesbitt et al. (2010; and later modified by Martínez et al., 2012) with the addition of three new characters; 2, using
the dataset for basal dinosauromorphs published by Bittencourt et al., 2014. Numbers above nodes are Bremer decay indices and below nodes
are Bootstrap values greater than 50%.
lasto Formation may provide further knowledge on the Is-
chigualasto lagerpetid that may help determining the rela-
tionship of the Carnian lagerpetids with the Dromomeron
clade and the diagnostic features of this genus.
Among Dromomeron specimens, PVSJ 898 shows three
femoral characters previously noted as autapomorphies of
D. romeri, which distinguished it from D. gregorii (Nesbitt
et al., 2009; Nesbitt, 2011): a) absence of a distinct ridge for
the attachment of the M. caudifemoralis longus (=fourth
trochanter), b) presence of a sharp ridge on the anterome-
dial edge of the distal end of the femur, and c) presence of a
lateral tuberosity on the anterolateral edge of the distal end
of the femur. These features originally proposed to diagnose
D. romeri are now synapomorphies shared by D. romeri and
D. gigas.
Despite these similarities we consider PVSJ 898 cannot
be identified as a large specimen of D. romeri because the
new specimen differs from this taxon (and all other basal
dinosauromorphs) by the presence of two autapomorphies:
a) rugose and blunt anterior trochanter on the anterolateral
surface of the proximal portion of the femur (Figs. 2.1, 3),
and b) pronounced muscle scar on the posteromedial sur-
face of the proximal portion of the femur (Fig. 2.2). As pre-
viously mentioned (see Description), we consider unlikely
that these features are ontogenetically related, so their
absence in the specimens of D. romeri is not regarded here
as due to a possible immaturity of these specimens. There-
fore, these mentioned autapomorphies demonstrate that
PVSJ 898 is in fact a new taxon. Although the medial
condyle of D. gigas is mostly lost, the wide intercondylar
groove suggests that this medial condyle is mediolaterally
small (Fig. 2.5), which is another possible feature shared
with D. romeri possibly reinforcing the close relationship
between D. romeri and D. gigas retrieved in the phylogenetic
analysis.
Distribution of Lagerpetidae
The oldest known skeletal record of Lagerpetidae is
Lagerpeton chanarensis from the Ladinian–earliest Carnian
Chañares Formation in NW Argentina (Desojo et al., 2011),
being this the only Middle Triassic record of this family. The
other record from NW Argentina is the unnamed lagerpetid
(PVSJ 883) from Carnian horizons of the Carnian–Norian
Ischigualasto Formation, which is the oldest record of a non-
dinosauriform dinosauromorph occurring with dinosaurs
(Martínez et al., 2012). All other lagerpetids are species of
the genus Dromomeron and Norian in age, i.e., Dromomeron
romeri and Dromomeron gregorii from the Chinle and Colo-
rado City formations of SW North America, and now Dromo-
meron gigas sp. nov. from Quebrada del Barro Formation of
NW Argentina.
The unnamed late Carnian lagerpetid PVSJ 883 is the
only Lagerpetidae found in layers closely associated with a
radioisotopic date (Martínez et al., 2012), as the age of other
lagerpetids is relative and therefore not as precise. Although
the Chañares Formation has never been radioisotopically
dated, there is a consensus about its Ladinian–earliest
Carnian age (Desojo et al., 2011), mostly based on two ar-
guments: (i) its faunal assemblage includes a majority of
cynodonts, and a small number of non-dinosaurian di-
nosauromorphs, basal archosaurs and pseudosuchians; (ii)
its stratigraphic position, located under 700 m of thickness
of the Ischichuca and Los Rastros formations, which in turn
underlie the Carnian–Norian Ischigualasto Formation (the
base of which has been dated at 231.4 Ma; Rogers et al.,
1993; Martínez et al., 2011). Among the youngest lager-
petids, on the basis of stratigraphic correlations D. gregorii
and D. romeri are supposed to be early to middle Norian
(Irmis et al., 2007; Nesbitt et al., 2009; Langer et al., 2013).
The age of D. gigas based on biostratigraphic correlations
with the La Esquina fauna from the Los Colorados Forma-
tion is inferred to be Norian, but possibly late Norian–
Raethian (Martínez et al., 2015).
