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This contribution concludes a general revision of genera of pectiniform bivalves (pectinoids) of the world's marine rocks near the Permo-Triassic boundary. The study includes the morphology, taxonomy, and distribution of some 20 families and 30 genera, several of them new (for new taxa see Contents). It is based on the best material available anywhere, mainly from the western United States. The known diversity of these early pectinoids declined gradually from about 23 genera in the Guadalupian (M. Permian) to a minimum of 5 in the Griesbachian (L. Triassic). After the biological decline, they did not recover substantial diversity until late Triassic (Norian) time.
So they shared the great mass extinction with most other groups of marine invertebrates. The crisis extended over some tens of millions of years and was slow, rather than catastrophic. This diversity pattern agrees well with carbon isotope ratios that seem to reflect a depressed oceanic productivity in early Triassic time.
