A Basal Dinosaur from the Dawn of the Dinosaur Era in Southwestern Pangaea

Abstract

Upper Triassic rocks in northwestern Argentina preserve the most complete record of dinosaurs before their rise to dominance in the Early Jurassic. Here, we describe a previously unidentified basal theropod, reassess its contemporary Eoraptor as a basal sauropodomorph, divide the faunal record of the Ischigualasto Formation with biozones, and bracket the formation with (40)Ar/(39)Ar ages. Some 230 million years ago in the Late Triassic (mid Carnian), the earliest dinosaurs were the dominant terrestrial carnivores and small herbivores in southwestern Pangaea. The extinction of nondinosaurian herbivores is sequential and is not linked to an increase in dinosaurian diversity, which weakens the predominant scenario for dinosaurian ascendancy as opportunistic replacement.

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DOI: 10.1126/science.1198467
, 206 (2011);331 Science , et al.Ricardo N. Martinez
Pangaea
A Basal Dinosaur from the Dawn of the Dinosaur Era in Southwestern
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A Basal Dinosaur from the Dawn of the
Dinosaur Era in Southwestern Pangaea
Ricardo N. Martinez,
1
Paul C. Sereno,
2
*Oscar A. Alcober,
1
Carina E. Colombi,
1,3
Paul R. Renne,
4,5
Isabel P. Montañez,
6
Brian S. Currie
7
Upper Triassic rocks in northwestern Argentina preserve the most complete record of
dinosaurs before their rise to dominance in the Early Jurassic. Here, we describe a previously
unidentified basal theropod, reassess its contemporary Eoraptor as a basal sauropodomorph, divide
the faunal record of the Ischigualasto Formation with biozones, and bracket the formation with
40
Ar/
39
Ar ages. Some 230 million years ago in the Late Triassic (mid Carnian), the earliest
dinosaurs were the dominant terrestrial carnivores and small herbivores in southwestern Pangaea.
The extinction of nondinosaurian herbivores is sequential and is not linked to an increase in
dinosaurian diversity, which weakens the predominant scenario for dinosaurian ascendancy as
opportunistic replacement.
An arid valley in northeastern Argentina
called Ischigualasto contains a well-exposed
fossiliferous Upper Triassic section from
the dawn of the dinosaur era. Some 50 years of
intensive collecting have yielded nearly complete
skeletons of the basal dinosaurs, Eoraptor (Fig. 1A)
and Herrerasaurus (1,2), and hundreds of ad-
ditional fossil vertebrates. Recent finds include
several previously unidentified dinosaurs based
on partial skeletons (3–5), and recent geologic
work has detailed stratigraphic and paleoclimatic
variation across the formation (6).
We describe here a nearly complete basal
dinosaur, Eodromaeus murphi nov. gen. nov. sp.
(7,8), which helps to reveal skeletal form and
function at the root of Theropoda, a clade that
includes all predatory dinosaurs (Fig. 2A) (9).
The skull is relatively low and lightly built with a
relatively spacious antorbital fenestra, which is
emarginated anteriorly by a relatively broad ant-
orbital fossa (Fig. 1B). On the snout, an accessory
pneumatic opening, the promaxillary fenestra, is
present near the anterior margin of the antorbital
fossa (specimen number PVSJ 560) (Fig. 1B).
The promaxillary fenestra is present in the basal
theropod Herrerasaurus (10) and in most later
theropods, although it is secondarily closed in the
early North American theropod Tawa and some
coelophysoids (11,12). The premaxilla-maxilla
suture is long and akinetic, and the jugal ramus
under the orbit is shallow (Fig. 1B). The sidewall
and ventral aspects of the braincase are marked by
well-defined tympanic and basisphenoid recesses,
respectively, and the basipterygoid processes are
transversely compressed (Fig. 1C). In the lower
jaw, the dentary is slender, and the retroarticular
process is well developed posteriorly (Fig. 1B).
