Paleozoic and post-Paleozoic marine faunas are strikingly different in composition. Paleozoic marine gastropods may be divided into archaic and modern groups based on taxonomic composition, ecological role, and morphology. Paleozoic assemblages were dominated by pleurotomariids (Eotomariidae and Phymatopleuridae), the Pseudozygopleuridae, and, to a lesser extent, the Euomphalidae, while Triassic assemblages were dominated by the Trochiina, Amberleyacea, and new groups of Loxonematoidea and Pleurotomariina. Several new groups of caenogastropods appeared as well. Yet the importance of the end-Permian mass extinction in generating these changes has been questioned. As part of a study of the diversity history of upper Paleozoic and Triassic gastropods, to test the extent to which taxonomic and morphologic trends established in the late Paleozoic are continued after the extinction, and to determine the patterns of selectivity operating during the extinction, I assembled generic and morphologic diversity data for 396 genera in 75 families from the Famennian through the Norian stages. Within this interval, gastropod genera underwent an adaptive radiation during the Visean and Namurian, largely of pleurotomariids, a subsequent period of dynamic stability through the Leonardian, a broad-based decline during the end-Permian mass extinction, and a two-phase post-extinction rebound during the Triassic. The patterns of generic diversity within superfamily-level clades were analyzed using Q-mode factor analysis and detrended correspondence analysis.The results demonstrate that taxonomic affinity, previous clade history, generic age, and gross morphology did not determine survival probability of genera during the end-Permian extinction, with the exception of the bellerophontids, nor did increasing diversity within clades or expansion of particular morphologies prior to the extinction facilitate survival during the extinction or success after it. The pleurotomariids diversified during the Lower Permian, but were heavily hit by the extinction. Similarly, trochiform and turriculate morphologies, among those which Vermeij (1987) has identified as having increased predation resistance, were expanding in the late Paleozoic, but suffered similar extinction rates to other nondiversifying clades. Survival was a consequence of broad geographic and environmental distribution, as was the case during background periods.
Estimating the normal background rate of species extinction
