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Skull of the dicynodont Placerias from the Upper Triassic of Arizona

Authors:
Spencer G. Lucas at New Mexico Museum of Natural History and Science
Spencer G. Lucas
Andrew B. Heckert at Appalachian State University
Andrew B. Heckert
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Abstract and Figures

We describe here a rare, incomplete but articulated skull of the dicynodont Placerias from the Blue Mesa Member of the Petrified Forest Formation in northern Arizona. This specimen comes from near the type locality of P. hesternus Lucas, 1904 in the vicinity of Ward's Terrace. Although the skull is crushed and somewhat distorted, it confirms previous reconstructions that posit the presence of a relatively short squamosal bar and dorso-ventrally robust, but relatively short, maxillary process, as opposed to the first reconstruction based on isolated bones from the Placerias quarry in east-central Arizona. With the exception of the Placerias quarry, dicynodonts in general are uncommon in the Chinle Group, but their occurrences are from strata of Otischalkian or Adamanian (late Carnian) age, and in part define a Placerias biochron of Otischalkian-Adamanian age.
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Heckert, A.B., and Lucas, S.G., eds., 2002, Upper Triassic Stratigraphy and Paleontology. New Mexico Museum of Natural History and Science Bulletin No. 21.
127
SKULL OF THE
DICYNODONT
PLACERIAS FROM THE UPPER TRIASSIC OF ARIZONA
SPENCERG. LUCAS
and
ANDREW
B. HECKERT
New
Mexico
Museum
of Natural History, 1801Mountain Road NW,Albuquerque, NM 87104-1375
Abstract-We
describe here a rare, incomplete
but
articulated skull of the dicynodont
Placerias
from
the Blue Mesa Member of the Petrified Forest Formation in northern Arizona. This specimen comes
from
near
the type locality of
P.
hesternusLucas, 1904in the vicinity of Ward's Terrace. Although the
skull is crushed
and
somewhat distorted, it confirms previous reconstructions
that
posit the presence
of a relatively
short
squamosal
bar
and
dorso-ventrally robust,
but
relatively short, maxillary process,
as
opposed
to the first reconstruction based on isolated bones from the
Placerias
quarry
in east-central
Arizona. With the exception of the
Placerias
quarry,dicynodontsin generalare uncommonin the Chinle
Group,
but
their occurrences are from strata of Otischalkian or
Adamanian
(late Carnian) age,
and
in
part
define a
Placerias
biochron of Otischalkian-Adamanian age.
Keywords:
Placerias,
dicynodont, topotype, Petrified Forest Formation, Blue Mesa Member
INTRODUCTION
Duringthe Permian
and
Triassic,dicynodonts werea wide-
spread
group
of terrestrial- herbivorous synapsids (King, 1986,
1988,1990). In Triassic nonmarine strata, they are
known
from all
of the continents
and
are particularly
abundant
(often the domi-
nant) tetrapod fossils at Lower
and
MiddleTriassic localities. The
last dicynodonts are of Late Triassic age, from
North
America,
SouthAmerica,Morocco
and
India (Lucas, 1995;Lucas
and
Wild,
1995).The most widelydistributed ofthe LateTriassicdicynodonts
is
Placerias
Lucas, 1904,
known
from the western United States,
eastern United States
and
from Morocco. Here, we describe an
incompleteskull of
Placerias
from the
upper
TriassicChinle
Group
in northernArizona.
Abbreviations:
MNA
=Museumof NorthernArizona,Flag-
staff; UCMP
==
University of California
Museum
of Paleontology,
Berkeley.
PROVENANCE
The skull described here is
MNA
V8464, collected from
MNA
locality 1454 by R. Kirby
and
P. Luttrell in August, 1991.
The locality is along Ward's Terrace, east of Cameron
and
just
north
of Landmark Wash at UTM Zone 12, 472000E,3964620N.
MNA
locality records state
the
locality is in the Petrified Forest
Memberof the ChinleFormationin strataoflate Carnianage (Fig.