There are 4 premaxillary and 11 maxillary
teeth, which are separated medially by interdental
plates. All crowns are laterally compressed, re-
curved, and very finely serrated mesially and
distally (nine serrations per millimeter), unlike
the more numerous maxillary teeth in Eoraptor,
which have larger denticles, less recurvature,
and a constriction at the base of the crown (Fig. 1,
D and E). Anterior maxillary crowns are cani-
niform, their ventral reach increased by a ven-
trally convex alveolar margin (Fig. 1, B and
E). Dentary teeth, on the other hand, are rela-
tively uniform in size, and the first tooth is lo-
cated at the anterior tip of the dentary, unlike
the condition in Eoraptor (Fig. 1, A and B). A
row of very small rudimentary teeth crosses the
palatal ramus of the pterygoid in Eodromaeus
(PVSJ 560), as in Eoraptor, the only dinosaurs
known to retain palatal teeth.
The cervical column is composed of pro-
atlantal neural arches followed by 10 cervical
vertebrae. Cervical vertebrae have spool-shaped
centra that are more elongate than in Eoraptor;
many centra have lengths more than three times
the centrum diameter (Fig. 2, A and B). Cervical
vertebrae have a low ventral keel and projecting
epipophyseal processes. Invaginated pleurocoels
are present in posterior cervicals, indicative of the
presence of pneumatic invasion by parasagittal
cervical air sacs (Fig. 2B). The pleurocoels open
posteriorly into a lateral groove, which is present
in most other vertebrae in the axial column. Thus,
the cervical air sac system may have extended
into the trunk, unlike in extant avians. There are
14 dorsal vertebrae in the trunk, the posterior of
which are stabilized by hyposphene-hypantrum
articulations (Fig. 2D). There appear to be three
sacral vertebrae—a dorsosacral followed by two
primordial sacrals with robust ribs—although the
dorsosacral rib is not preserved in articulation.
The elongate tail, which is composed of ~ 45
caudal vertebrae, has long anterior chevrons
(Fig. 2A). Mid and distal caudal centra are cy-
lindrical and have elongate prezygapophyses
(Fig.2,AandE).
1
Instituto y Museo de Ciencias Naturales, Universidad Nacional
de San Juan, San Juan 5400, Argentina.
2
Department of
Organismal Biology and Anatomy and Committee of Evolu-
tionary Biology, University of Chicago, Chicago, IL 60637, USA.
3
Consejo Nacional de Investigaciones Científicas y Técnicas,
Buenos Aires, Argentina.
4
Berkeley Geochronology Center,
2455 Ridge Road, Berkeley, CA 94709, USA.
5
Department of
Earth and Planetary Science, University of California, Berkeley,
CA 94720, USA.
6
Department of Geology, University of Cal-
ifornia, Davis, CA 95616, USA.
7
Department of Geology, Miami
University, Oxford, OH 45056, USA.
*To whom correspondence should be addressed. E-mail:
dinosaur@uchicago.edu
Table 1. Skull and long-bone lengths (in millimeters, upper portion of table) and proportions (in
percent, lower portion) of Eodromaeus murphi,Eoraptor lunensis, and other basal dinosaurs (32,33).
Parentheses indicate estimate. Skull length is measured from the anterior tip of the premaxilla to the
posterior extremity of the occipital condyle. Measurements average long-bone lengths when both sides
are available.