Considering Lagerpetidae as a whole, the current knowl-
edge of its skeletal record shows a scenario in which basal
lagerpetids (Lagerpeton and PVSJ 883) are restricted to SW
Pangea from Ladinian to Late Carnian, whereas derived
lagerpetids (Dromomeron species) are restricted temporally
to the early–middle Norian, possibly extending to the late
Norian–Raethian (Martínez et al., 2015) based on the new
record, but geographically widespread across western
Pangea. The skeletal record seems to show that lagerpetids
first evolved in southwestern Pangea, remaining there as
an endemic group until the late Carnian–early Norian when
they spread to the northern hemisphere. However, it is im-
portant to bear in mind that the record of basal dinosauro-
morphs is scarce and the incompleteness of the record
and the possible taphonomic biases may mean that this
MARTÍNEZ ET AL.: NORIAN LAGERPETID DINOSAUROMORPH
9
scenario is not correct. For instance, the footprint record
suggests the possible presence of dinosauromorphs (e.g.,
Prorotodactylus, Rotodactylus) in earlier beds (Olenekian–La-
dinian) in the northern hemisphere (Brusatte et al., 2011;
Niedzwiedzki et al., 2014), although the precise taxonomic
identity of the producer of these footprints is hard to es-
tablish and therefore cannot be regarded as a definitive
evidence on the presence of lagerpetids in the Early–Middle
Triassic in the northern hemisphere.
Nevertheless, the long temporal (from 242 to 208?
Ma) and wide geographical distribution (western Pangea,
approximately from latitude 10° N to 40° S) evidences the
success of this group and that they coexisted in the same
ecosystems with more derived ornithodirans for a period of
over 25 million years.
Estimated femur length and body size
Despite the incompleteness of the femur of D. gigas, its
length can be estimated using the formula by Barrett et al.
(2015). Substituting the values of w1 (maximum laterome-
dial width of the proximal end of the femur) and w2 (maxi-
mum lateromedial width of the distal end) in that formula
for those of D. gigas (w1 = 31.3 mm, w2 = 32.6 mm) yields
a calculated femoral length of 190 mm for this new taxon
(Table 1).
The trend towards an increase in body size over time for
evolutionary lineages, Cope’s Rule (Benton, 2002), is fre-
quently cited as one of the key adaptations propping non-
avian dinosaur success (e.g., Sander et al., 2011; O´Gorman
and Hone, 2012). Most of the oldest known dinosaurs were
small animals, with a femur length under 170 mm (e.g., Eo-
raptor, Eodromaeus, Pampadromaeus; Sereno et al., 2012;
Martínez et al., 2012; Cabreira et al., 2011), being herrera-
saurids the only basal dinosaurs reaching femoral lengths
of 400 mm (e.g., Sanjuansaurus Alcober and Martínez, 2010)
and even more in Herrerasaurus (PVSJ 053 = Frenguellisaurus).
Langer et al. (2013) suggested that the small non-di-
nosaurian dinosauromorphs display a similar but more
muted increase in body size as did most of the non-avian
lineages of dinosaurs. Nevertheless, this assumption was
not demonstrated until now for the two most diverse
groups of non-dinosaurian dinosauromorphs: lagerpetids
and silesaurids.
Silesauridae is considered the closest relative of Di-
nosauria (Nesbitt et al., 2010; Brusatte et al., 2010a,b; Nes-
bitt, 2011; Langer et al., 2013; Benton et al., 2014), although
some authors have considered them as ornithischians
(Ferigolo and Langer, 2007; Langer, 2014). Silesaurids have
a highly variable femoral length, ranging from 74 to 345
mm. The late Anisian Asilisaurus kongwe, the oldest known
AMEGHINIANA - 2016 - Volume 53 (1): 1 – 13
10
TABLE 1. Femoral measurements (mm) of lagerpetid dinosauromorphs. Measurements are from the type specimens of Lagerpeton chanarensis, the
lagerpetid from Ischigualasto Formation (PVSJ 883), and Dromomeron gigas sp. nov. The measured specimens of Dromomeron gregorii (TMM
31100-1306) and Dromomeron romeri (GR 238) are the largest known specimens based on Barret et al. (2015) for the former and Nesbitt et al.