1).Cooleyet al. (1969,pl. I, sheet2)
map
thesestrataas lower
part
of the Petrified Forest Member, which is the Blue Mesa Member
of the Petrified Forest Formation of Lucas (1993).Thus,
MNA
lo-
cality 1454 is from the same stratigraphic interval (and close to
the same locality) as the holotype specimen of
Placerias
hesternus
Lucas, 1904 (d.
Camp
and
Welles, 1956,fig. 1; Lucas
and
Hunt,
1993a;Lucas et al., 1997;Lucas, 1998)
and
is effectivelya topotype
(Lucas et al., 1999).The Blue Mesa Member hosts the "type as-
semblage" of the
Adamanian
land-vertebrate faunachron (lvf) of
Lucas
and
Hunt
(1993b),which is of latest Carnian age through-
out
its distributionin Arizona
and
New
Mexico (Lucas
and
Hunt,
1993b; Lucas
and
Heckert, 1996).Thus,
MNA
locality 1454 is of
Adamanian
(latest Carnian) age.
DESCRIPTION
MNA
V8464(Figs. 2-3)is an incomplete skull
that
has been
distorted dorso-ventrally
and
also compressed laterally
toward
the
right side. The specimenis thoroughly fractured
and
sits in a
Wyoming
•
Placerias localities:
• type locality
Colorado
• other localities
(see text)
(
•
)
•
Arizona
•
•
New Mexico
•
I
FIGURE1.Index
map
showingthe distribution of Chinle Group
Placerias
localities.
plaster jacket, so
that
its ventral side is
not
visible.
What
is pre-
served is 580
mm
long antero-posteriorly
and
includes
the
right
and
left maxillae, the left orbit
and
the left parietal-squamosal
region. The rostrum has been splayed anterior to the orbits so
that
both
the left
and
right maxillae are in essentially one plane
(Figs. 2-3).
Both maxillae are thick, rugose bones,
and
on
the
right
maxillae the base of a stout "tusk" can be
seen
along its antero-
externaledge (Figs. 2-3).The nasal, prefrontal
and
lacrimal (their
separate sutures are
not
visible) unite to form a thick,
deep
ros-
128
FIGURE 2.
MNA
V8464,incomplete skull of
Placerias
hesternusLucas in lateral view. A, Overview; B·C, Stereophoto.
trum
dorsal
to
the
left maxilla
and
anterior
to
the
orbit. The
orbit
is nearly
round
and
is
bordered
dorsallyby a thick frontal, a promi-
nent
postorbital
ridge
posteriorly
and
a
more
gracile jugal-maxil-
lary
ridge
ventrally. The
squamosal
has
a thick,
blunt,
flange-like
jugal
bar
that
projectslaterally.
It
is confluent
with
the
dorsal
crest
of
the
squamosal,
which
is thick
and
rugose. The fossae
dorsal
and
ventral
to
the
jugal
bar
are
deep
and
concave. The parietal
edge
of
the
skull
is a
broad
arc of
bone
that
rises
sharply
just
pos-
terior
to
the
orbit.
DISCUSSION
Although
Camp
and
Welles (1956)
and
Cox (1965) recon-
structed
the
skull
of
Placerias
from extensive
material
collectedat
the
Placerias
quarry
near
St. Johns,
Arizona
(d.
Long
and
Murry,
1995;Lucaset al., 1997),
the
specimensavailable to
them
were
not
as
complete
as
MNA
V8464.
Indeed,
the
dicynodont
specimens
from
the
Placerias
quarry
consist
almost
entirelyof disarticulated
skull
and
postcranial elements,
and
skull reconstructions
were
necessarily
based
on
composite
specimens
(Camp
and
Welles,
D
skull
openings
0
missing
bone
•
hole
in
specimen
(lacket
exposed)
- - - -
midline
FIGURE3.Interpretivesketchof MNA V8464,incompleteskull of
Placerias
hesternus. Abbreviations: mx =: maxilla, 0 =: orbit, p =: parietal, sq =:
squamosal, t =: "tusk."