Measure or ratio Eodromaeus
PVSJ 562
Herrerasaurus
PVSJ 373
Eoraptor
PVSJ 512
Heterodontosaurus
SAM-PK-K337
Skull* (120) 282|| 114 115
Humerus 85 (175) 85 83
Radius 64 153 63 58
Metacarpal 3 28 62 21 22
Femur 160 345 152 112
Tibia 165 315 156 145
Metatarsal 3 (100)§ 165 81 68
Humerus/forelimb†48% 45% 50% 51%
Radius/forelimb 36% 39% 37% 36%
Metacarpal 3/forelimb 16% 16% 12% 14%
Tibia/femur 106%¶ 91% 103% 130%
Femur/hind limb‡38% 42% 39% 35%
Tibia/hind limb 39% 38% 40% 45%
Metatarsal 3/hind
limb 24% 20% 21% 21%
Humerus/femur 53% 51% 56% 74%
Forelimb/hind limb 42% 47% 43% 50%
*Skulllength was measured between the anterior tip of the premaxilla and posterior extremity of the occipital condyle. †Forelimb
length equals the sum of the lengths of the humerus, radius, and metacarpal 3. ‡Hind-limblength equalssum of the femur, tibia,
and metatarsal 3lengths. §Metatarsal 3 is not preserved in PVSJ 562. In PVSJ 560, the distal end is missing; the length estimate
is based on a comparison to digit I (plus 10% to account for the size differential for the major long bones of the hind limb). ||S kull
lengthis based on the comparably sized specimen PVSJ407, because theskull is not preserved in PVSJ 373. ¶Average of 103 and
109%, based on PVSJ 560 and 562, respectively.
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In the pectoral girdle, the coracoid has deep
proportions with a relatively short posterior pro-
cess, and the scapula has a relatively narrow neck
between a prominent acromial process and a
distally expanded blade (Fig. 2C). The straight-
shafted humerus has a broad proximal end, a
subrectangular deltopectoral crest, and a distal
end with a hemispherical radial condyle (Fig. 2F).
The ulna and radius have shafts in close contact,
the former with a prominent olecranon process
unlike the condition in Eoraptor (Fig.2G).The
well-ossified carpus is composed of a radiale,
ulnare, centrale, and four distal carpals (Fig. 1G).
There are five manual digits with a phalangeal
formula of 2-3-4-1-1 (Fig. 1G). The manus has a
pronounced lateral metacarpal arch, distal exten-
sor depressions on metacarpals 1 to 3, and elon-
gate penultimate phalanges on digits I to III.
In the pelvic girdle, the preacetabular process
is proportionately deep, and the postacetabular
process has an arched brevis fossa (Fig. 1A). The
pubic blades taper distally, where a small para-
sagittal pubic foot is located (Fig. 2H). Proxi-
mally, the femur has a dorsallyprojecting anterior
trochanter and trochanteric shelf (Fig. 2, K and L).
Distally, the end of the femur is marked by a
broad rugose depression for femoral extensor
musculature (Fig. 2M). The tibia is slightly lon-
ger than the femur, which, like other intra- and
interlimb proportions, is similar to that in Eoraptor
(Table 1). The fibula has a raised scar proximally
for strong ligamentous attachment to the tibia
(Fig. 2I). The astragalus has a rectangular medial
margin and a wedge-shaped ascending process,
and the calcaneum retains a prong-shaped pos-
terior heel (Fig. 2J).
Phylogenetic analysis of basal dinosaurs po-
sitions the contemporaneous and similar-sized
Eodromaeus and Eoraptor at the base of Therop-
oda and Sauropodomorpha, respectively (Fig. 3)
(8). Basal theropod status for Eodromaeus is
supported by a suite of derived attributes in the
skull (promaxillary fenestra, basisphenoid fossa),
axial skeleton (cervical pleurocoels, elongate caudal
prezygapophyses), forelimb (radioulnar shaft ap-
position, elongate penultimate phalanges), pelvic
girdle (distally tapering pubic blade, pubic foot),
and hind limb (femoral extensor depression, tibial
crest for fibula) (8). Whereas herrerasaurids appear
to be more basal in position among theropods
(Fig. 3), Eodromaeus is only marginally more de-
rived, has few specializations (autapomorphies) (8),
and thus approximates the hypothetical ancestral
theropod in body size and morphology. The North
American basal theropod Tawa is allied with
coelophysoids in our analysis but with a single
added step can be repositioned outside Neo-
theropoda as originally proposed (11).
Not only does Eoraptor lack all of the afore-
mentioned theropod attributes in Eodromaeus,
but it also exhibits features previously seen only
among basal sauropodomorphs. In the skull, these
features include an enlarged narial opening, a
Fig. 1. Skull, carpus, and manus of the basal dino-
saurs Eoraptor lunensis and Eodromaeus murphi.
