Abstract

A recent taxonomic revision of the prolific pelecypod faunas occurring in the type region of the Chemung Stage has provided basic data for biostratigraphic interpretations in Upper Devonian marine rocks of the Appalachian region. Application of the concepts of population systematics to the Chemung pelecypods has reduced by 70 per cent the number of species names formerly applied to the fauna. The taxonomic revision has therefore greatly simplified the practical problem of identifying pelecypod species in these rocks. Many species have restricted time ranges. More than one third of the species have been found in only a single stage, and more than two thirds are known from only one or two of the four stages of the Appalachian Upper Devonian. In addition, a quantitative comparison of species abundance within the Chemung Stage reveals significant differences in species distribution at different Stratigraphic horizons. The degree of Stratigraphic restriction of the Chemung pelecypods compares favorably with that found in other stratigraphically valuable fossil groups. This indicates that these abundant and widely distributed pelecypod faunas may prove useful in correlating Upper Devonian strata throughout the Appalachian region.

Most Chemung pelecypod species show little or no evidence of evolutionary change throughout their range of occurrence in the Appalachian Devonian. This suggests that the Stratigraphic restriction observed in many species was ultimately caused by environmental variation and not by evolutionary modification. A comparison of the observed time ranges of the Chemung species with ranges reported for late Cenozoic pelecypods indicates that the Chemung Stage represented too short a time (approximately 5 m. y.) to encompass significant evolutionary change in the pelecypod faunas.

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