A Possible New Species of Dimetrodon (Eupelycosauria: Sphenacodontidae) from the Lower Permian Abo Formation, Socorro County, New Mexico

Abstract

We add to the growing record of Dimetrodon fossils from the Lower Permian Abo Formation in New Mexico with the addition of a potential new species from the Gallina Well locality in Socorro County that previously yielded other material of Dimetrodon in 2010. During a visit to the site in 2014, one of us (KLM Jr.) collected a large vertebra and associated fragments including a 15 cm long neural spine. The neural spine morphology of this specimen is of the more primitive round cross section, most similar to Dimetrodon milleri , the oldest known Dimetrodon from Texas. A significant difference is larger size being at least 50% larger than D. milleri . We also see a temporal difference with the Gallina Well Dimetrodon being late Asselian or early Sakmarian in age and D. milleri being younger in the Sakmarian. The Discovery of this specimen raises several important questions. The first relates to size of early Dimetrodon species. Until the discovery of this specimen all known early Dimetrodon were small. Indeed, all early species, cf. D. milleri from the middle Asselian of New Mexico, D. occidentalis from the upper Asselian of New Mexico, and D. milleri from the Sakmarian of Texas, are all small species. This was thought advantageous to life in an inland and upland habitat, and that larger size arose to take advantage of deltaic habitats. The new species from Gallina Well demonstrates that larger size arose much earlier than previous thought. The diversity of Dimetrodon is also in question with regard to how many species were there in New Mexico deposits. The new Gallina Well Dimetrodon suggest the presence of two contemporaneous species in the Abo Formation. Research since 2009 has revealed Dimetrodon to be a more common, though not the dominant predator, on the Abo floodplains of Permian New Mexico. Much more research is needed to fully understand Dimetrodon from the Lower Permian of New Mexico.