Skull reconstruction in lateral view of (A)E. lunensis
(based on PVSJ 512) and (B)E. murphi (based on
PVSJ 560 to 562). (C)BraincaseofE. murphi (PVSJ
562) in ventrolateral view (reversed) and in two
computed-tomographic (CT) cross sections (at ver-
tical lines). Anterior left maxillary teeth are shown at
thesamescalein(D)E. lunensis [PVSJ 512, drawing
of right maxillary tooth 2 in lateral view (reversed)]
and (E)E. murphi [PVSJ 561, cast of left maxillary
tooth 3 in medial view (reversed)]. Reconstruction of
the left carpus and manus in dorsal view with
enlarged views of the proximal and distal ends of
phalanx 1 of digit I in (F)E. lunensis (based on PVSJ
512) and (G)E. murphi (based on PVSJ 560 and
562). The distal condyles in E. lunensis show 35°
clockwise rotation with the proximal dorsal extensor
process positioned dorsally. Abbreviations: I to V,
manualdigitsItoV;a,angular;antfe,antorbital
fenestra;antfo,antorbitalfossa;ar,articular;asaf,
anterior surangular foramen; bo, basioccipital; bpt,
basipterygoid process; bsf, basisphenoid fossa; bt,
basal tubera; ce, centrale; d, dentary; d1, d12, and
d16, dentary teeth 1, 12, and 16; dc1 to dc5, distal
carpals 1 to 5; dip, dorsal intercondylar process; ec,
ectopterygoid; emf, external mandibular fenestra;
en,externalnaris;f,frontal;j,jugal;l,lacrimal;ldc,
lateral distal condyle; m, maxilla; m1, m11, and
m17, maxillary teeth 1, 11, and 17; mcot, medial
cotylus; mdc, medial distal condyle; n, nasal; nf,
narial fossa; p, parietal; pl, palatine; pm, premaxilla;
pm1 and pm4, premaxillary teeth 1 and 4; pmfo,
promaxillary fossa; po, postorbital; popr, par-
occipital process; prf, prefrontal; ps, parasphenoid;
pt, pterygoid; q, quadrate; qf, quadrate foramen; qj,
quadratojugal; rae, radiale; ri, ridge; sa, surangular;
saf, surangular foramen; snf, subnarial foramen; sq,
squamosal; ule, ulnare; vip, ventral intercondylar
process. Dashed lines indicate a missing margin;
hatching indicates a broken surface. Scale bars, 2
cm in (C) (for braincase); 3 mm in (D) and (E); 5 mm
in (F) and (G) (enlarged views).
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slender ventral process of the squamosal, and
the inset position of the first dentary tooth
(Fig. 1B). The toothless anterior end of the
dentary, which is flanked by a conspicuous pair
of vascular foramina, may have supported a
small lower bill as in other basal sauropodo-
morphs (13). In addition, the form of the crowns
(basal constriction, lateral crest, larger inclined
denticles) strongly suggests that Eoraptor had an
omnivorous, if not wholly herbivorous, diet. In
the postcranial skeleton, sauropodomorph fea-
tures include substantial medial rotation in the
shaft of the first phalanx of the thumb (digit I)
that directs the tip of the ungual inward (Fig. 1F)
(13) and an astragalus with a characteristic shape
(anteriorly projecting anteromedial corner) (3).
Reinterpretation of Eoraptor as a basal sau-
ropodomorph closely related to Panphagia (Fig.
3) differs from previous phylogenetic assess-
ments of this early dinosaur as a basal saurischian
(3,5,14–16) or basal theropod (1,11,12,17). The
phylogenetic analysis is decisive in this regard,
requiring nine additional steps to reposition Eoraptor
at the base of Theropoda. With Eoraptor as a basal
sauropodomorph, the three principal clades of di-
nosaurs (ornithischians, sauropodomorphs, thero-
pods) now appear to be converging on an ancestral
skeletal plan—a bipedal cursor (tibia longer than
femur) with body length less than 2 m. (Fig. 2A
and Table 1). In our analysis, heterodontosaurids
remain nested among basal ornithischians (Fig. 3).
