CNCing Is Believing

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Science  19 Aug 2011:
Vol. 333, Issue 6045, pp. 946-947
DOI: 10.1126/science.1210771

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In 1984, paleontologist Jack Sepkoski of the University of Chicago published a now-famous paper documenting three successive waves of animal groups that dominated marine ecosystems during the past half-billion years (see the figure) (1). The earliest, the Cambrian fauna, was composed of trilobites, inarticulate brachiopods, and a host of lesser-known animals. Within 90 million years, these groups were largely replaced by the Paleozoic fauna, which included crinoids, ammonites, and articulate brachiopods. This second assemblage dominated for another 200 million years before suffering a catastrophic decline during the Permo-Triassic mass extinction about 250 million years ago. The Modern fauna, which includes familiar groups such as bony fishes, gastropod mollusks, and sea urchins, filled the void and has dominated marine ecosystems to this day.