1956;Cox, 1965). Likewise, cranial material of
Placerias
from the
Newark
Supergroupin
North
Carolinais not as completeas
MNA
V8464(Lucas, 1998).
MNA
V8464is
thus
the
most
complete skull
of
Placerias
found
in
North
America.
This
most
complete skull conforms in a general
way
to the
revised reconstruction of the skull of
Placerias
published
by Cox
(1965).
Indeed,it confirmsCox'sreconstruction of a relativelyshort
squamosal
bar
and
dorso-ventrally
robust
but
relatively
short
maxillary process,
which
contrasts strikingly
with
the skull re-
construction of
Camp
and
Welles (1956,pl. 31).
Outside
of the
Placerias
quarry
in Arizona, specimens of
Placerias
are relatively rare in the Chinle Group. Lucas
and
Hunt
(1993a)
reviewed
records of
Placerias
from
the
Upper
Triassic
Chinle
Group
of the westernUnitedStates. Thereare eightoccur-
rences
that
span
the Otischalkian-Adamanian (Lucas
and
Hunt,
1993a; Lucas, 1994;Long
and
Murry, 1995;Lucas et al., 1997):
1. The
type
locality of
Placerias
hesternusLucas just
northeast of Cameron, Arizona is in the Blue Mesa
Member
of the Petrified Forest Formation (Lucas,
1904;Lucas
and
Heckert, 1996).
129
2. The occurrence
documented
here (MNA V8464),
whichis
very
close to the typelocality
and
also in the
Blue Mesa Member.
3.At CarrizoArroyonearAdamana,Arizona,
Placerias
also is present in the Blue Mesa
Member
(Lucas
and
Hunt,
1993a; Long
and
Murry, 1995).
4.The
Placerias
specimenfrom the Blue Forest
quarry
in
the
Petrified Forest NationalPark, Arizona, also is
in the Blue Mesa Member (Long
and
Murry, 1995).
5.
At
the
Blue
Hills
north
of St.
Johns,
Arizona,
Placerias
occurs in the Blue Mesa
Member
(Long
and
Murry, 1995).
6. The
Placerias
quarry
near St. Johns, Arizona, is in
the
basal
Bluewater
Creek
Formation
(Camp
and
Welles, 1956;Cox, 1965;Lucas et al., 1997).
7.The
middle
part
of the BluewaterCreekFormation
at Fort Wingate, McKinley County,
New
Mexico,
has
produced
isolated, large postcranial fragments of a
dicynodont,probably
Placerias
(Heckert, 1997,p. 160-
161).
8. The
Popo
Agie Formation in Wyoming
yielded
dicynodontpostcraniafor whichWilliston (1905)cre-
ated
two
taxa,
Brachybrachium breoipes
and
Eubrachiosaurus
broumi.
The holotype of B.
breoipes,
a
humerus,
has
been lost,
but
the illustration indicates
it is
very
similar to the
humerus
of
Placerias.
Techni-
cally, B.
brevipes
thus
should
be considered a nomen
dubium
that
is
most
likely a
synonym
of P. hesternus
(Lucas
and
Hunt,
1993a).
Placerias
localities in
North
America comefrom a relatively
restricted stratigraphic interval
that
can be
demonstrated
to be
upper
Carnian. This interval is
part
of the biostratigraphic basis
of the
Placerias
biochronof Lucas (1995,1998)
and
Lucas
and
Wild
(1995).This biochron is less precise (emcompassingOtischalkian
and
Adamanian
time)
than
many
phytosaur- or aetosaur-based
biochrons,
but
is still useful for correlating
upper
Carnian
strata.
ACKNOWLEDGMENTS
We
thank
Deb Hill for access to the
MNA
collection; the
late JohnEstepfor photography;
and
A.P.