If heterodontosaurids are repositioned as basal-
most ornithischians (18), which is only slightly
less parsimonious, the ancestral body plan for di-
nosaurs would have included a proportionately long
forelimb (~45% hind-limb length) and sharp-
Fig. 3. Phylogenetic re-
lationships among basal
dinosaurs. Consensus clado-
gram summarizing three
minimum-length trees based
on maximum-parsimony
analysis of 139 characters
in successive outgroups
(Lagerpetidae, Marasuchus,
Silesauridae) and 16 basal
dinosaur taxa (246 steps;
consistency index = 0.618,
retention index = 0.800).
Outgroup taxa were con-
strained as shown; numbers
at nodes indicate decay in-
dex. Suprageneric terminal
taxa are scored on the basis
of two or more included
species. Suprageneric taxa that label the cladogram are positioned on the basis of phylogenetic
definitions (8).
Fig. 2. Postcranial features of the Late Triassic basal
theropod Eodromaeus murphi. Skeletal silhouette
basedonPVSJ534,560,and562;postcranialbones
are from PVSJ 562, except where indicated; bone
casts and pencil drawings were used to eliminate
color distraction and reversed as needed to left lateral
view. (A) Skeletal silhouette showing preserved bones
(missing portions in red). (B) Posterior cervical
vertebrae (C7 and C8 anterior part) in lateral view
(reversed) with enlarged cross-sectional view across
the pleurocoel from a CT scan (at vertical line). (C)
Scapulocoracoid in lateral view. (D) Posterior dorsal
vertebra (~D11) in lateral view. (E) Distal caudal
vertebrae (CA27 and CA28) in lateral view (PVSJ 560).
(F) Humerus in anterior view. (G) Ulna in lateral view.
(H) Pubes in anterior and lateral (reversed) views. (I)
Proximal fibula in medial view. (J)Astragalusand
calcaneum (right) in dorsal view (PVSJ 534). (Kto M)
Femur in lateral view, proximal end in lateral view, and
distal end in anterior view, respectively. Abbreviations:
ac, acromion; ach, articular surface for the chevron;
afi, articular surface for the fibula; ap, ambiens pro-
cess; arhd, articular surface for the radial head; arsh,
articular surface for the radial shaft; as, astragalus;
asp, ascending process; at, anterior trochanter; ati,
articular surface for the tibia; c, centrum; C2 and C10,
cervicalvertebrae2and10;ca,calcaneum;CA1,cau-
dal vertebra 1; co, coracoid; ct, calcaneal tuber; D1
andD14,dorsalvertebrae1and14;di,diapophysis;
dpc, deltopectoral crest; ep, epipophysis; gl, glenoid;
gr, goove; hd, head; in, interpubic notch; k, keel; lco,
lateralcondyle;lp,lateralprocess;nc,neuralcanal;
nds, nonarticular dorsal surface; ns, neural spine; ol,
olecranon; pa, parapophysis; pf, pubic foot; pl, pleuroceol;
poz, postzygapophysis; ppr, posterior process; prz,
prezygapophysis; rco, radial condyle; rr, raised rugosity
for attachment; S1 and S3, sacral vertebrae 1 and 3; scb,
scapular blade; tp, transverse process; ts, trochanteric shelf. Dashed lines indicate a missing margin; hatching indicates a broken surface. Scale bars, 2 cm in (B)
and (D); 2 cm in (C); 5 mm in (E); 2 cm in (F) and (G); 3 cm in (H); 2 cm in (I); 1 cm in (J); 3 cm in (K); 2 cm in (L) and (M).
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clawed manus with pits to accommodate digital
hyperextension.
The Ischigualasto Formation, a richly fossil-
iferous fluvial succession within a Triassic con-
tinental rift basin (6,19), provides a window to
faunal dynamics at the dawn of the dinosaur era.
