DISCOVERY OF A EUPELYCOSAUR BODY FOSSIL IN THE LOWER PERMIAN YESO GROUP, CENTRAL NEW MEXICO

The lower Permian (Leonardian) Yeso Group, which until recently was considered to have few fossils, records arid coastal plain, shallow marine, and evaporitic deposition northwest of the Permian basin, across much of central New Mexico. The recently discovered fossil record in Yeso strata includes marine micro-organisms (mostly algae and foraminiferans), terrestrial plants, and tetrapod footprints. In November of 2016, the first tetrapod body fossil of the Yeso Group was discovered at Salinas Pueblo Missions National Monument. This fossil is a partial skeleton of a basal synapsid, varanopid eupelycosaur, and is preserved as natural casts of the bones in part and counterpart in a lower sandstone bed of the Arroyo de Alamillo Formation in the southern Manzano Mountains. This quartz-rich sandstone is fine-grained and pale reddish brown to grayish red when unweathered, weathers to blackish red, and is partially encrusted with white caliche. The casts preserve in close articulation the lower half of a single individual including part of the pelvis(?), 18 caudal vertebral centra, both femora, tibiae, fibulae, and most of the pedes. With a femur length of 62 mm and a total length of the preserved cast from the pelvis to the tip of the incomplete tail of 325 mm, this skeleton is of a relatively small and gracile eupelycosaur most similar to Varanops. Older strata in New Mexico, including the Bursum Formation, Abo Formation, and Cutler Group are known to contain eupelycosaurs, but this discovery extends that fossil record into the younger part of the early Permian section. The arid coastal environment recorded in the Arroyo de Alamillo Formation must have cultivated a substantial terrestrial food chain in order to sustain a relatively large, early Permian predator like Varanops. This animal likely fed on smaller vertebrates and arthropods, which have yet to be discovered in the Yeso Group strata.