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The Evolution of Dinosaurs
Paul C. Sereno,
et al.
Science
284
, 2137 (1999);
DOI: 10.1126/science.284.5423.2137
www.sciencemag.org (this information is current as of September 11, 2007 ):
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E VOLUTION
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dock for help with illustrations; H. Bode, E. David-
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son, N. Shubin, A. Adoutte, E. Davidson, J. Grenier,
and G. Halder for comments on the manuscript; and
J. Wilson for help with its preparation. A.H.K. is
supported in part by the NASA Astrobiology Insti-
tute. S.B.C. is an investigator of the Howard Hughes
Medical Institute.
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d
REVIEW
The Evolution of Dinosaurs
Paul C. Sereno
The ascendancy of dinosaurs on land near the close of the Triassic now
appears to have been as accidental and opportunistic as their demise and
replacement by therian mammals at the end of the Cretaceous. The
dinosaurian radiation, launched by 1-meter-long bipeds, was slower in
tempo and more restricted in adaptive scope than that of therian mam-
mals. A notable exception was the evolution of birds from small-bodied
predatory dinosaurs, which involved a dramatic decrease in body size.
Recurring phylogenetic trends among dinosaurs include, to the contrary,
increase in body size. There is no evidence for co-evolution between
predators and prey or between herbivores and ßowering plants. As the
major land masses drifted apart, dinosaurian biogeography was molded
more by regional extinction and intercontinental dispersal than by the
breakup sequence of Pangaea.
million-year interval when virtually all ani-
mals1mormore in length in dry land
habitats were dinosaurs.
Dinosaurs, the descendants of a single com-
mon ancestor, first appeared at least 15 million
years earlier but were limited in diversity and
abundance (Fig. 1). Well-preserved skeletons
discovered recently in 230-million-year-old
rocks (mid-Carnian in age) provide a glimpse
of a land radiation already underway ( 12 ). The
most fundamental adaptations for herbivory
and carnivory among dinosaurs had already
evolved. A novel means for slicing plant matter,
utilizing inclined tooth-to-tooth wear facets, is
fully developed in the meter-long herbivore
Pisanosaurus , the oldest known ornithischian
(Fig. 1, left; Fig. 2, node 1; Fig. 3A, feature 4).
Jointed lower jaws and a grasping hyperextend-
able manus for subduing and eviscerating prey
are present in the contemporary predators
Eoraptor and Herrerasaurus , which are the
oldest well-preserved theropods (Fig. 1, right;
Fig. 2, node 41; Fig. 3B, features 11 and 12).
Traditional scenarios for the ascendancy of
dinosaurs that invoke competitive advantage
( 13 ) have difficulty accommodating the sub-
stantial temporal gap (15 million years or more)
between the initial radiation of dinosaurs and
their subsequent global dominance during the
latest Triassic and Early Jurassic ( 14 ). Oppor-
tunistic replacement of a diverse array of ter-
restrial tetrapods (nonmammalian synapsids,
basal archosaurs, and rhynchosaurs) by dino-
saurs is now the most plausible hypothesis ( 11 ,
14 , 15 ). This pattern is broadly similar to the
replacement of nonavian dinosaurs by therian
mammals at the end of the Cretaceous. Recent
During the past 30 years, intensified paleon-
tological exploration has doubled recorded
dinosaurian diversity ( 1 ) and extended their
geographic range into polar regions ( 2 ). Ex-
ceptional fossil preservation has revealed
eggshell microstructure ( 3 ), nesting patterns
and brooding posture among predators ( 4 ),
and epidermal structures such as downy fila-
ments and feathers ( 5 , 6 ). Analysis of bone
microstructure and isotopic composition has
shed light on embryonic and posthatching
growth patterns and thermophysiology ( 7 ).
Footprint and track sites have yielded new
clues regarding posture ( 8 ), locomotion ( 9 ),
and herding among large-bodied herbivores
( 10 ). And the main lines of dinosaurian de-
scent have been charted, placing the afore-
mentioned discoveries in phylogenetic con-
text ( 11 ).
The most important impact of this en-
riched perspective on dinosaurs may be its
contribution to the study of large-scale evo-
lutionary patterns. What triggers or drives
major replacements in the history of life?
How do novel and demanding functional
capabilities, such as powered flight, first
evolve? And how does the breakup of a
supercontinent affect land-based life? The
critical evidence resides in the fossil
record—in the structure, timing, and geog-
raphy of evolutionary radiations such as
that of dinosaurs.
Early Dinosaurs: Victors by Accident
Did dinosaurs outcompete their rivals or sim-
ply take advantage of vacant ecological
space? The ascendancy of dinosaurs on land
transpired rather rapidly some 215 million
years ago, before the close of the Triassic.
