PTEROTOCRINUS WING PLATES A RELIABLE BIOSTRATIGRAPHIC TOOL FOR THE ELVIRAN STAGE, CHESTERIAN SERIES, ILLINOIS BASIN

Pterotocrinus is a unique camerate crinoid genus which includes five distinctive tegumen plates called wing plates. Pterotocrinus, an index fossil for Chesterian limestones and shales, ranges from the Renault to the Grove Church Formations. This crinoid has a long documented history of its potential as a biostrat-marker within Chesterian strata. The “winged” crinoid has also been questioned by some researchers as not being useful as a biostratigraphic tool in the middle part of the Chesterian Series, Hombergian Stage. Cases have been made that the wing plates may not reflect true species. This is based on the differing morphology of the plates that occur on similar calyces in the same horizon. Also, morphological variations within named species may reflect ecophenotypic variation. Finally, there are ontogenetic variations in morphology.

After 31 years of geologic mapping in southern Illinois under the auspices of the COGEOMAP and STATEMAP cooperative USGS mapping programs, useful wing plates of Pterotocrinus have come to light in the Elviran Stage. The most useful wing plates found thus far are Pterotocrinus menardensis (Weller) from the Menard Limestone and Clore Formation, Pterotocrinus tridecibrachiatus (Gutschick) from the Kinkaid Limestone and Pterotocrinus pegasus (Gutschick) from a horizon in the basal Grove Church Formation. These wing plates all occur in the dark gray shales and gray wackestones of the Elvirian Stage.

When mapping the rocks in highly faulted terrain like that of southeastern Illinois, it is difficult to know which limestone or shale unit outcrops especially within the Elviran Stage. These dark gray, argillaceous, fossil wackestones and shales look quite similar. Therefore, the aforementioned wing plates, if found, can help in determining small fault displacements when mapping Elviran carbonate rocks.