of both more and larger bipedal carnivores and the first gigantism among early sauropods (such as Plateosaurus of the Upper Triassic).

Early to Middle Jurassic Continuation of the trend of the latest Triassic, where saurischian bipeds and quadrupeds dominated faunas, characterized this phase. During this time, however, the ornithischians, while remaining small in size and few in number, diversified into the major stocks that would ultimately dominate dinosaur diversity later in time, in the Cretaceous. These stocks included the appearance of heavily armored forms (such as the thyreophorans). These were quadrupeds and included the first stegosaurs of the middle Jurassic. A second group was the unarmored neornischians (which included ornithopods, hypsilophondontids, iguanodons, and duck bills), marginocephalians (the ceratopsians, which did not appear until the Cretaceous), and bone-headed pachycephalans. But it was the sauropods that were most evident in numbers. They split into two groups in the latest Triassic, the prosauropods and true sauropods, and in the early and middle Jurassic the prosauropods were far more diverse than sauropods but went extinct in middle Jurassic time, leading to a vast radiation of sauropods into the late Jurassic.

Finally, the bipedal saurischians also showed diversity and success in the lower and middle Jurassic. In the latest Triassic time they split into two groups (the ceratosaurs and tetanurans). The ceratosaurs dominated the early Jurassic, but by middle Jurassic time the tetanurans increased in number at the expense of the ceratosaurs. They too split into two groups, the ceratosauroids and the coelophysids. The latter group eventually produced the most famous dinosaur of all, the late Cretaceous Tyrannosaurus rex, although its middle Jurassic members were considerably smaller. Their most important development in the Jurassic was evolution of the stock that gave rise to birds.

Upper Jurassic This was the time of the giants. The largest sauropods came from late Jurassic rocks, and their dominance continued into the early part of the Cretaceous. Keeping pace with this large size were the saurischian carnivores, with giants such as Allosaurus typical. Thus, the most notable aspect of this interval was the appearance of sizes far larger than in the early and middle Jurassic. This was not only among the saurischians. During the late Jurassic the armored ornithischians also increased in size, most notably among the heavily ar-

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