Benton M.J. Introduction to paleobiology and the fossil record

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448 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 16.9 Classiﬁ cation of the reptiles

Reptiles are a paraphyletic group of Amniota that excludes the reptile descendants, birds and

mammals. The basic classiﬁ cation is founded on skull pattern (Fig. 16.18).

Class REPTILIA

Subclass ANAPSIDA: no temporal openings

• Various basal groups, such as procolophonids

• Permian to Triassic

Order TESTUDINES (Chelonia)

• Turtles; bony carapace, retractable neck and limbs

• Late Triassic to Recent

Subclass SYNAPSIDA: one (lower) temporal opening

Order “PELYCOSAURIA”

• The sail-backed “mammal-like reptiles” and relatives

• Late Carboniferous to Early Permian

Order THERAPSIDA

• Synapsids with differentiated teeth, including the mammals

• Late Permian to Recent

Subclass DIAPSIDA: two temporal openings

Infraclass ARCHOSAURIA

• Thecodontians, crocodilians, pterosaurs, dinosaurs and birds; characterized by an antorbital

fenestra

• Late Permian to Recent

Infraclass LEPIDOSAURIA

• Lizards, snakes and their ancestors

• Late Triassic to Recent

The Anapsida: turtles and relatives

The oldest anapsids were small insect eaters.

During the Permian and Triassic, some

unusual anapsids came on the scene. The most

diverse of these were the procolophonids (Fig.

16.19a), small animals with triangular skulls

and broad teeth adapted to a diet of tough

plants and insects.

The turtles appeared ﬁ rst in the Late Trias-

sic, being represented by Proganochelys (Fig.

16.19b). Modern turtles have no teeth, but

Proganochelys still had some on its palate.

The skull is solid, and the body is covered

above and below by a bony shell. Turtles live

on land, in ponds (Fig. 16.19c) and in the sea.

Some marine turtles of the Cretaceous reached

3 m in length.

Our current understanding of amniote evo-

lution (see Fig. 16.18) has been challenged by

new molecular studies that suggest turtles

might in fact be modiﬁ ed diapsid reptiles; if

this is so, and it is still controversial (Lee et

al. 2004), then the clade Anapsida might no

longer have any meaning.

A world of synapsids

The ﬁ rst synapsids, known from the Late Car-

boniferous and Early Permian, are grouped

loosely as “pelycosaurs”. Most of these were

small- to medium-sized insectivores and carni-

vores with powerful skulls and sharp, ﬂ esh-

piercing teeth. Some later pelycosaurs, like

Dimetrodon (Fig. 16.20a), had vast sails sup-

ported on vertical spines growing up from the

5 mm

large orbit

(a) (b)

(c)

20 mm

toothless

jaw margin

Figure 16.19 Fossil and recent anapsid reptiles: (a) skull of the Triassic procolophonid Procolophon;

(b) skull of the Triassic turtle Proganochelys; (c) a fossilized snapping turtle, with the head (bottom

right) and skeleton separated from the carapace, from pond sediments ﬁ lling an impact crater at

Steinheim, Germany. (a, based on Carroll & Lindsay 1985; b, based on Gaffney & Meeker 1983.)

100 mm

elongate neural spines

to support a sail

carnivorous

dentition

(a)

(b) (c)

Figure 16.20 Synapsids of the Permian: (a) the carnivorous pelycosaur Dimetrodon; (b) the carnivorous

gorgonopsian Lycaenops; and (c) the herbivorous dicynodont Dicynodon. (a, based on Gregory

1951/1957; b, c, courtesy of Gillian King.)

450 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

vertebrae, perhaps used in controlling body

temperature. The pelycosaurs also include a

number of groups that adapted to plant eating,

among the ﬁ rst herbivorous land vertebrates.

Synapsids radiated dramatically in the Late

Permian as a new clade, the Therapsida. The

most astonishing carnivores were the gor-

gonopsians (Fig. 16.20b) with their large,

wolf-like bodies and massive saber teeth that

they probably used to attack the larger thick-

skinned herbivores. The dicynodonts had

bodies shaped like overstuffed sausages, and

no teeth at all, or only two tusks (Fig. 16.20c).