Hunt,
K.E. Zeigler,
and
R.M.Sullivanfor commentson an earlierdraftof thismanuscript.
REFERENCES
Camp,
C. L.,
and
Welles,S.P.,1956,Triassic dicynodontreptiles: Memoirs
of the University of California, v. 13, p. 255-348.
Cooley, M.
E., Harshbarger, J.W.,Akers, J.P.,
and
Hardt, W.E, 1969, Re-
gional hydrogeology of the Navajo
and
Hopi
Indian
Reservations,
Arizona,
New
Mexico,
and
Utah: U.S.Geological Survey Professional
Paper
421A, p. 1-61.
Cox,
C. B.,1965,
New
Triassic dicynodontsfrom SouthAmerica, their ori-
gins
and
relationships: Philosophical Transactions of the Royal Soci-
ety of London, v.248,
P:457-516.
Heckert,
A B.,1997,Litho-andbiostratigraphyof the lowerChinleGroup,
east-central Arizona
and
west-central
New
Mexico,
with
a descrip-
tion of a
new
theropod (Dinosauria.Theropoda) from the Bluewater
Creek
Formation
[M.s. thesis}:
Albuquerque,
University
of
New
Mexico, 278 p.
King,
G. M., 1986,Distribution
and
diversityof dicynodonts: an introduc-
tory approach:
Modern
Geology, v. 10, p. 109-120.
King,
G. M., 1988, Anomodontia: Encyclopedia of Paleoherpetology, v.
17C, p. 1-174.
King,
G. M., 1990, The dicynodonts: a
study
in paleobiology: London,
Chapman
and
Hall, 233 p.
Long,
R.
A,
and
Murry, P.
A,
1995, Late Triassic (Carnian
and
Norian)
tetrapodsfrom the southwesternUnitedStates:
New
Mexico
Museum
of
Natural
History
and
Science, Bulletin 4, 254 p.
Lucas, E
A,
1904,A
new
batrachian
and
a
new
reptile from the Trias of
Arizona: Proceedings of the U.S. National Museum, v. 27, no. 1353,p.
193-197.
Lucas, S. G., 1993,The Chinle Group: Revised stratigraphy
and
bio-chro-
nology of
Upper
Triassic stratain the western United States:
Museum
130
of
Northern
Arizona, Bulletin 59, P:27-50.
Lucas, S. G., 1994, The beginning of the age of dinosaurs in Wyoming:
Wyoming Geological Association Guidebook, v. 44, p.
105-113.
Lucas.S.G., 1995,Triassic dicynodontbiochronology:Albertiana,v.16, p.
33-40.
Lucas, S. G., 1998,
Placerias
(Reptilia, Dicynodontia) from the
Upper
Tri-
assic of
the
Newark
Supergroup,
North
Carolina, U.S.A.,
and
its
biochronological significance:
Neues
[ahrbuch
fur
Geologie
und
Palaontologie Monatshefte, v.2998, p. 432-448.
Lucas, S. G.,
and
Heckert, A. B., 1996, Vertebrate biochronology of the
Late TriassicofArizona: Proceedings of the Fossils ofArizona Sympo-
sium, v.4, p. 63-81.
Lucas.S. G.,
and
Hunt,
A. P.,1993a,A dicynodontfrom the
Upper
Triassic
of
New
Mexico
and
its biochronologic significance:
New
Mexico Mu-
seum
of
Natural
History
and
Science, Bulletin 3, p. 321-325.
Lucas, S.G.,
and
Hunt,
A. P.,1993b,Tetrapodbiochronologyofthe Chinle
Group(UpperTriassic),westernUnitedStates.:
New
Mexico
Museum
of
Natural
History
and
Science, Bulletin 3, P:327-329.
Lucas, S. G.,
and
Wild, R., 1995,A Middle Triassic dicynodont from Ger-
many
and
the biochronology of Triassic dicynodonts:
Stuttgarter
Beitrage
zur
Naturkunde
Serie B(Geologie
und
Palaontologie),v.220,
p.1-16.
