PECTINOID BIVALVES OF THE PERMIAN-TRIASSIC CRISIS

This contribution concludes a general revision of genera of pectinifor m bivalves (pectinoids) of the world's marine rocks near the Permo-Tri assic boundary. The study includes the morphology, taxonomy, and distr ibution of some 20 families and 30 genera, several of them new (for ne w taxa see Contents). It is based on the best material available anywh ere, mainly from the western United States. The known diversity of the se early pectinoids declined gradually from about 23 genera in the Gua dalupian (M. Permian) to a minimum of 5 in the Griesbachian (L. Triass ic). After the biological decline, they did not recover substantial di versity until Late Triassic (Norian) time. So they shared the great ma ss extinction with most other groups of marine invertebrates. The cris is extended over some tens of millions of years and was slow, rather t han catastrophic. This diversity pattern agrees well with carbon isoto pe ratios that seem to reflect a depressed oceanic productivity in Ear ly Triassic time.