USING GEOMETRIC MORPHOMETRICS TO TEST SPECIES IDENTIFICATIONS AND INTERPRETATIONS OF CONODONT EVOLUTION IN THE ILLINOIS BASIN, MIDDLE PENNSYLVANIAN

Conodont fossils are highly valuable for biostratigraphically correlating Paleozoic formations and for interpreting evolutionary change through time. Despite this well-established history, conodont species designations are prone to continual debate. Quantitative methods can provide a more objective and robust approach to identifying species and tracking evolutionary changes in this significant fossil group.

Research objectives are to use geometric morphometrics (GM), a quantitative method, to test species identifications of the conodont genus Neognathodus and to interpret Neognathodus evolution throughout the Middle Pennsylvanian in the Illinois Basin. A total of 390 specimens were analyzed, including 210 samples from seven Neognathodus species and 180 samples spanning six lithologic intervals.

GM groups of Neognathodus are congruent with some established species and incongruent with others. GM analysis of N. bassleri, N. bothrops and N. roundyi are statistically different from each other and thus congruent with established species designations. In contrast, there is no statistically significant difference between the species pairs of N. medadultimus and N. medexultimus, and N. dilatus and N. metanodosus, suggesting these GM group pairs are incongruent with established species designations.

Interpretations of Neognathodus evolution show patterns of drift and stability through the Middle Pennsylvanian in the Illinois Basin. Statistically significant differences in shape indicate morphologic drift from the Perth Limestone Member, Staunton Formation to the Velpen Limestone and Mecca Quarry Shale Members, Linton Formation. Non-statistically significant differences in shape show morphologic stability proceeds to the Alum Cave Member and into the Providence Member, Dugger Formation. Drift again occurs into the West Franklin Limestone Member of the Shelburn Formation. GM evolutionary trends are similar in direction to previous interpretations, but show much smaller changes in magnitude.

This GM analysis highlights areas of conodont taxonomy and spatial correlation that require additional study, and may ultimately lead to a highly robust and objective method to correlate Middle Pennsylvanian strata globally and to ascertain evolutionary trends in this significant chordate group.