The wealth of paleontological data we now have shows us a dynamic history of animal radiations and extinctions through time (Chapter 23). The graph shown in Fig. 44.41 is a tabulation of fossil occurrences through time, compiled from paleontological literature by American paleontologist Jack Sepkoski since the 1970s. In this figure, we see that despite the burst of body plan evolution recorded by Cambrian fossils, there were still relatively few species at the end of the Cambrian Period.

The following Ordovician Period (485–443 million years ago) was a time of renewed animal diversification, especially the evolution of heavily skeletonized animals in the world’s oceans. The number of genera recorded by fossils increased fivefold, suggesting that species diversity might have increased by an order of magnitude (most genera contain multiple species). The Ordovician radiation established a marine ecosystem that persisted for more than 200 million years. Interestingly, however, if you had walked along an Ordovician beach, the shells washing about your feet would have been far different from the ones you see today. The dominant shells were those of brachiopods, not clams. Broken corals in the surf were the skeletons of now-extinct cnidarians only distantly related to modern reef-forming corals. And arthropod shells, molted during growth, were those of now-extinct trilobites, not lobsters or crabs. Why was the Paleozoic world so distinct from our own?

Another look at Fig. 44.41 provides the answer. At the end of the Permian Period, 252 million years ago, environmental catastrophe eliminated most genera in the oceans. Paleozoic coral-like cnidarians become extinct, as did the trilobites. Brachiopods survived as a group, but most species disappeared. As noted in Chapter 23, the trigger for this devastation was massive volcanism that unleashed global warming, ocean acidification, and oxygen loss from subsurface oceans.

As ecosystems recovered from this mass extinction, they came to be dominated by new groups descended from survivors of the extinction: Bivalves and gastropods diversified; new groups of arthropods radiated, including the ancestors of the crabs and shrimps we see today; and surviving cnidarians evolved a new capacity to make skeletons of calcium carbonate, resulting in the corals that build modern reefs. In short, mass extinction reset the course of evolution, as it did four other times during the past 500 million years (Fig. 44.41).