Geometric morphometric analysis of the early Mississippian conodont Gnathodus punctatus from its type area, Oklahoma, USA

The Gnathodus punctatus (Cooper 1939) conodont biozone is often used to correlate the early Mississippian Kinderhookian–Osagean boundary in the US Midcontinent. It is unclear whether G. punctatus represents a complex of multiple diagnosable species, or a single species with highly variable morphology. Therefore, recognition of the G. punctatus biozone and the boundary position can vary by worker. The issue is complicated by poor documentation of specimens from the type area in Oklahoma, which is also stratigraphically condensed. This study uses landmark-based geometric morphometrics to assess shape variation of G. punctatus from its type area to better characterize its morphology. Most of Cooper's types plot as outliers when compared with the average shape of G. punctatus and two are juveniles. This analysis finds no straightforward separation in morphospace with which to confidently diagnose new species. However, three lobe ornamentation morphotypes are statistically distinct, including two that were recognized by other workers. This suggests that lobe ornamentation measurably influences cup shape. Additional specimens from other regions are needed to clarify geographic and stratigraphic distribution of the morphotypes.