Changes In Mississippian Crinoid Diversity during Deposition of the St. Louis Limestone (Meramecian) In the Illinois Basin

A significant transition in the macroevolution of crinoids occurred during the Mississippian Period. Following the Frasnian-Famennian extinction event, the diversity of crinoid taxa changed substantially from faunas dominated by camerates and primitive cladids to those dominated by advanced cladids. Much of this change took place during the Meramecian, which includes the St. Louis Limestone.

A preliminary analysis of the crinoid fauna of the St. Louis Limestone indicates co-dominance by camerates and advanced cladids. Yet the overlying Ste. Genevieve Limestone shows a clear dominance by advanced cladids. Analysis of the ranges of those two taxa between the Late Osagean and the Early Chesterian reveals a shift in the genera.

Camerates not only decrease in number of genera but also change in the distribution of families. Advanced cladids also have a shift in genera but are not as consistent with a change in the distribution of families. Although not as obvious with camerates as with advanced cladids, both groups incurred a decrease in diversity in the St. Louis Limestone compared to the preceding Salem Limestone and the subsequent Ste. Genevieve Limestone. These decreases in diversity probably reflect both macroevolutionary changes and environmental limitation during St. Louis time

Further study will be facilitated by two developments that have occurred making a study of the St. Louis Limestone crinoid macroevolution more plausible. The boundary between the St. Louis and Ste. Genevieve formations is defined as the top of the Lost River Chert Bed. In addition, a collection of crinoids collected from the St. Louis Limestone in Putnam County, Indiana, is in the Indiana State Museum and is currently under study. This will serve as a reference for separating out Ste. Genevieve-age taxa erroneously reported from the St. Louis Limestone.