To quantify and temporally calibrate faunal abun-
dance in the Ischigualasto Formation, we logged
nearly 800 vertebrate specimens and obtained
radioisotopic ages that bracket the formation be-
tween 231.4 and 225.9 million years ago (Ma)
(Fig. 4) (8,20).
All of the early dinosaurs from the Ischigua-
lasto Formation thrived during what we identify
here as the Scaphonyx-Exaeretodon-Herrerasaurus
biozone, a narrow temporal interval from 231.5
to 229 Ma in the latter half of the Carnian stage
(Fig. 4). During this interval, dinosaurs account
for 11% of recorded vertebrate specimens, which
is about twice that previously estimated (20).
This biozone is dominated by mid-sized non-
dinosaurian herbivores (rhynchosaurs, traverso-
dontid cynodonts) (Fig. 4). Herrerasaurids and
Eodromaeus, however, represent ~70% of all
nonaquatic carnivores in the faunal assemblage.
Similarly, all small-bodied omnivores or herbi-
vores(<2minbodylengthand/or<15kg)are
dinosaurs (Pisanosaurus,Eoraptor,Panphagia,
Chromogisaurus). Dinosaurs are also taxonomi-
cally diverse within this biozone, making up
33% of recorded genera, a percentage equaling
that in the overlying Los Colorados Formation
near the close of the Late Triassic (late Norian)
(Fig. 4, top pie chart).
A major faunal change seems to have oc-
curred near the Carnian-Norian boundary ~ 229 Ma,
when humidity increased as inferred from sedi-
mentological (argillic paleosols) and paleofloral
(palynomorph-cuticle-trunk) evidence (8). The
rhynchosaur Scaphonyx, most therapsids, and all
dinosaurs disappeared, leaving a depauperate
fauna dominated by the traversodontid cynodont
Exaeretodon (Exaeretodon biozone). A less con-
strained but consistent faunal record from south-
ern Brazil (21,22) suggests that this may have been
a regional extinction event across southwestern
Pangaea. Higher in the section, Exaeretodon is not
present, and the mid-sized dicynodont Jachaleria
is the dominant vertebrate (Jachaleria biozone).
Although taphonomic bias against preservation of
smaller vertebrates may play some role in the up-
per biozone, Exaeretodon is clearly not the dom-
inant large vertebrate herbivore.
The discovery of Eodromaeus, the reinterpre-
tation of Eoraptor as a sauropodomorph, and
the faunal record of the Ischigualasto Forma-
tion provide additional evidence that, by mid
Carnian time (~232 Ma), the earliest dinosaurs
had already evolved the most functionally
important trophic and locomotor features char-
acterizing ornithischians, sauropodomorphs,
and theropods (17,23). These attributes are thus
unlikely to have functioned as the competitive
Fig. 4. Calibrated stratigraphic profile across the
Ischigualasto Formation (northwest Argentina)
showingtetrapoddiversityandabundance.(Left)
The chart shows a simplified stratigraphic section
divided into four formational members (Roman
numerals I to IV) as well as the stratigraphic po-
sitions of the holotype of Eodromaeus murphi and
a bracketing pair of radiometric dates using re-
cently revised radioisotopic decay constants (30).
(Middle) Three biozones, three large pie charts
summarizing faunal abundance for each biozone,
and two smaller pie charts pie showing taxonom-
ic diversity before (Los Chañares Fm.) and after
(Los Colorados Fm.) the Ischigualasto Formation.
(Right) A current geologic time scale (31), which
assumes an average rate of sedimentation be-
tween radioisotopically dated horizons.
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advantage to account for the dominance of dino-
saurs in abundance and diversity in terrestrial
habitats some 30 million years later in the earliest
Jurassic (~ 202 Ma) . Eodromaeus increases the
range of salient theropod features present in
the earliest dinosaurs, and Eoraptor shows that
the enlarged naris, basally constricted crowns,
and a twisted pollex were present in the earliest
sauropodomorphs.
