Abstracts Earth and Environmental Sciences

Paleontology and geology of an upper Pennsylvanian tetrapod locality from the Ada Formation: Seminole County, Oklahoma

by Richard Anthony Kissel

The paleontology, taphonomy, stratigraphy, depositional setting, and paleozoogeographic significance of an Upper Pennsylvanian tetrapod-bearing fossil locality is presented. The site, OMNH Locality V1005, is located within the Virgilian Ada Formation in southcentral Oklahoma, north of the town of Ada. The fossiliferous outcrop represents a meandering stream deposit, with the majority of the exposure consisting of fine-grained overbank (floodplain) deposits. The fossil material is found within a single horizon that stretches 11 meters along the face of the outcrop. This horizon is located within a gray mudstone unit and is 22.4 meters above the contact between the Ada Formation and the imderlying Vamoosa Formation. The bone-bearing interval is further characterized by the presence of granule-sized carbonate rock clasts and carbonized plant material. Tetrapod fossils recovered fi-om the Ada locality include the remains of at least six taxa, with remains of the pelycosaurian-grade synapsid Ophiacodon cf O. mirus constituting the majority (90 percent) of the fossil material collected and identified. Other taxa include the diadectid Diasparactus zenos, the temnospondyl ?Eryops sp., and three unidentified taxa known only from jaw fragments. An absolute minimum of five Ophiacodon individuals were recovered from the site, including the remains of both mature and immature individuals. Moreover, the material recovered from the Ada locality suggests, for the first time, the presence of size-based sexual dimorphism within the genus Ophiacodon. Linear measurements of the larger dimorph are approximately 1.5 times as great as those of the small form. No gender assignment is suggested. Of the five remaining taxa, no evidence supports the presence of more than one individual of each taxon. Except for four articulated vertebral segments, all of the fossil material collected in situ was disarticulated. The presence of articulated vertebral sequences, while all other skeletal elements are isolated and disarticulated, suggests that the vertebral column represents the last stage of the disarticulation sequence of an Ophiacodon corpse. The vertebral segments are straight and relaxed, which indicates that the corpses decomposed in water. All of the in situ bones are complete except for the anterior margin of a scapulocoracoid and the puboischiadic plate of a pelvic girdle. The bone surfaces exhibit no indication of cracking or flaking and, thus, were not exposed to prolonged periods of weathering at the surface. All fractures are indicative of post-burial breakage. The hypothesis that the bones were buried relatively quickly after the individuals' deaths is also supported by the lack of tooth marks and other evidence of scavenging. The fossil material represents a transported assemblage and the orientation of the long bones indicates that flow direction was either northwest or southeast during the time of deposition. Furthermore, the dip of several bones relative to the stratification suggests rapid deposition under flood conditions. …