Systematics and evolutionary paleoecology of crinoids from the St. Louis Limestone (Mississippian, Meramecian) of the Illinois Basin

Very little attention has been paid to crinoids of the St. Louis Limestone since the 19th century. Now that the stratigraphic boundaries of the formation are well defined and a collection of crinoids specific to the St. Louis has become available, updated systematics and evolutionary paleoecology is possible.;Previous work recognized a significant transition in the macroevolution of crinoids that occurred during the Mississippian Period. The diversity of crinoid taxa changed substantially from faunas dominated by camerates and primitive cladids to those dominated by advanced cladids. Much of that change took place during the Meramecian Stage, which includes the St. Louis Limestone. The preceding Salem Limestone contained a prevalence of camerates whereas the following superjacent Ste. Genevieve Limestone contained a more advanced cladids. Questions that are considered include: what changes in morphology of crinoids accompanied that transition; what, if any, environmental change occurred that influenced the major transition in crinoids; and what was the precise change in the distribution of crinoid taxa throughout the deposition of the St. Louis Limestone? The approach to providing a plausible description and explanation includes an evaluation of the geologic setting, a re-evaluation of crinoid systematics, and an investigation of the evolutionary paleoecology of the St. Louis Limestone crinoids.;Lithology of the St. Louis Limestone is compared to the Salem Limestone below and the Ste. Genevieve limestone above. A change in environment occurred within the St. Louis. The Salem and Ste. Genevieve were deposited in a deeper water setting, whereas St. Louis deposition took place in a low-energy, mostly lagoonal setting. This is reflected in microscopic study by the prevalence of mudstone and wackestone in the St. Louis while the Salem and St. Genevieve are granistones.;Systematic evaluation confirmations 17 species of crinoids previously assigned to the St. Louis. Three crinoid species that were identified within other formations, but not in the St. Louis, were added. One new species, Armenocrinus howelli was identified. Three species were synonymized within the St. Louis. Eight species previously listed as belonging to the St. Louis were found to be incorrectly assigned.;Crinoids had been adapting in their overall morphology prior to the Meramecian. Various changes in feeding structures, predatory defenses and mobility occurred from the Ordovician through the early Meramecian. Those adaptations allowed crinoids with the most advantageous morphology to survive adverse environmental conditions. In fact, two events took place during the Meramecian that would have impacted the survival of crinoids. Durophagous predators increased in numbers and a prolonged regression reduced habitat area during the deposition of the St. Louis. Those two factors impacted crinoids in a negative way. That evolutionary bottleneck resulted in the better adapted advanced cladids radiating, whereas the previously dominant camerates saw a substantial decrease in diversity