LATE PALEOZOIC EVOLUTION OF THE CAENOGASTROPODA: LARVAL SHELL MORPHOLOGY AND IMPLICATIONS FOR THE PERMIAN/TRIASSIC MASS EXTINCTION EVENT

Abstract

Representatives of several genera of late Paleozoic Caenogastropoda have been studied based on exceptionally well-preserved material from the latest Permian of south China, the Pennsylvanian of the United States, and the Early Carboniferous of Australia. This material has well-preserved protoconchs which are relatively rare in Paleozoic gastropods. The morphology of the protoconch and especially that of planktotrophic larval shells gives crucial taxonomic and phylogenetic information. It helps to recognize convergence, which is common in Gastropoda. The larval shell morphology of the following taxa is discussed and illustrated: Platyzona (Goniasmatidae), Erwinispira n. gen. (Erwinispirinae n. subfam.), Propupaspira (Propupaspiridae), Soleniscus (Soleniscidae), Meekospiridae, and Chlorozyga (Imoglobidae). The new species Meekospira? solenisiciforma is described. The main diagnostic feature of the order Procaenogastropoda, a fluent protoconch/teleoconch transition, probably represents a preservational artifact. An abrupt transition indicating metamorphosis can be shown for several genera which have been assigned to the Procaenogastropoda. Therefore, the monophyly of the Procaenogastropoda is questioned, and the taxon is refuted. While the caenogastropods of latest Permian gastropod faunas are dominated by typical late Paleozoic genera and families, the Early Triassic is characterized by an almost complete faunal turnover of caenogastropod genera.