Lucas.S, G.,Heckert, A.B.,and Estep.], W.,1999,Topotype skull
ofPlacerias
(Synapsida:Dicynodontia) from the
Upper
Triassic Chinle Group of
northernArizona: SouthwestPaleontologicalSymposiumProceedings,
v.6, p. 21.
Lucas, S. G., Heckert, A. B.,
and
Hunt,
A. P.,1997,Lithostratigraphy
and
biostratigraphic
significance of
the
Placerias
quarry,
east-central
Arizona: Neues [ahrbuch fur Geologie
und
Palaontologie, v. 203, p.
23-46.
Williston, S.W.,1905,Notice of some
new
reptilesfrom the
Upper
Trias of
Wyoming: Journal of Geology, v. 12, p. 688-697.
... Placerias hesternus (hereafter referred to simply as Placerias) is one of the largest known species of dicynodonts, and among the most massive herbivores known from the Chinle Formation, reaching three meters in length and possibly weighing over one tonne [29]. Placerias has a farily broad geographic distribution, having been found in the Chinle Formation in Arizona [30] and New Mexico [31], the Pekin Formation in North Carolina [32], and possibly the Dockum Group in Texas [33], but it is mostly known from fragmentary remains. The one exception is the Placerias Quarry, a small death assemblage site in the Blue Mesa Member of the Chinle Formation, near the boundaries of Petrified Forest National Park, southwest of the town of St. Johns, Arizona, USA, which has produced over 1700 elements from at least 41 individuals of Placerias [34]. ...
... The reconstructed skull was modified in a later publication by Cox [14], with revisions including replacing the maxillae with slightly smaller ones (UCMP 25317 and UCMP 25318), and changing their angle to improve articulation with the premaxilla (Fig 1). An articulated partial skull of Placerias (MNA.V.8464) was later described from a different locality and supports Cox's interpretation of the position of the maxilla, but it is poorly preserved and crushed, making anatomical analysis difficult [22,31]. ...
... Currently the best preserved articulated skull of Placerias comes from a site outside of the Placerias Quarry, in another Chinle locality near Cameron, AZ [31]. The specimen consists of a fully articulated right half of a skull, including the maxilla. ...
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... They are also thought to be geographically restricted: until recently only a single Late Triassic dicynodont species was known outside of South America. That species, Placerias hesternus, is usually listed as the only Late Triassic North American dicynodont [17,18]. Placerias is best known from the Placerias Quarry in eastern Arizona (Blue Mesa Member, Chinle Formation), which yielded roughly 1600 Placerias elements representing at least 41 individuals [19,20]. ...
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  • Jan 2007
Parasuchus (= Paleorhinus) is the most primitive known phytosaur, and its fossils define a Carnian biochron recognizable across much of Pangea. The largest known specimen of this primitive taxon, an incomplete skull from the Popo Agie Formation in northwestern Wyoming, demonstrates that the nares remain anterior to the antorbital fenestra throughout the ontogeny of Parasuchus, an observation confirmed by an analysis of a broad database. The fact that this character is not variable through the ontogeny of this phytosaur genus, as some previous authors have speculated, helps to cement the taxonomic validity of Parasuchus. For the past century, systematists have attempted to establish a classification of organisms rooted in some form of a biological species concept. Cladotaxonomy, on the other hand, is the recognition of cladotaxa, which are low-level taxa (genera and species) that correspond to clades in a cladistic analysis. Cladotaxonomic relegation of all primitive phytosaurs to a metataxon is based on a posteriori evaluation of character polarity that fails to acknowledge the existence of a biotaxon regardless of how a cladist evaluates character polarities millions of years later. We reject assignment of primitive phytosaurs to a metataxon as uninformative, and recognize Parasuchus as a diagnosable phytosaur genus.
