The oreodont Merychyus major major (Mammalia: Artiodactyla: Oreodontidae) from the Miocene Popotosa Formation, Bosque del Apache National Wildlife Refuge, Socorro County, New Mexico

Abstract

A partial fossilized skeleton of an oreodont (Mammalia: Artiodactyla: Oreodontidae) was discovered in 2008 eroding from the wall of an arroyo on the Bosque del Apache National Wildlife Refuge near San Antonio in Socorro County, central New Mexico. The fossil was preserved in a semi-indurated, sandy stream deposit in a distal piedmont facies of the Miocene Popotosa Formation. The specimen includes a nearly intact skull and attached mandible that were associated with a partial articulated postcranial skeleton. The postcranial skeleton consists of a complete set of thoracic, lumbar, and sacral vertebrae, most of the ribs, which are still attached to their respective thoracic vertebrae, both wrists (distal radius and ulna, carpals, proximal metacarpals), left innominate, and an incomplete left hind limb (femur, tibia, fibula, tarsals). A highly unusual feature is the preservation of an ossified larynx and the delicate hyoid bones. The Bosque del Apache oreodont is identified as Merychyus major major, a member of the subfamily Ticholeptinae. It is the only record of an oreodont from the Popotosa Formation. The specimen represents one of the largest individuals of the genus currently known and the most derived and tapir-like in the degree of posterior retraction of its external narial opening. Merychyus major major is restricted to faunas of early late Miocene (latest Clarendonian) age. Geologic age constraints for the Popotosa Formation in the general vicinity of the oreodont site are provided by two 40Ar/39Ar radioisotopic ages. An age of 14.59 ± 0.05 Ma was obtained on pumice clasts in fluvial sandstones near the base of the local section, and an age of 8.57 ± 0.26 Ma was determined for a basalt flow interbedded with conglomerates near the top. The oreodont skeleton was found in strata somewhat below the dated basalt. The radiometric dates for the Popotosa Formation are consistent with those from the Dove Spring Formation of southern California where the stratigraphic range of M. major major is bracketed by smaller-bodied subspecies of M. major and by vitric tuff units that have been deter mined to be 11.19 ± 0.10 Ma in age, below, and 9.36 ± 0.20 Ma, above.