COMPARATIVE GEOMETRIC MORPHOMETRICS OF THE OSAGEAN (MISSISSIPPIAN) INDEX CONODONT GNATHODUS PUNCTATUS FROM THE 'PRE-WELDEN SHALE' AND WELDEN LIMESTONE OF OKLAHOMA AND THE SAN ANDRES MOUNTAINS, NEW MEXICO

Conodont biostratigraphy is based on visually deciphering morphological change through time and space. Landmark based geometric morphometric analysis is relatively new tool that has been applied to try to solve some of the complexities of conodont morphology. Gnathodus punctatus (Cooper, 1939) is commonly used as an index fossil for the basal Osagean in the Midcontinent. Previous studies considered it to be a morphologically variable species, broadly distinguished from other Gnathodus species by the basal cup shape as well as lobe ornamentation. In another view, G. punctatus may form a complex of stratigraphically significant morphotypes (Boardman et al., 2013), where a division between 'primitive' and 'advanced' marks the Kinderhookian-Osagean boundary in southwest Missouri. Large sample sizes are required to assess inter- vs. intraspecific variation and biostratigraphic utility.

A preliminary geometric morphometric analysis of G. punctatus (Hauf and McAdams, 2023) tested the idea that 'advanced' (with a concentric, radial pattern of nodes on the outer lobe) and 'primitive' (v-shaped lines of nodes) morphotypes represent unique morphologies. Using a dataset of 135 adult specimens from the Welden Limestone of Oklahoma and 14 landmarks, we concluded that G. punctatus likely represents a highly morphologically variable species, i.e., there are no distinct groups separated in morphospace. Both 'primitive' and 'advanced' forms occur within the Welden samples, however there is a notable dominance of ‘advanced’ forms. The present study expands the dataset to include specimens from the 'pre-Welden Shale' of OK (type horizon of G. punctatus) and Osagean strata in the San Andres Mountains, NM. It assesses 1) variation within topotype material, 2) variation between topotype material and that from the Welden Limestone and from New Mexico, and 3) whether there is a shift to increasing abundance of the 'advanced' morphotype in younger strata.