Herbivorous prosauropods and carnivorous
coelophysoid ceratosaurs spread across Pan-
gaea, ushering in the “dinosaur era”: a 150-
Department of Organismal Biology and Anatomy,
University of Chicago, 1027 East 57th Street, Chicago,
IL 60637, USA.
www.sciencemag.org SCIENCE VOL 284 25 JUNE 1999
2137
CH 2 O
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E VOLUTION
Fig. 1. Temporally calibrated phylogeny of Dinosauria, showing known tem-
poral durations (solid bars), missing ranges (shaded bars), and ranges extend-
ed by fragmentary or undescribed specimens (dashed bars). At left is tabu-
lated the number of recorded nonavian dinosaurian genera per stage and an
estimated curve of generic diversity, taking in to account available outcrop
area ( 87 ). Basal or primitive taxa, in general, appear earlier in time than more
derived members of a clade. Long missing ranges result from preservational
bias against small body size (less than 2 m), which truncates the early record
of many clades, and from intervals for which there is little corresponding
exposed terrestrial rock (such as the Middle Jurassic). The shaded zone
(bottom) indicates the initial stage of the dinosaurian radiation before their
dominance of land faunas in taxonomic diversity and abundance.
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E VOLUTION
evidence, moreover, has implicated similar
primary agents of extinction, namely global
climatic change (seasonal aridity) ( 16 ) and,
possibly, flood basalts associated with the
opening of the Atlantic Ocean and extrater-
restrial impacts ( 17 ).
Although the timing of end-Triassic extinc-
tions remains less resolved than events at the
end of the Cretaceous ( 18 ), dinosaurian and
mammalian radiations cannot be explained as
the result of niche subdivision, increased com-
petition, or progressive specialization (escala-
tion), or as taxonomic, taphonomic, or stochas-
tic artefacts ( 19 ). These two great land radia-
tions, the conventional signposts for the sub-
division of Phanerozoic time, constitute oppor-
tunistic infilling of vacant ecospace after phys-
ical perturbation on a global scale.
horny bill and then sliced by tooth rows
composed of expanded overlapping crowns
with inclined wear facets (Fig. 3A, features 1
through 4). The predentary, a neomorphic
bone, provided a stable platform for the lower
bill while allowing the dentaries to rotate
during (isognathus) occlusion ( 20 ). A holding
space, or cheek, lateral to the tooth rows also
suggests increased oral processing of plant
matter ( 21 ).
Ornithischians were extremely rare during
Ornithischians: Bird-Hipped Croppers
Ornithischians processed plant matter by
novel means. Vegetation was cropped by a
Fig. 2. Phylogeny of Dinosauria, showing the relationships among orni-
thischians (left) and saurischians (right). Thickened internal branches are
scaled to reßect the number of supporting synapomorphies (scale bar
equals 20 synapomorphies). Phylogenetic structure and internal branch
lengths are based on minimum-length trees from maximum-parsimony
analyses of approximately 1100 characters under delayed character-
state optimization (Table 1). The evolution of hadrosaurids within Orni-
thopoda (nodes 11 through 18) and birds within Tetanurae (nodes 46
through 57) provide the best examples of sustained skeletal trans-
formation. Numbered nodes are listed here, with normal and bold text
indicating stem- and node-based taxa, respectively ( 88 ): 1, Ornithis-
chia; 2, Genasauria ; 3, Thyreophora; 4, Eurypoda ; 5, Stegosauria; 6,
Stegosauridae; 7, Ankylosauria; 8, Nodosauridae; 9, Ankylosauridae;
10, Neornithischia; 11, Ornithopoda ; 12, Euornithopoda; 13, Iguan-
odontia; 14, Ankylopollexia ; 15, Styracosterna; 16, Hadrosauri-
formes ; 17, Hadrosauroidea; 18, Hadrosauridae ; 19, Marginocepha-
lia ; 20, Pachycephalosauria; 21, Pachycephalosauridae ; 22,
Pachycephalosaurinae; 23, Ceratopsia; 24, Neoceratopsia; 25, Coro-
nosauria ; 26, Ceratopsoidea; 27, Ceratopsidae ; 28, Saurischia; 29,
Sauropodmorpha ; 30, Prosauropoda; 31, Plateosauria ; 32, Mas-
sospondylidae; 33, Plateosauridae; 34, Sauropoda; 35, Eusauropoda;
36, Neosauropoda ; 37, Diplodocoidea; 38, Macronaria; 39, Titano-
sauriformes ; 40, Somphospondyli; 41, Theropoda; 42, Neothero-
poda ; 43, Ceratosauria; 44, Ceratosauroidea; 45, Coelophysoidea; 46,
Tetanurae; 47, Spinosauroidea ; 48, Neotetanurae ; 49, Coelurosau-
ria; 50, Maniraptoriformes , 51, Ornithomimosauria; 52, Ornithomi-
moidea; 53, Tyrannoraptora; 54, Maniraptora ; 55, Paraves; 56,
Deinonychosauria ; 57, Aves ; 58, Ornithurae; 59, Ornithothoraces .
