THE “AGE OF CRINOIDS”: A MISSISSIPPIAN BIODIVERSITY SPIKE COINCIDENT WITH WIDESPREAD CARBONATE RAMPS

Phanerozoic crinoids reached their highest generic richness and overall abundance during the Mississippian, which has thus been dubbed the “Age of Crinoids.” Generic richness during the Visean (175) was 260% of the stage average (67) during the Ordovician-Permian, based on Sepkoski’s (2002) compilation. The causes are from the coincidence of two factors. First, in the wake of the Late Devonian mass extinction event, the five major crinoid clades re-radiated in the Early Mississippian, particularly the camerates, which reached their Paleozoic peak (52) in the Visean. The advanced cladids also continued to radiate from their origin in the Early Devonian and reached a peak in the Visean (76) that was not exceeded again until the Moscovian (100). The simultaneous radiation of these two groups were responsible for the biodiversity spike in the Mississippian. The minor crinoids clades, disparids, primitive cladids, and flexibles, showed a similar pattern of contraction during the Late Devonian and subsequent expansion that was not as pronounced as for the camerates and advanced cladids. Second, the Late Devonian mass extinction event destroyed the extensive coral-stromatoporoid platform-edge reefs that had restricted circulation on carbonate platforms and limited the abundance of crinoids, which are stenohaline. Widespread evaporite deposits are associated with Silurian-Devonian barrier reefs indicating their effect on circulation. During the Mississippian, the resulting carbonate ramps had improved circulation producing stenohaline conditions that resulted in an abundance peak for crinoids as recorded by widespread regional encrinites. The “Age of Crinoids” ended with a major drop in sea level at the end of the Mississippian as massive glaciers formed on Gondwana.