One explanation for the rise of dinosaurs has
been that a few key features led gradually to the
competitive dominance of dinosaurs (20,24).
This view has been overtaken by a hypothesis
of noncompetitive replacement, in which their
rise is split into two successive episodes of ex-
tinction and noncompetitive infilling of vacant
ecospace (25,26). In the replacement hypoth-
esis, the earliest dinosaurs are regarded as par-
ticularly rare (1 to 3% of terrestrial vertebrates),
their abundance and diversity increasing succes-
sively at the Carnian-Norian and Triassic-Jurassic
boundaries coincident with mass extinction of
rhynchosaurs, traversodontid cynodonts, and
dicynodonts and later of (noncrocodyliform)
crurotarsal archosaurs.
In contrast, the fossil record from Ischigua-
lasto indicates that early dinosaurs in the latter
half of the Carnian (231 to 228 Ma) were more
common and diverse than previously thought,
equaling the percentage of dinosaurian genera in
the late Norian fauna from the overlying Los
Colorados Formation (Fig. 4). Thus, in terms of
taxonomic diversity, dinosaurs did not increase
their percentage among terrestrial vertebrates
toward the end of the Triassic in southwestern
Pangaea.
The record also shows that extinction of rhyn-
chosaurs and other large-bodied herbivores was
not synchronous but rather spread out across 4 to
5 million years (Fig. 4). The disappearance of
rhynchosaurs at the Carnian-Norian boundary was
not linked to an increase in dinosaur diversity but
rather coincided with the local extinction of dino-
saurs. The most substantial change between the
earliest dinosaurs in the Carnian and those in the
late Norian is that the latter have expanded into
the upper register of body size as both carnivores
(Zupaysaurus) and herbivores (Lessemsaurus).
Increased body size probably enhanced the pres-
ervation potential of late Norian dinosaurs, which
are also recorded from many more sites than late
Carnian dinosaurs (6,27). We cannot evaluate
whether the increase in body size was gradual or
rapid, as there are no dinosaurs in the section
between late Carnian and late Norian faunas.
Furthermore, increasing body size among dino-
saurs was not limited to the Triassic but con-
tinued throughout the Jurassic (23,28)longafter
the extinction of Triassic synapsids and (non-
crocodyliform) crurotarsal archosaurs.
The earliest dinosaurs are currently restricted
to late Carnian sites in southern Pangaea (Argen-
tina, Brazil). The earliest dinosaur from northern
Pangaea (Taw a , western North America) has re-
cently been dated to the mid Norian (~214 Ma)
(11), making it some 15 million years younger in
age. This has led to the view that the major dino-
saurian clades (Ornithischia, Sauropodomorpha,
Theropoda) may have originated in southern
Pangaea (14,16). The paleogeographic impor-
tance of this distribution, however, is compromised
by the absence of well-preserved skeletal remains
of Carnian age from northern locales (northern
Africa, Europe, North America) (14,21,29). Dis-
cerning global patterns of replacement or areas
of paleogeographic origin for particular groups
in terrestrial ecosystems in the Triassic requires
greater temporal and geographic control than is
currently available.
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34. We thank C. Abraczinskas for final drafts of all figures,
personnel of the Fossil Lab at the Univ. of Chicago
for preparation of fossil material, personnel of the
High-Resolution X-ray Computed Tomography Facility
at The Univ. of Texas at Austin for assistance with
computed tomography imaging, and S. Nesbitt and
R. Irmis for examination of fossil material in their care.
This work was supported by the Whitten-Newman
Foundation, the Island Fund of the New York Community
Trust, Earthwatch Institute, Consejo Nacional de
Investigaciones Científicas y Técnicas, the Ann and
Gordon Getty Foundation, and the National Geographic
Society.
Supporting Online Material
www.sciencemag.org/cgi/content/full/331/6014/206/DC1
Materials and Methods
Fig. S1
Tables S1 to S6
References
29 September 2010; accepted 8 December 2010
10.1126/science.1198467
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