bipedal forms, and we are thankful that most of the obvious carnivores are relatively small, for they zip about, running rings around the other vertebrates. But soon a much larger form, 12 feet in length, strides into view, a staurikosaur, the top carnivore among the dinosaurs. We watch this animal, puzzled at how different it seems from the nondinosaurs. Despite its size, it is very active.

This dichotomy extends as well into the swampy and freshwater habitats, where untold numbers of crocodile-like phytosaurs loll on the banks. They too seem not disposed to frolic in any fashion. It is only the dinosaurs that move about with speed, grace, and purpose.

In the oceans we see a major change. Largely gone are the brachiopods, the bivalved invertebrates so dominant in Paleozoic oceans. In their place is another kind of bivalve—the more familiar clams. Few burrow. Most rest on the surface of the sea bottom, sometimes in huge numbers. Swimming above the bottom are many varieties of fish, among them a host of ammonites. This latter group just missed total extinction in the Permian extinction. From the few spared species, however, a host of new species has evolved until now. In the latter parts of the Triassic period, they are even more numerous than anytime before.

There are also coral reefs again, but like the bottom communities of invertebrates found on the sandier and muddier bottoms, the reefs are composed of an entirely new suite of corals. Gone are the tabulate

Reconstruction of a ceratite ammonite, a group found only during the Triassic. These forms evolved from the few ammonites that survived the Permian extinction, an event that decimated the cephalopods.

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