They were successful herbivores and some of

the ﬁ rst animals to have a complex chewing

cycle that allowed them to tackle a wide

variety of plant foods. Late Permian therap-

sids are common in the continental sediments

of the Karoo Basin in South Africa and the

Urals in Russia. At the end of the Permian, at

the time of a major mass extinction in the sea

(see p. 170), most of these animals died out.

The gorgonopsians disappeared and the dicyn-

odonts were nearly wiped out – extinction on

a huge scale.

The cynodonts were an important Triassic

synapsid group. The Early Triassic form Thri-

naxodon (Fig. 16.21a) looked dog-like. In the

snout area of the skull, there are numerous

small canals that indicate small nerves serving

the roots of sensory whiskers. If Thrinaxodon

had whiskers, it clearly also had the potential

for hair on other parts of its body, and this

implies insulation and temperature control.

Cynodonts evolved along several lines during

the Triassic, and gave rise to mammals, such

as Megazostrodon (Fig. 16.21b), in the Late

Triassic and Early Jurassic.

The transition from basal synapsid to

mammal is marked by an extraordinary shift

of the jaw joint into the middle ear. Reptiles

typically have six bones in the lower jaw and

the articular bone articulates with the quad-

rate in the skull (Fig. 16.21c). In mammals,

on the other hand, there is a single bone in

the lower jaw, the dentary, which articulates

with the squamosal (Fig. 16.21d). The reptil-

ian articular–quadrate jaw joint became

reduced in Triassic cynodonts, and moved

into the middle ear passage. That is why we

have three tiny ear bones, the hammer, anvil

and stirrup, which transmit sound from the

ear drum to the brain, while reptiles have only

one, the stirrup or stapes.

10 mm

lumbar vertebrae

20 mm

(a)

(b)

(c)

(d)

articular

quadrate

dentary

squamosal

100 mm

Figure 16.21 Transition to the mammals: (a) the

Early Triassic cynodont Thrinaxodon; (b) the

Early Jurassic mammal Megazostrodon; and

(c, d) skulls of an early synapsid (c) and a

mammal (d) to show the reduction in elements

in the lower jaw and switch of the jaw joint. (a,

based on Jenkins 1971; b, based on Jenkins &

Parrington 1976; c, d, based on Gregory

1951/1957.)

FISHES AND BASAL TETRAPODS 451

Dinosaurs and mammals

People usually think of the dinosaurs as pre-

cursors of the mammals. Dinosaurs famously

ruled the Earth for 160 Myr of the Mesozoic,

and then were replaced by the mammals

65 Ma. However, as we have seen, the

mammals arose in the Late Triassic, about the

same time as the ﬁ rst dinosaurs. So, both

groups evolved side by side through the Late

Triassic, Jurassic and Cretaceous – the dino-

saurs as large to very large beasts, and the

mammals generally scuttling unobtrusively

through the undergrowth. Our understanding

of how both groups evolved has changed

enormously in recent years, and this is dis-

cussed in Chapter 17.

Review questions

1 How has the application of cladistics (see

p. 129) affected our ideas about basal ver-

tebrate phylogeny? Look at older books

and papers, and compile simple trees of

Agnatha, Placodermi, Chondrichthyes,

Acanthodii and Osteichthyes as accepted

in 1960, 1970, 1980, 1990 and 2000.

2 Read about the typical Devonian ﬁ shes of

either the Orcadian Basin in Scotland or

Miguasha in Canada, and attempt to

reconstruct a food web (see p. 88): what

eats what?

3 How has the discovery of seven- and eight-

ﬁ ngered tetrapod fossils from the Late

Devonian changed our views about the

development of ﬁ ngers and toes? Read

about older and newer views on develop-

ment (embryology) of limb buds and

digits, and ﬁ nd out about the Hox genes

(see p. 148).

4 How did global environments change

through the Carboniferous and Permian,

and how did this affect tetrapod

evolution?

5 How did mammal-like characters appear

in the synapsids of the Permian and Trias-

sic? Draw up a simple cladogram of 10

key synapsid genera, and mark on the

acquisition of key apomorphies.

Further reading

Armstrong, H.A. & Brasier, M. 2004. Microfossils, 2nd

edn. Blackwell, Oxford. (Chapter on conodonts.)

Benton, M.J. 1991. The Reign of the Reptiles. Crescent,

New York.

