Abstract

There were two periods of adaptive radiation of gastropods in the Upper Paleozoic, the latest culminating in the Guadalupian. The last of the well-balanced, diverse marine faunas of the Paleozoic are Guadalupian in age. There are some 100 gastropod genera distributed through 32 families in these faunas. Only 26 genera are known from the Dzhulfian and these represent quite conservative families. In addition, the Dzhulfian faunas are abnormal in that they are composed of a few species with large numbers of individuals, suggesting hypersaline conditions.

Scythian gastropod faunas are quite rare and, like the Dzhulfian, are depauperate with few species and many individuals. Ten of the 13 Scythian genera are Paleozoic in origin and represent conservative families. The earliest Mesozoic fauna which is well balanced and diverse is in the Ladinian, where there are about 106 genera in 32 families. It is significant that this fauna has more in common with the Permian than the Jurassic, with about 58 genera which belong to Paleozoic or Paleozoic-derived families. Thirty-two of these 58 genera are rather common in the Upper Paleozoic but are not found in either the Dzhulfian or the Scythian. The principal faunal turnover occurred following the Rhaetian.