The fossil record is rich in information on past changes in biodiversity and patterns of extinction. Echinoderms such as those on the cover (maximum test diameter 8 centimeters; arms may exceed 1 meter), Uintacrinus socialis Grinnell from Kansas (Smoky Hill Member, Niobrara Formation), are thought to have adopted a planktonic existence, and they were abundant and globally distributed in Late Cretaceous seas. The Uintacrinida order suddenly became extinct well before the end of the Cretaceous. See p. 754. [Photo by John Weinstein, courtesy of the Field Museum of Natural History]

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