Benton, M.J. 2003. When Life Nearly Died. Norton,

New York.

Benton, M.J. 2005. Vertebrate Paleontology, 3rd edn.

Blackwell, Oxford.

Carroll, R.L. 1987. Vertebrate Paleontology and Evolu-

tion. Freeman, San Francisco.

Clack, J.A. 2002. Gaining Ground: The Origin and

Evolution of Tetrapods. Indiana University Press,

Bloomington, IN.

Cracraft, J. & Donoghue, M.J. 2004. Assembling the

Tree of Life. Oxford University Press, New York.

Gould, S.J. (ed.) 2001. The Book of Life. Norton, New

York.

Long, J. 1996. The Rise of Fishes. Johns Hopkins

University Press, Baltimore.

Shubin, N.H. 2008. Your Inner Fish. Pantheon, New

York.

Zimmer, C. 1999. At the Water’s Edge. Touchstone,

New York.

References

Aldridge, R.J., Briggs, D.E.G., Smith, M.P. et al. 1993a.

The anatomy of conodonts. Philosophical Transac-

tions of the Royal Society of London B 340,

405–21.

Aldridge, R.J., Jeppsson, L. & Dorning, K.J. 1993b.

Early Silurian oceanic episodes and events. Journal

of the Geological Society of London 150, 501–

13.

Alexander, R.McN. 1975. The Chordates. Cambridge

University Press, Cambridge.

Armstrong, H.A. & Brasier, M. 2004. Microfossils, 2nd

edn. Blackwell, Oxford.

Carroll, R.L. 1987. Vertebrate Paleontology and Evolu-

tion. Freeman, San Francisco.

Carroll, R.L. & Lindsay, W. 1985. The cranial anatomy

of the primitive reptile Procolophon. Canadian

Journal of Earth Sciences 22, 1571–87.

Coates, M.I., Jeffery, J.E. & Ruta, M. 2002. Fins to

limbs: what the fossils say. Evolution and Develop-

ment 4, 390–401.

Daeschler, E.B., Shubin, N.H. & Jenkins Jr., F.A. 2006.

A Devonian tetrapod-like ﬁ sh and the evolution

of the tetrapod body plan. Nature 440, 757–

63.

Donoghue, P.C.J. & Purnell, M.A. 2005. Genome dupli-

cation, extinction and vertebrate evolution. Trends

in Ecology and Evolution 20, 312–19.

Furlong, R.F. & Holland, P.W.H. 2004. Polyploidy in

vertebrate ancestry: Ohno and beyond. Biological

Journal of the Linnean Society 82, 425–30.

Gaffney, E.S. & Meeker, L.J. 1983. Skull morphology

of the oldest turtles: a preliminary description of

Proganochelys quenstedti. Journal of Vertebrate

Paleontology 3, 25–8.

452 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Gagnier, P.-Y. 1993. Sacabambaspis janvieri, vertébré

Ordovicien de Bolivie: 1. analyse morphologique.

Annales de Paléontologie 79, 19–69.

Grande, L. 1988. A well preserved paracanthopterygian

ﬁ sh (Teleostei) from freshwater lower Paleocene

deposits of Montana. Journal of Vertebrate Paleon-

tology 8, 117–30.

Gregory, W.K. 1951/1957. Evolution Emerging, Vols 1

and 2. Macmillan, New York.

Hou, X.-G., Aldridge, R.J., Siveter, D.J. et al. 2002.

New evidence on the anatomy and phylogeny of the

earliest vertebrates. Proceedings of the Royal Society

B 269, 1865–9.

Jenkins Jr., F.A., 1971. The postcranial skeleton of

African cynodonts. Bulletin of the Peabody Museum

of Natural History 36, 1–216.

Jenkins Jr., F.A. & Parrington, F.R. 1976. The postcra-

nial skeletons of the Triassic mammals Eozostrodon,

Megazostrodon and Erythrotherium. Philosophical

Transactions of the Royal Society B 173, 387–431.

Lee, M.S.Y., Reeder, T.W., Slowinski, J.B. & Lawson,

R. 2004. Resolving reptile relationships: molecular

and morphological markers. In J. Cracraft & M.J.

