A NEOGNATHODUS-BASED BIOZONATION FOR THE DESMOINESIAN SERIES (PENNSYLVANIAN) OF THE ILLINOIS BASIN
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.