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E VOLUTION
the Late Triassic; their remains consist largely
of isolated teeth. The first well-preserved orni-
thischian skeletons are Early Jurassic in age
( 20 , 22 ), by which time the major clades of
ornithischians were already established (Fig. 1).
The small-bodied bipeds Pisanosaurus and Le-
sothosaurus constitute successive sister taxa to
other ornithischians (Fig. 2, node 1). The “bird-
hipped” configuration of the pelvic girdle (with
the pubis rotated posteriorly) characterizes Le-
sothosaurus and later ornithischians (Fig. 3A,
feature 9). Possibly before the end of the Trias-
sic, the remaining ornithischians split into ar-
mored thyreophorans and unarmored neorni-
thischians, which include ornithopods and mar-
ginocephalians (Fig. 1; Fig. 2, nodes 3, 10, 11,
and 19). This phylogenetic scheme is based on
few characters, which may indicate that these
early divergences occurred within a short inter-
val of time.
Thyreophoran body armor was originally
composed of parasagittal rows of keeled
scutes as in Scutellosaurus ( 23 ), a small Early
Jurassic thyreophoran from western North
America. More advanced thyreophorans,
such as Emausaurus ( 24 ) and Scelidosaurus
( 25 ) from the Lower Jurassic of Europe, ap-
pear to have reverted to a quadrupedal pos-
ture, as evidenced by hoof-shaped manual
unguals. The larger bodied stegosaurs and
ankylosaurs constitute the “broad-footed”
thyreophorans (Eurypoda), named for the
spreading (versus compact) arrangement of
metatarsals in their elephantine hind feet
(Fig. 1; Fig. 2, node 4).
The earliest and most primitive stego-
saurs, such as Huayangosaurus from the
Middle Jurassic of China ( 26 , 27 ), have re-
duced the lateral osteoderm rows while elab-
orating the pair flanking the midline into
erect plates (over the neck) that grade into
pointed spines (over the tail) (Fig. 1; Fig. 2,
nodes 5 and 6). Stegosaurs more advanced
than Huayangosaurus have low narrow skulls
and long hindlimbs as compared to their fore-
limbs ( 27 , 28 ).
Ankylosaurs elaborated the dermal armor
of the trunk in another direction, filling the
spaces between scute rows with smaller os-
sicles to create a solid shield over the neck
and trunk. Several skull openings are closed
by surrounding cranial bones and accessory
ossifications, as in the basal ankylosaurid
Gargoyleosaurus , discovered recently in Up-
per Jurassic rocks in western North America
( 29 ) (Fig. 2, node 9). Before the close of the
Jurassic, ankylosaurs had split into two dis-
tinctive subgroups—nodosaurids and ankylo-
saurids— both of which diversified for the
most part on northern continents during the
Cretaceous ( 30 , 31 ). The nodosaurid skull is
proportionately low and held with the snout
tipped downward. Cranial sutures completely
fuse with maturity, as in the North American
genera Pawpawsaurus and Panoplosaurus
( 32 ). In most ankylosaurids, the skull is very
broad, and the snout is gently domed. The
wedge-shaped osteoderms that project from the
back corners of the ankylosaurid skull are short
in basal forms such as Gastonia , Shamosaurus ,
and Minmi ( 33 ) but form prominent plates in
other ankylosaurids. A terminal tail club,
composed largely of two pairs of wedge-
shaped osteoderms, also distinguishes all
known ankylosaurids.
Ornithopods split into three distinct clades
during the Jurassic: heterodontosaurids, hyp-
silophodontids, and iguanodontians (Fig. 1;
Fig. 2, nodes 11 through 13). Heterodonto-
saurids, named for their prominent lower ca-
nines, were small fleet-footed ornithopods
that first appear in the Early Jurassic. Al-
though undoubted herbivores, heterodonto-
saurids have elongate forelimbs with large
grasping hands tipped with trenchant claws,
as seen in the southern African genera Het-
erodontosaurus and Abrictosaurus ( 34 ).
Hypsilophodontids, the most conservative
ornithopods, underwent little modification
during their long fossil record from the Mid-
dle Jurassic to the end of the Cretaceous ( 35 ).