Donoghue (eds) Assembling the Tree of Life. Oxford

University Press, Oxford, pp. 451–67.

Moy-Thomas, J.A. & Miles, R.S. 1971. Palaeozoic

Fishes, 2nd edn. Chapman and Hall, London.

Shu, D.-G., Luo, H.L., Conway Morris, S. et al. 1999.

Lower Cambrian vertebrates from South China.

Nature 402, 42–6.

Shubin, N.H., Daeschler, E.B. & Jenkins Jr., F.A. 2006.

The pectoral ﬁ n of Tiktaalik roseae and the origin of

the tetrapod limb. Nature 440, 764–70.

Sweet, W.C. & Donoghue, P.C.J. 2001. Conodonts:

past, present and future. Journal of Paleontology 75,

1174–84.

Trewin, N.H. 1986. Palaeoecology and sedimentology

of the Achanarras ﬁ sh bed of the Middle Old Red

Sandstone, Scotland. Transactions of the Royal

Society of Edinburgh: Earth Sciences 77, 21–46.

Chapter 17

Dinosaurs and mammals

Key points

• After the end-Permian mass extinction event 251 Ma, diapsid reptiles diversiﬁ ed in the

Triassic.

• Dinosaurs were a hugely successful group for 160 myr of the Mesozoic.

• Pterosaurs were key Mesozoic ﬂ yers, and the most important marine reptiles were the

plesiosaurs and ichthyosaurs.

• Birds evolved from dinosaurs, and radiated particularly during the Tertiary.

• The ﬁ rst mammals were small insect eaters of the latest Triassic; the mammals achieved

great diversity and abundance only after the extinction of the dinosaurs.

• Molecular and paleontological evidence show that modern mammals radiated in the

Late Jurassic and Early Cretaceous, and that some basic splits were geographic – with

major clades separated in South America, Africa, Australasia and the northern

hemisphere.

• Humans arose 6–8 Ma, and fossil evidence points to repeated human migrations out of

Africa.

The dinosaur’s eloquent lesson is that if some bigness is good, an overabundance of

bigness is not necessarily better.

Eric Johnston, President of the US Chamber of Commerce (1896–1963)

454 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

True; not true. Dinosaurs were big – and some

were very big. To politicians, the word “dino-

saur” is often a term of abuse, hurled at their

enemies to characterize them as old-fashioned,

over-blown, past it. Paleontologists (and 4-

year-old kids) know better, since dinosaurs

were of course one of the most successful

animal groups of all time. Nonetheless, they

were certainly big, and yet in their day large

size was clearly a great advantage for them:

after all, dinosaurs and mammals, the subject

of this chapter, both arose at the same time,

in the Late Triassic, and yet dinosaurs

somehow dominated ecosystems in terms of

diversity and size, and kept our hairy little

ancestors on the fringes.

The tetrapods moved onto land some

380 Ma in the Devonian, and they diversiﬁ ed

and occupied more and more ecospace

through the Carboniferous and Permian (see

pp. 442–52). The most successful tetrapod

clade, the Amniota, became most diverse by

the end of the Paleozoic, dominating many

terrestrial habitats. Two amniote groups in

particular rose to prominence. First were the

synapsids, which dominated Permian lands

but were then hit very hard by the end-Permian

mass extinction (see p. 170). The synapsids

recovered and gave rise to the mammals in the

Late Triassic. The second major amniote

group, the diapsids, were minor components

of Permian ecosystems, but they diversiﬁ ed in

the Triassic, giving rise to the dinosaurs in the

Late Triassic. Perhaps the devastating end-

Permian mass extinction gave diapsids, and

especially the dinosaurs, their chance to

diversify.

In this chapter, we take the evolution of

vertebrates forward into the Mesozoic and

Cenozoic. We explore ﬁ rst the rise of the diap-

sids, and especially the dinosaurs and their

descendants, the birds. Then, we look at how

the modest evolution of mammals through

the Mesozoic set the scene for their explosive

radiation at the beginning of the Cenozoic,

after the dinosaurs had gone.

