Recovery of Benthos Versus Nekton after the End-Permian Mass Extinction Event. A Preliminary Comparison

Benthic organisms are known to have recovered much slower after the end-Permian mass extinction event than ammonoids, but hitherto the poor age-control of most Early Triassic benthic faunas prevented detailed comparisons. A new dataset that includes occurrences of bivalve and gastropod genera at the substage level reveals previously unknown details of the recovery of these most important benthic groups during the Early Triassic, which allows for a preliminary comparison with ammonoid recovery. We found a distinct increase of bivalve diversity already in the Smithian, which was chiefly due to newly evolved taxa but also includes the reappearance of Lazarus taxa. A similar trend is observed in gastropods, although there is a likely bias caused by an exceptional gastropod lagerstätte of the Sinbad Limestone Member (Utah). However, even the Sinbad gastropod fauna is dominated by newly evolved taxa. This first diversity peak of benthic organisms coincides with a diversity burst of ammonoids and conodonts, suggesting a similar cause in spite of the different order of magnitude. In contrast to ammonoids and conodonts, present data do not indicate a distinct bivalve or gastropod end-Smithian extinction. This apparent absence of extinction needs further substantiation, especially by gaining better resolved diversity time series. The Spathian saw doubling of newly evolved bivalve taxa, the reappearance of most Lazarus taxa, and the first occurrence of deep-infaunal bivalves during the Early Triassic. Several bivalve and gastropod genera with Paleozoic affinity disappeared around the Spathian-Anisian boundary. However, new field data are necessary to distinguish between progressive outcompeting or a distinct extinction event. The global trends in bivalve and gastropod diversity described herein are likely to be robust, but finer patterns may change as new data will become available.