Permo-Triassic Extinctions: Relation to Sea-Floor Spreading

An analysis of the Permo-Triassic fauna and paleogeography on a stage-by-stage basis supports previous more general summaries by concluding that the number of families of marine invertebrates was significantly reduced (essentially halved) during the last three Permian stages (32 m.y.), and that the extinctions were not markedly selective. Shallow marine seas were reduced from a coverage of 40% of their possible distribution in the early Permian to less than 15% coverage in latest Permian, and then were expanded to 34% coverage in the lower Triassic. If this apparent reduction in shallow marine seas is due to selective erosion of late Permian sediments, there seems no way to account for it unless there was significant reduction in sea level. Alternatively, late Permian marine seas may never have been widespread, again due to lowering of sea level. The apparent late Permian marine regression is attributed to water withdrawing into a deepening ocean basin. The basaltic ocean bottom continued to sink during an unusually long period of cooling (and decreased buoyancy) of laterally spreading ocean ridges during a time of increased linking together of plates, and corresponding decreased rate of sea-floor spreading. Extinctions during the Recent are likely to result from competition and predation, often aided by habitat alteration. The effects of competition and predation are greatly enhanced if extensive habitat regions are reduced to habitat islands. With significant lowering of sea level, shallow epicontinental seas would become limited to continental margins. Geographic ranges of shallow marine species would correspondingly be reduced, and the probability of extinction therefore increased. Perhaps no extraordinary biologic process, atmospheric change, or astronomical event is necessary to account for "paleontology's outstanding dilemma."