DINOSAURS AND THEIR KIN

The diapsids take over

The diapsids (see p. 447) were initially small

to medium-sized carnivores that never matched

the abundance of the synapsids in the Car-

boniferous or Permian. Things began to

change during the Triassic, perhaps as a result

of the end-Permian extinction event, which

had such a devastating effect on therapsid

communities. Small and large meat eaters

such as Erythrosuchus (Fig. 17.1a) appeared,

one of the ﬁ rst of the archosaurs, a group that

was later to include the dinosaurs, pterosaurs,

crocodilians and birds. Archosaurs are

characterized by an additional skull opening

between the orbit and the naris, termed

the antorbital fenestra, whose function is

unclear.

During the Triassic, diapsids diversiﬁ ed

widely, some on land and some in the sea.

Some archosaurs became large carnivores,

others became specialized ﬁ sh eaters, others

adopted a specialized grubbing herbivorous

lifestyle, yet others were small, two-limbed,

fast-moving insectivores (crocodilians and

dinosaurs), and some became proﬁ cient ﬂ yers

(pterosaurs). It took another extinction event,

near the beginning of the Late Triassic (about

220 Ma) to set the new age of diapsids fully

in motion. Most of the synapsids died out

then, as did various basal archosaur groups.

Many new kinds of land tetrapods then radi-

ated: the dinosaurs, pterosaurs, crocodilians

and lizard ancestors, as well as the turtles,

modern amphibians and true mammals.

The pterosaurs were proﬁ cient ﬂ apping

ﬂ yers (Fig. 17.1b), with a lightweight

body, narrow hatchet-shaped skull and a long

narrow wing supported on a spectacularly

elongated fourth ﬁ nger of the hand. The bones

of the arm and ﬁ nger supported a tough ﬂ ex-

ible membrane that could fold away when the

animal was at rest, and stretch out for ﬂ ight.

Pterosaurs were covered with hair, and were

almost certainly endothermic. Some later

pterosaurs were much larger than any known

bird, such as Pteranodon with a wingspan of

5–8 m, and Quetzalcoatlus with a wingspan

of 11–15 m. Most pterosaurs fed on ﬁ shes

caught in coastal seas, but others were

insectivorous.

Early crocodilians were largely terrestrial

in habits, walked on all fours and had an

extensive armor of bony plates (Fig. 17.1c).

Crocodilians were more diverse and abundant

during the Jurassic and Cretaceous than they

are now. Some even became fully marine in

adaptations, to the extent of having paddles

instead of hands and feet, and a deep tail ﬁ n

DINOSAURS AND MAMMALS 455

(a)

(b)

(c)

Figure 17.1 Archosaurs: (a) skull of the Early Triassic archosaur Erythrosuchus (×0.1); (b) the Late

Jurassic pterosaur Rhamphorhynchus, showing the elongated wing ﬁ nger on each side, and the long tail

with its terminal “sail” made from skin (×0.3); and (c) the Late Jurassic crocodilian Crocodilemus,

showing the skeleton and armor covering (×0.2). (Courtesy of David Unwin and Danny Grange.)

456 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

to speed their swimming. The modern croco-

dilians – crocodiles, alligators and gavials – all

arose in the Late Cretaceous.

The second major diapsid clade, the lepi-

dosaurs, represented today by lizards and

snakes, diversiﬁ ed in the Late Triassic. The

key forms then were sphenodontids – snub-

nosed, lizard-sized animals (Fig. 17.2a) that

fed on plants and insects. The group dwindled

after the Jurassic, except for a single living

representative, Sphenodon, the tuatara of

New Zealand, a famous “living fossil”. The

ﬁ rst true lizards are known from the Mid and

Late Jurassic (Fig. 17.2b), and they show

characteristic mobility of the skull: the bar

beneath the lower temporal opening is broken,

the quadrate is mobile, and the snout portion

of the skull can tilt up and down (Fig. 17.2c).

This process of loosening of the skull was

taken even further in the snakes, a group

known ﬁ rst in the Early Cretaceous. Snakes

have such mobile skulls that they can open

their jaws to swallow prey animals that

are several times the diameter of the head

(Fig. 17.2d).

The age of dinosaurs

Dinosaurs were the most important of the

new diapsid groups of the Triassic, both in

terms of their abundance and diversity, and in

terms of the vast size reached by some of

them. The ﬁ rst dinosaurs were modest-sized

bipedal carnivores. After the Late Triassic

extinction, a new group of herbivorous dino-

saurs, the sauropodomorphs, radiated dra-

matically, some like Plateosaurus (Fig. 17.3a)

reaching a length of 5–10 m during the Late

Triassic. Later sauropodomorphs were mainly

large and very large animals; some of them,

such as Brachiosaurus (Fig. 17.3b) reaching

lengths of 23 m or more and heights of 12 m.

These giant dinosaurs pose fascinating bio-

logical problems (Box 17.1).

The theropods include all the carnivorous

dinosaurs, and in the Jurassic and Cretaceous

the group diversiﬁ ed to include many special-

ized small and large forms. Deinonychus (Fig.

17.5a) was human-sized, but immensely agile

and intelligent (it had a bird-sized brain). Its

key feature was a huge claw on its hindfoot,

which it almost certainly used to slash at prey

animals. Tyrannosaurus (Fig. 17.5b) is famous

as probably the largest land predator of all

time, reaching a body length of 14 m, and

having a gape of nearly 1 m. The theropods

and sauropodomorphs share the primitive

reptilian hip pattern, in which the two lower

elements point in opposite directions, the

pubis forwards and the ischium backwards

(Fig. 17.5b). They also share derived charac-

thyroid fenestra

10 mm

(a)

(b)

(c)

(d)

streptostylic quadrate

pterygoideus muscle

fang

venom gland

Figure 17.2 Lepidosaurs: (a) the Late Triassic

sphenodontid Planocephalosaurus; (b) the Late

Jurassic lizard Ardeosaurus; and (c, d) skulls of a

modern lizard (c) and snake (d), showing the

points of mobility that permit wide jaw opening.

(a, based on Fraser & Walkden 1984; b, based

on Estes 1983.)

DINOSAURS AND MAMMALS 457

and these are termed the Ornithischia, all of

which were herbivores. Two groups of

armored ornithischians are the stegosaurs and

the ankylosaurs. Stegosaurus (Fig. 17.6a) has

a row of bony plates along the middle of its

back that may have had a temperature control

or display function. Euoplocephalus (Fig.

17.6b) is a massive, tank-like animal with a

solid armor of small plates of bone set in the

skin over its back, tail, neck and skull: it even

had a bony eyelid. The tail club was a useful

defensive weapon that it used to whack threat-

ening predators such as Tyrannosaurus.

Most ornithischians were ornithopods,

bipedal forms, initially small, but later often

large. In the Late Cretaceous, the hadrosaurs

were successful fast-moving plant eaters.

Many of them have bizarre crests on top of

their heads that may have been used for

species-speciﬁ c signaling, and their duck-

billed jaws are lined by multiple rows of

grinding teeth (Fig. 17.7). Close relatives of

the ornithopods were the ceratopsians (“horn-

faces”), like Centrosaurus (Fig. 17.6c), which

had a single, long nose-horn and a great bony

frill over the neck.

There has been a continuing debate about

whether the dinosaurs were warm-blooded

(endothermic) or not. Evidence for warm-

bloodedness is strongest for the small active

predators like Deinonychus that might have

required the added stamina and speed.

However, endothermy is costly in terms of the

extra food required as fuel, and it is not clear

whether the larger dinosaurs could have eaten

fast enough. Indeed, larger dinosaurs would

have maintained a fairly constant core body

temperature simply because of their size,

whether they were endothermic or not.

Dinosaur reproductive habits have also

come under scrutiny recently. Discoveries of

eggs and nests in North America and Mongo-

lia have shown that many dinosaurs practiced

parental care. They laid their eggs in earth

nests scooped in the soil, and returned to feed

the young when they hatched out. Some of

the most spectacular ﬁ nds are unhatched eggs

with the tiny bones of the dinosaur embryos

still inside (Box 17.2).

Dragons of the deeps

During the Mesozoic, several reptile

groups became key marine predators. The

(a)

(b)

Figure 17.3 Sauropodomorph dinosaurs: (a) the

Late Triassic prosauropod Plateosaurus; and

(b) the Late Jurassic sauropod Brachiosaurus.

(Courtesy of David Weishampel.)

ters of the skull and limbs that show they

form a clade, the Saurischia.

All other dinosaurs share a unique hip

pattern in which the pubis has swung back

and runs parallel to the ischium (Fig. 17.6a),