Discovery of a tetrapod body fossil in the Lower Permian Yeso Group, central New Mexico

Lucas, Spencer; Thorpe, Emily; Berman, David; Rinehart, Larry; Santucci, Vincent; Henrici, Amy

The lower Permian Yeso Group records arid coastal plain, shallow marine and evaporitic deposition across much of central New Mexico during Leonardian time. Generally considered to have few fossils, recent study of Yeso Group strata has uncovered a diverse fossil record of marine microorganisms (mostly algae and foraminiferans), terrestrial plants and tetrapod footprints. We document the discovery of a tetrapod body fossil in the Yeso Group—a partial skeleton of a varanopid eupelycosaur. The fossil consists of the natural molds of articulated bones in two pieces, part and counterpart, that were preserved in a sandstone bed of the lower part of the Arroyo de Alamillo Formation in the southern Manzano Mountains. The fossil-bearing sandstone is interpreted as an eolian sheet sand. The casts preserve part of the pelvis(?), 18 caudal vertebral centra, both femora, tibia-fibulae, and most of the pedes, largely in close articulation, of a single individual. The skeleton is of a relatively small (femur length = 62 mm, total length of the preserved cast from the pelvis to tip of the incomplete tail = 325 mm) and gracile eupelycosaur most similar to Varanops. In New Mexico, various early Permian eupelycosaurs are known from the older strata of the Bursum Formation, Abo Formation, and Cutler Group, so this discovery extends the eupelycosaur fossil record in the state into younger early Permian strata. It also indicates that a substantial community must have been present on the arid coastal plain during deposition of the Arroyo de Alamillo Formation, as the varanopid is a generally large, early Permian predator that likely fed on smaller vertebrates and arthropods. Furthermore, this discovery indicates the potential for additional discoveries of tetrapod body fossils in Yeso Group strata.