(2009) for the latter. Parentheses indicate estimated measurement; incomplete measurements due damages are indicate by d; em dash indicates
unknown or unpublished data.
Bone measurement
Lagerpeton
chanarensis
PVSJ 883 D. gigas D. gregorii D. Romeri
Femur
Femur length 77.8 (109-139) (190) 96 127
Maximum length of femoral head — — 31.3 — —
Head to apex of fourth trochanter 20.5 — — — —
Length of fourth trochanter 15.3 — — — —
Minimum diameter of the shaft — — 10.3 — —
Max. lateromedial width of proximal end — — 27.7 — —
Max. lateromedial width of distal end — 25.1d 32.6 — —
Max. anteroposterior width of distal end — 16.1d 22.02 — —
silesaurid, reached a maximum femoral length of 144 mm,
although specimen NHMUK R16303 (a putative new sile-
saurid from the same horizon) is estimated to have a
femoral length of 345 mm (Barrett et al., 2015). Among No-
rian silesaurids, the early Norian Silesaurus opolensis has a
femoral length of 200 mm, and the early–middle Norian
Sacisaurus agudoensis has a variable femoral length ranging
from 87 to 110 mm (Ferigolo and Langer, 2007). When we
place those specimens in a phylogenetic context, the result
shows a slight increase in size over time (Fig. 5; Supple-
mentary Online Information), as was suggested by Langer
et al. (2013).
It has been suggested that among Lagerpetidae, the
most basal members of Dinosauromorpha, the plesiomor-
phic condition of small body-size was retained with little
variation from the hypothesized size of the last common
ancestor of crocodylians and birds (Barrett et al., 2015).
Conversely, the estimated femur length of D. gigas seems
to show a different scenario. The femur length of D. gigas is
much larger than that of any known lagerpetid dinosauro-
morph, i.e., 67% longer than the longest known lagerpetid
femur (i.e., D. romeri). The most basal member of Lager-
petidae, the Ladinian–earliest Carnian Lagerpeton chanaren-
sis, reached a maximum femoral length of 78 mm. The late
Carnian Ischigualasto form (PVSJ 883) has an estimated
femur length of 110–140 mm (Martínez et al., 2012). Among
the youngest members, the largest specimens of D. gregorii
and D. romeri measure 96 and 127 mm long, respectively
(Nesbitt et al., 2009). The new specimen reported here, D.
gigas is estimated to have a femoral length of 190 mm
(Tab. 1). These values, although based on a few samples,
show that comparing the first appearance of lagerpetids in
the fossil record (Ladinian–earliest Carnian) with their last
appearance (middle–late Norian), the femoral length in-
creased by a factor of 2.43. Within a phylogenetic context,
the inclusion of D. gigas among Lagerpetidae, as well as
PVSJ 883, suggests that within this linage the body size in-
creased through the time (Figs. 4, 5; Supplementary Online
Information). Despite the sparse record of lagerpetids and
the possible sensitivity of this trend to incomplete taxon
sampling, the new lagerpetid dinosauromorph from the
late Norian Quebrada del Barro Formation, Dromomeron
gigas sp. nov., undoubtedly shows that by the Late Triassic
basal dinosauromorphs reached body sizes much larger
than previously thought.
ACKNOWLEDGMENTS
We thank Instituto y Museo de Ciencias Naturales of San Juan
Province for its continue support to our research. Fine preparation
was done by D. Abelín. We specially thank to the reviewers S.
Brusatte and J. Bittencourt, and the Editor D. Pol for their sugges-
tions and comments which greatly increased the quality of this
manuscript. This research was possible thanks to PICTO UNSJ
2009-0144 (to RNM) and PICT 2013-1920 (to CA) both granted by
FONCyT. Special thanks to Secretaría de Ciencia, Técnica e Innova-
ción, Dirección Provincial de Vialidad, and Dirección de Patrimonio
Cultural of the San Juan Province.
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doi: 10.5710/AMGH.21.06.2015.2894
Submitted: February 13
th
, 2015
Accepted: June 21
st
, 2015
MARTÍNEZ ET AL.: NORIAN LAGERPETID DINOSAUROMORPH
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