Evolution of Exogyra in the Late Cretaceous of the southeastern United States

Morphological variation in three species of the ostreid pelecypod Exogyra-E. ponderosa Roemer, E. cancellata Stephenson and E. costata Say, occurring in the Late Cretaceous (late Campanian-early Maestrichtian) marine sediments of the southeastern states, Tennessee to North Carolina, was investigated by means of biometric techniques and interpreted in terms of its relationship to the evolution of the species and the nature of the sediments. Application of multivariate statistical distance functions illustrates that the main differentiation between the species, as well as between the conspecific populations, follows the stratigraphic sequence. Geographic variation within one stratigraphic zone is relatively small. An 'average' rate of speciation of Exogyra in the Cretaceous of the Gulf and Atlantic Coastal Plains was calculated as 1 species in 3-4 m.y.

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