As a consequence, their monophyly is less
certain ( 30 , 36 ). Iguanodontians, in contrast,
underwent marked transformation during the
Late Jurassic and Early Cretaceous, from bas-
al forms such as Muttaburrasaurus and Ten-
ontosaurus to more derived genera such as
Dryosaurus , Camptosaurus , Probactrosau-
rus , and Iguanodon ( 37 ) (Fig. 2, nodes 13
through 17; Fig. 3A, features 5, 6, and 8).
Ornithopods achieved their greatest diversity
in the Late Cretaceous with the radiation of
duck-billed hadrosaurids ( 38 ).
Marginocephalians, a group characterized
by a bony shelf on the posterior margin of the
skull, are composed of two distinct subgroups:
the thick-headed pachycephalosaurs ( 39 , 40 )
and frilled ceratopsians ( 41 , 42 ). Both clades
are known exclusively from northern continents
and primarily from the Upper Cretaceous of
western North America and Asia (Fig. 1; Fig. 2,
nodes 20 through 27). In all pachycephalosaurs,
the skull roof is thickened and ornamented with
lateral and posterior rows of tubercles. In prim-
itive forms such as Goyocephale , the skull roof
is flat with open supratemporal fenestrae. In
more derived forms, the frontoparietal portion
of the skull roof thickens further into a dome,
which eventually incorporates all elements of
the skull roof. The largest of these domed
forms, Pachycephalosaurus and Stygimoloch ,
have swollen tubercles or horns projecting from
the posterior corners of the skull ( 40 ) and con-
stitute the only ornithischians to maintain an
obligatory bipedal posture at large body size
(more than 1 ton) ( 11 ). Some researchers have
united flat-headed pachycephalosaurs as a clade
( 43 ), but this condition is primitive, with some
flat-headed genera being more closely related to
domed forms ( 11 , 30 ).
Psittacosaurids, the most primitive cera-
topsians, are small-bodied parrot-beaked her-
bivores from Asia assigned to the single ge-
nus Psittacosaurus . As in all ceratopsians, the
anterior margin of the psittacosaurid snout is
capped by the rostral, a neomorphic bone
sheathed by the upper bill. Although they
show remarkably little skeletal variation,
psittacosaurids persisted throughout most of
the Early Cretaceous.
Remaining ceratopsians (neoceratopsians)
also date back to the earliest Cretaceous of
China and include Chaoyangsaurus and Ar-
chaeoceratops ( 42 ). Archaeoceratops and
more derived neoceratopsians are distin-
guished by very large skulls relative to their
postcranial skeletons and may have already
reverted to a quadrupedal posture. In Late
Cretaceous neoceratopsians, such as the
abundant Asian form Protoceratops , the pos-
terior margin of the skull extends posterodor-
sally as a thin shield pierced by a pair of
fenestrae. Ceratopsids, a diverse subgroup of
large-bodied neoceratopsians, were restricted
to western North America, ranging from
Mexico to the north slope of Alaska. Their
many cranial and postcranial modifications
include slicing dental batteries composed of
stacked columns of two-rooted teeth and post-
orbital horns and frill processes of variable
length and shape ( 41 ).
Sauropodomorphs: Long-Necked Titans
Sauropodomorphs constitute the second great
radiation of dinosaurian herbivores. Although
their origin is as ancient as that of ornithis-
chians, their diversification followed a differ-
ent time course ( 44 , 45 ). As a group, sau-
ropodomorphs are united by only a few char-
acteristics, such as an enlarged narial opening
and an unusual position for the longest pedal
claw— on the first digit, or hallux, rather than
the middle toe (Fig. 3C, features 21 and 29).
Unlike ornithischians, there are no singleton
genera at the base of the clade. By the Late
Triassic, sauropodomorphs had already split
into two distinctive groups: prosauropods and
sauropods (Fig. 2, nodes 29, 30, and 34).
Prosauropods diversified rapidly with only
minor skeletal modification to become the
dominant large-bodied herbivores on land
from the Late Triassic through the Early Ju-
rassic. Sauropods, in contrast, were rare in the
Early Jurassic, when ornithischians appear to
have undergone their major radiation, but
diversified rapidly during the Middle Jurassic
after prosauropods had gone extinct (Fig. 1).
A succession of basal sauropods lies outside
the main neosauropod radiation, which split
during the Middle Jurassic into diplodocoids
and macronarians, a clade composed of ca-
marasaurids, brachiosaurids, and titanosaurs
(Fig. 2, nodes 37 through 40). Neosauropods
became the dominant large-bodied herbivores
during the Middle and Late Jurassic and, on
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