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The Evolution of a Permian Vertebrate Chronofauna

Everett C. Olson
Evolution
Vol. 6, No. 2 (Jun., 1952), pp. 181-196
DOI: 10.2307/2405622
Stable URL: http://www.jstor.org/stable/2405622
Page Count: 16
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The Evolution of a Permian Vertebrate Chronofauna
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Abstract

An analysis of the interrelated changes of the species of a chronofauna with time and altering environmental conditions has been based upon studies of an Early Permian fauna of fish, amphibians and reptiles from beds of Clear Fork age of Texas. This chronofauna evolved with only minor influences of migration from adjacent areas. It existed on a delta that underwent modifications in topography and rainfall during the period of time studied. Four ecological subzones inhabited by the populations of the chronofauna have been recognized from physical criteria: streams, ponds, pond margins, and flood plains and divides, called uplands. The changes of the subzones and their inhabitants through the Clear Fork show the importance of considering evolution from an ecological viewpoint. Stability of certain species and changes of others with respect to habitat and/or morphology are related to biological and physical events correlated with interrelated courses followed by populations under environmental change. The general tendency for stability of a chronofauna under moderate environmental change stands out. As long as the various ecological niches are effectively occupied little evolution occurs. Conversely, changes that have a marked effect upon but one or a few species may initiate a chain of events that can alter the internal balance radically and result in rapid evolution. Moderately drastic changes of physical environment do not appear to have been met by adaptive modification but rather to have set in motion a series of events that led to the extinction of the chronofauna through elimination of several species essential to the maintenance of its internal balance.

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