EFFECTS OF EUSTATIC SEALEVEL LOWERING ON TERRESTRIAL AND SHALLOW MARINE BIOTAS

Shallow inland seas create equable climates for nearby land areas. Their withdrawal at times such as the Permian has led to increased seasonality and profound biotic changes on continents. Paleontologists have also correlated extinctions of endemic shallow marine life with the demise of epeiric seas, but the proximal causation of such crises has remained unclear because accessible habitats persisted along nearby shallow shelves. A nutritional change is unlikely to have caused extinction of marine invertebrates, which as endotherms require little food, and species/area studies have shown that reduction of seafloor area should also have had little effect. On the other hand, elimination of the highly seasonal habitat of epeiric marine organisms should have wreaked havoc on them. The key physical factor here is the difference in heat capacity between land and water. Not only does an epeiric sea stabilize the regional terrestrial climate, but also, in complementary fashion, a seasonal terrestrial climate elevates the thermal seasonality of an epeiric sea relative to that of marine waters bordering nearby open shelves. It might seem that epeiric species would have encountered an equable, and therefore hospitable, marine climate when forced into marginal shelf areas during a eustatic sealevel decline, and yet reproduction should have been impossible for many such immigrants. Spawning of many shallow-water marine species is triggered when seasonally rising or falling water temperatures reach a critical level. Thus, eustatic sea level lowering should have caused heavy extinction within previously “perched” faunas by eliminating the reproductive trigger for numerous species. In the thermally stable central tropics, where many species presumably spawned year-round, a eustatic drop should have posed a different problem: species adapted to the perennial warmth of epeiric tropical seas would have been forced into cooler, physiologically intolerable waters. This problem must have been especially severe on the west coasts of continents, where upwelling and equatorward transport of water maintain relatively low open-ocean temperatures. Presumably, the thermal consequences of obligatory biogeographic shifts caused much extinction of marine life during intervals such as the Late Ordovician and Late Permian.