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Periodicity in Invertebrate Evolution

Norman D. Newell
Journal of Paleontology
Vol. 26, No. 3 (May, 1952), pp. 371-385
Stable URL: http://www.jstor.org/stable/1299949
Page Count: 15
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Periodicity in Invertebrate Evolution
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Abstract

Varying rates of evolution in the main groups of fossil invertebrates are estimated by means of graphs showing distributions of genera through geologic time. There are from one to three peaks in each group which appear to represent times of relatively rapid differentiation. These are followed by depressions when evolutionary activity was reduced. Several groups show similarity of pattern which suggests common causes. In the majority of groups there was a particularly marked depression in evolutionary rates in the late Paleozoic. Nearly all of the geologic periods were marked by peaks of evolutionary activity in one or more major groups, but the Ordovician, Mississippian and Jurassic periods were times of peak activity in many groups. Causes for these fluctuations undoubtedly are complex and include both biological and physical factors. Of the latter, probably the most important were the successive expansions and restrictions of shallow epicontinental seas. These always have been the chief centers of evolution of marine invertebrates and they were greatly reduced or eliminated by simultaneous emergence of the continents. It is believed that the evolutionary peaks coincide with times of rapid expansion into evolutionary niches previously vacated by extensive extinctions. Diastrophism influences the course of evolution by causing extinction, migrations and modifications in habitats, but there is no evidence that there is increased evolutionary activity during diastrophic disturbances; in fact, the converse may well be true.

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