Evolution, 4th Edition

456 CHAPTER 17

The other major diapsid group, the archosaurs, includes the most

spectacular and diverse of the Mesozoic amniotes. Most of the late

Permian and Triassic archosaurs were fairly generalized predators

1 m or so in length (FIGURE 17.25B). From this generalized body

plan, numerous specialized forms evolved. Among the most highly

modified archosaurs are the pterosaurs, one of the three major ver-

tebrate groups that evolved powered flight. The wing consisted of a

membrane extending to the body from the rear edge of a greatly

elongated fourth finger (FIGURE 17.25C). The pterosaurs diversified

greatly: one, with a wingspan of 11 m (36 ft), was the largest flying

vertebrate known, while others were as small as sparrows.

Dinosaurs evolved from archosaurs related to the one pictured in Fig-

ure 17.25B. Dinosaurs are not simply any old large, extinct reptiles, but

members of the orders Saurischia and Ornithischia, which differ in the

form of the pelvis. Both orders included bipedal forms and quadrupeds

that were derived from bipedal ancestors. Both orders arose in the Trias-

sic and became diverse in the Jurassic.

More than 39 families, about 550 genera, and well over 1000 species of

dinosaurs are recognized (FIGURE 17.26). The Ornithischia—herbivores

with specialized, sometimes very numerous, teeth—included the well-

known stegosaurs, with dorsal plates that probably served for thermo-

regulation, and the ceratopsians (horned dinosaurs), of which Tr ic e rat o p s

is the most widely known. Among the Saurischia, the sauropods, herbi-

vores with small heads and long necks, include the largest animals that

have ever lived on land, such as Apatosaurus (= Brontosaurus); Brachio-

saurus, which weighed more than 80,000 kg; and Argentinosaurus, which

reached about 40 m in length. Saurischians also included carnivorous, bipedal

theropods, such as Velociraptor, the renowned Tyrannosaurus rex (late Cretaceous),

which stood 15 m high and weighed about 7000 kg, and many smaller theropods.

All dinosaurs became extinct at the end of the Cretaceous except for a single group of the-

ropods. They radiated extensively in the Cenozoic and today include about 10,000 species

that we know as the birds. Aside from birds, the only living archosaurs are the 22

species of crocodilians.

The late Paleozoic also included the synapsids (see Figure 17.24), with a single

temporal opening. They gave rise to the therapsids, sometimes called “mammal-

like reptiles,” which flourished until the middle Jurassic, and gave rise in the late

Triassic and early Jurassic to forms that are almost fully mammalian in their features

(see Chapter 20). Many clades of mammals, including more than 300 known gen-

era, arose and became extinct in the Jurassic and especially the Cretaceous (FIGURE

17. 27). Although most were small in size, many were ecologically and morphologi-
    cally specialized, and convergent with living mammals that evolved similar traits
    independently (FIGURE 17.28) [56]. Only three of the many Mesozoic mammal

Futuyma Kirkpatrick Evolution, 4e

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(A) An ichthyosaur

(B) Lagosuchus talampayensis

Tail

Elongated

fourth ngers

Tail

Elongated

fourth ngers

(C) Pterodactylus

FIGURE 17.25 Some Mesozoic reptiles. (A) A marine

ichthyosaur (Greek, “fish lizard”), convergent in form with

sharks and porpoises. (B) Lagosuchus, a Triassic thecodont

archosaur (Greek, “ruling lizard”), showing the body form

of the stem group from which dinosaurs evolved. (C) A

pterosaur (Greek, “wing lizard”).

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