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Triassic vertebrate fossils in Arizona
Article
Full-text available
  • Jan 2005
The Triassic System in Arizona has yielded numerous world-class fossil specimens, includ-ing numerous type specimens. The oldest Triassic vertebrates from Arizona are footprints and (largely) temnospondyl bones from the Nonesian (Early Triassic: Spathian) Wupatki Member of the Moenkopi Formation. The Perovkan (early Anisian) faunas of the Holbrook Member of the Moenkopi Formation are exceptional in that they yield both body-and trace fossils of Middle Triassic vertebrates and are almost certainly the best-known faunas of this age in the Americas. Vertebrate fossils of Late Triassic age in Arizona are overwhelmingly body fossils of temnospondyl amphibians and archosaurian reptiles, with trace fossils largely restricted to coprolites. Late Triassic faunas in Arizona include rich assemblages of Adamanian (Carnian) and Revueltian (early-mid Norian) age, with less noteworthy older (Otischalkian) assemblages. The Adamanian records of Arizona are spectacular, and include the "type" Adamanian assemblage in the Petrified Forest National Park, the world's most diverse Late Triassic vertebrate fauna (that of the Placerias/Downs' quarries), and other world-class records such as at Ward's Terrace, the Blue Hills, and Stinking Springs Mountain. The late Adamanian (Lamyan) assemblage of the Sonsela Member promises to yield new and important information on the Adamanian-Revueltian transition. Revueltian records are nearly as impressive as those of the Adamanian, including extensive exposures in the vicinity of the Petrified Forest National Park and the best-known tetrapod assemblages from the Owl Rock Formation. The combination of an exceptionally rich record and outstanding exposures of sedimentary sections that allow the correlation of tetrapod faunas means that Arizona will remain a hotbed of research on Middle and Late Triassic vertebrates for the foreseeable future. INTRODUCTION The Triassic System in Arizona is known worldwide for its vertebrate fossils. The record of Middle Triassic vertebrates from the Holbrook Member of the Moenkopi Formation is one of the best of its age (Perovkan—Anisian, see Lucas, 1998; Lucas and Schoch, 2002) anywhere in North America. The Moenkopi For-mation also yields a substantial vertebrate ichnofauna (Peabody, 1948). Still, the Middle Triassic vertebrates of Arizona pale in comparison to the rich, diverse, and storied collections of its Upper Triassic vertebrates. Highlights of the Upper Triassic of Arizona include the most diverse Upper Triassic vertebrate locality in the world (the Placerias quarry), the type fauna of the Adamanian (late-latest Carnian) land vertebrate faunachron (lvf), and the single best "laboratory" for studying Upper Triassic vertebrate evolution in stratigraphic and biostratigraphic context (Petrified Forest National Park). Both the Middle and Upper Triassic series are also important because of the large number of type specimens, particularly of Middle Triassic temnospondyls and Upper Trias-sic archosaurs (especially phytosaurs) from these strata (Table 1). This record is even more remarkable considering that the vast majority of it was gleaned from a few outcrop belts between the edge of the Colorado Plateau to the south and the Navajo and Hopi nations to the north. Thus, while the Moenkopi and Chinle have already yielded rich, diverse, and in some cases, magnificent collections, the possibility for even greater growth in the future is enormous. Abbreviations: Throughout this article AMNH = American Museum of Natural History, New York; FMNH = Field Museum of Natural History, Chicago; MCZ = Museum of Comparative Zoology, Harvard University, Cambridge; MNA = Museum of Northern Arizona, Flagstaff; NMMNH = New Mexico Museum of Natural History and Science, Albuquerque; PFNP = Petrified For-est National Park (and PEFO to its collections); SMU = Southern Methodist University, Dallas; UCMP = University of California Museum of Paleontology, Berkeley; and USNM = United States National Museum of Natural History, Washington, D.C.
