PENNSYLVANIAN BRACHIOPODS: SURVIVORSHIP, POPULATION STRUCTURE, AND RESPONSE TO INFLUX OF MUD IN THE MIDCONTINENT OF NORTH AMERICA

The Coal Creek Limestone Member of the Topeka Limestone (Pennsylvanian, Virgilian) provides a unique setting for investigating the survivorship of the midcontinental brachiopods and its effects on community structure in shallow-marine settings. Three articulated brachiopod species were chosen for study on the basis of relative abundance and identifiable features: Composita subtilita, Hustedia mormoni, and Punctospirifer kentuckyensis. While the Coal Creek Member occupies the same position in its cyclothem as the well-studied Plattsmouth Limestone, Beil Limestone, and Ervine Creek Limestone Members, fine-grained terrigenous sediment in the Coal Creek is appreciably more abundant. This is expected to affect both survivorship and species diversity. The median sizes of C. subtilita and P. kentuckyensis but not H. mormoni are significantly positively correlated with the percentage of mud in the samples collected. With regression of the sea, a statistically significant decline in abundance of all three species occurred toward the top of the unit. Finally, the proportion of juvenile specimens increases significantly up the section due to higher infant mortality in response to environmental stress with shoaling water. Also examined were the size-frequency distributions and survivorship curves for the three species throughout the unit. The brachiopods in the Coal Creek produce a typical soft-substrate size-frequency pattern.