16 August 1991
Vol 253, Issue 5021

About The Cover

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]