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Placerias (Reptilia, Dicynodontia) from the Upper Triassic of the Newark Supergroup, North Carolina, USA, and its biochronological significance
Article
  • Jul 1998
  • Neues Jahrbuch Geol Palaontol Monatsh
Numerous cranial and postcranial bones of a large dicynodont from the Pekin Formation (Newark Supergroup) in North Carolina are assigned to Placerias hesternus LUCAS. Placerias occurrences in Upper Triassic strata in the western United States (Chinle Group), North Carolina and Morocco (Argana Group) define a Placerias biochron of late Carnian (Tuvalian) age.
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New Triassic Dicynodonts from South America, Their Origins and Relationships
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Richly fossiliferous deposits have been found in the Ischigualasto region of Argentina in the last few years. The only known dicynodont from this area is the new genus Ischigualastia , of which a diagnosis and fully illustrated description are given. A specimen from Brazil, which had earlier been referred to the genus Stahleckeria as S. lenzii , is shown to be very similar to Ischigualastia , but not generically identical with it; this specimen is therefore placed in the new genus Barysoma . The only South American dicynodont which had previously been fully described is Stahleckeria , from Brazil. A diagnosis and fully illustrated description are now given of the complete skeleton of the genus Dinodontosaurus , also from Brazil. Earlier Brazilian material which had been referred to the African genus Dicynodon is shown to belong to Dinodontosaurus . A very large skull from the same deposits is identical with Dinodontosaurus , except that it has a much more massive snout and tusks, and a wider occiput. The dicynodonts are herbivorous, and may well have lived in herds; it is suggested that the massive skull may belong to the old male of such a herd of Dinodontosaurus , and it is therefore not given separate taxonomic status. The skull of Placerias , the only dicynodont known from North America, had previously been restored by Camp & Welles (1956) from the broken remains of about forty individuals. Comparison of the restored skull with that of Ischigualastia has suggested various modifications in the reconstruction, and illustrations of the new reconstruction are given. The relationships of the Triassic dicynodonts are discussed. It is suggested that, excluding the specialized genus Lystrosaurus , they show two main divergent adaptations, which are probably related to their mode of feeding. A pointed snout and high occiput is thought to characterize the family Kannemeyeriidae (which includes the forms Kannemeyeria, ‘Kannemeyeria’ vanhoepeni, Sinokannemeyeria, Parakannemeyeria, Ischigualastia, Barysoma and Placerias ). A blunt snout and wide occiput is thought to characterize the family Stahleckeriidae (which includes the genera Stahleckeria and Dinodontosaurus ). A similar distinction is found today between the browsing black rhinoceros and the grazing white rhinoceros. The most primitive kannemeyeriids are found in the Lower Triassic of China, and these forms may also be ancestral to the stahleckeriids. The only other Triassic dicynodonts, Shansiodon and ‘ Dicynodon’ njalilus , may be placed in a separate family, the Shansiodontidae. All these Triassic genera have two features in common: the presence of a separately ossified olecranon process on the ulna, and a shortened interpterygoid vacuity. It is possible that this may indicate a common ancestry for them all, but no Upper Permian or Basal Triassic genera now known appear to be possible ancestors for them. The lack of any Middle Triassic vertebrate fauna in the northern hemisphere makes it very difficult to date the Argentinian and Brazilian faunas, which include gomphodont cynodonts, dicynodonts, rhynchosaurs, pseudosuchians and a few saurischians. It is not felt that the presence of rhynchosaurs necessarily indicates a Middle Triassic age, as the group is known from the mid-Norian of India. It is possible that the presence of several saurischians and of a pseudosuchian closely related to the German Norian genus Aëtosaurus , may indicate a Carnian age for the Argentine fauna. The Brazilian fauna is somewhat dissimilar to that of Argentina and contains no genera in common with it; it may therefore be of earlier, Ladinian, age. The fauna of the Manda Beds of East Africa is similar in composition to that of Brazil, but contains no genera in common with it. It also lacks saurischians and includes a dicynodont, Kannemeyeria , that is otherwise typical of the Lower Triassic Cynognathus zone of South Africa. It may therefore be Anisian in age.
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