New Mexico Geological Society Annual Spring Meeting — Abstracts

The most complete postcranial skeleton known of the Late Pennsylvanian pelycosaurian-grade synapsid Ophiacodon navajovicus is from the Upper Pennsylvanian interval of the El Cobre Canyon Formation (Cutler Group) at Cañon del Cobre, Rio Arriba County, New Mexico. The skeleton was preserved in brownish-red mudstone with blue concretions located approximately 80 m below the contact of the El Cobre Canyon Formation and the overlying Arroyo del Agua Formation. Cranial elements are represented by fragments of the right maxilla and dentary. The postcranial elements include vertebrae representative of all regions of the vertebral column, a nearly complete pelvis, a complete right femur and portions of other limb bones, as well as bones of the right pes. The completeness of the skeleton contrasts with most other collections of O. navajovicus, which consist of isolated elements. O. navajovicus differs from all other known species of Ophiacodon in the retention of the following primitive characters: ventral ridge of the postaxial presacral centra are flat with well-defined longitudinal borders, adductor ridge of the femur is weakly developed and positioned mid-ventrally along the shaft, and the neck of the astragalus is almost half of the proximodistal length of the element. The unique morphology of the ventral surface of the centrum in O. navajovicus is potentially advantageous in the identification of isolated thoracic centra when present in combination with the typical ophiacodontid wing-shaped transverse process, in which a web of bone extends from the diapophyses to the parapophysis and the centrum has a wedge-shaped cross-sectional outline. Therefore, O. navajovicus can be considered of potential use in Late Pennsylvanian tetrapod biostratigraphy. As currently understood, the stratigraphic distribution of O. navajovicus in New Mexico, Utah and Colorado indicate that O. navajovicus characterizes the Cobrean landvertebrate faunachron.