Biotic Associations and Extinction

Abstract

THE ORIGINATION and extinction of animals and plants are continually taking place on the earth, but at least twice since the Cambrian period these processes have occurred, at least among animals, at a greatly accelerated rate. These post-Cambrian faunal revolutions occurred at about the end of the Permian and beginning of the Triassic and again at about the end of the Cretaceous and beginning of the Tertiary. In other words, these two times of relatively rapid extinction and replacement mark the boundaries of the Paleozoic and Mesozoic eras and the Mesozoic and Cenozoic eras. Thus, the study of extinction, besides having historical interest, is fundamental to stratigraphy; indeed, one might ask, What would stratigraphy be without the occurrence of extinctions among animals and plants? time interval between the two major faunal revolutions is approximately 125 million years, or the duration of the Mesozoic era, but there is no indication that such events occur at regularly spaced intervals. Some major groups-classes, orders, and families-became extinct at times other than at the end of the Permian and the end of the Cretaceous (for example, the graptolites during the early Mississippian, the conulariids in the late Triassic, and the receptaculitids in the Devonian); but such occurrences were few as compared with those of the two great faunal revolutions. At the Permian-Triassic boundary and again at the Cretaceous-Tertiary boundary the following phenomena occurred: (1) unusually large numbers of major groups became extinct; (2) other major groups rapidly declined to the state of relicts, although in some instances relict representatives remained extant for long periods of time; and (3) during and following these total and partial extinctions, large numbers of other major groups originated. One of the most puzzling things is that during these faunal revolutions, rapid extinction happened over a wide range of habitats on land and in the water. Simpson, Pittendrigh, and Tiffany (1957:774) refer to the end of the Cretaceous as The Great Dying and under this heading make the following observations: