2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 126-4
Presentation Time: 9:00 AM-6:30 PM


ZIMMERMAN, Alexander N., Department of Geological Sciences, Indiana University, 1001 East 10th Street, Bloomington, IN 47405-1405, BROWN, Lewis M., Department of Geology and Physics, Lake Superior State University, 650 W. Easterday Ave, Sault Ste. Marie, MI 49783 and REXROAD, Carl B., Indiana Geological Survey, 611 N. Walnut Grove Ave, Bloomington, IN 47405, alexzimm@indiana.edu

We developed the Neognathodus Index (N.I.) as a primary biostratigraphic indicator for the Desmoinesian. It is based on the gradual evolutionary morphotypic variations, or morphotypes, of Pa elements of named Neognathodus species. The morphotypic variations define bell curves that shifted evolutionarily through time, generally in a regular progression so that the peak of the curve shifted gradually from the distinct morphology of one named species to the next, suggesting overall genetic drift leading to the simplification of the platform configuration. The morphotypes are numbered in sequence starting with bassleri as #1, bothrops as #2 and so on. The average of the morphotypes portrays the position of the composite on the continuum. The species name assigned is the one closest to the NI, or average.

NI’s for the Illinois Basin show morphologic drift in Indiana from the Perth Limestone Member of the Staunton Formation (NI = 2.03) to the Velpen Limestone and Mecca Quarry Shale Members of the Linton Formation (NI = 5.04) followed by stability through the Alum Cave (NI = 4.83). Further morphologic drift then occurred through the Providence Limestone Member of the Dugger Formation (NI = 5.43) and continued through the West Franklin Limestone Member of the Shelburn Formation (NI = 6.32) until the extinction of Neognathodus at the end of Desmoinesian. Localized Neognathodus Index variations are due to morphologic shifts and differential evolution in response to localized environmental conditions.

Examination of the NI differences between Illinois, Kentucky, and Indiana reflect similar morphologic advancement from Perth to Velpen and Mecca Quarry in Indiana (NI= 2.03 – 5.04) and from Seville to Brereton in Illinois and Kentucky (NI = 2.80 – 5.00). Post-Brereton in Illinois and Kentucky (NI = 5.00 – 5.65) and post-Providence in Indiana (NI = 5.43 – 6.32) saw progressive evolutionary advancement but it was much more pronounced in Indiana.