New Mexico Geological Society Annual Spring Meeting — Abstracts

Vertebrate fossils were first reported in 1936 from Arroyo de la Parida, about 6 Ian northeast of Socorro, Socorro County, central New Mexico. The Arroyo de la Parida Local Fauna is derived from a 70 m thick sequence of sands and gravels that constitute the axial river (ancestral Rio Grande) facies of the Palomas Formation of Gordon (1910, USGS Prof. Paper 68). The strata in the vicinity of Arroyo de la Parida are located at the northern end of the Socorro basin, representing one of the northernmost occurrences of the Palomas Formation, which has its type area about 100 km farther south in Palomas Creek near Truth or Consequences in Sierra County.

The Arroyo de la Parida Local Fauna is composed of ten species of vertebrates, including the land tortoise Hesperotestudo; the ground sloth Megalonyx cf. M. leptostomus; three species of horses, Equus cf. E. cumminsii, E. scotti, and E. simplicidens; two camels, a large species of Camelops and a small species of Hemiauchenia; the small pronghorn antilocaprid Capromeryx; and two proboscideans, Rhynchotherium falconeri and Stegomastodon sp. This is a fairly typical faunal assemblage found in New Mexico Blancan sites, mostly consisting of large grazing ungulates and dominated by horses of the genus Equus. Five of these species are restricted to Blancan faunas, including Megalonyx leptostomus, Equus cumminsii, E. simplicidens, the large Camelops, and Rhynchotherium falconeri. The most biostratigraphically diagnostic of these taxa is Rhynchotherium, a proboscidean characterized by a strongly downturned mandibular symphysis and the presence of lower tusks. Rhynchotherium became extinct in the late Pliocene at about 2.2 Ma, together with several other characteristic genera of Blancan mammals. The lower jaws of R. falconeri from this site were collected near the top of the local section of the Palomas Formation on the south side of Arroyo de la Parida, suggesting that the entire fauna, most of which occurs some 40 m lower in the section, is older than 2.2 Ma. An early Blancan age for the Arroyo de la Parida Local Fauna is excluded by the presence of E. scotti, Camelops sp.,
and the small Hemiauchenia, all of which first appear in New Mexico faunas during the medial Blancan (2.6-3.7 Ma). The absence of South American immigrants that arrived following the Great American Interchange suggests that the Arroyo de la Parida Local Fauna is older than 2.7 Ma. Megalonyx is the only marrunal of South American origin in southwestern Blancan faunas that was not a participant in the Great American Interchange. Megalonyx or its progenitor arrived from South America in the late Miocene, and M. leptostomus is fairly widespread in early through late Blancan faunas. The Arroyo de la Parida Local Fauna is thus interpreted to be medial Blancan (2.7-3.7 Ma), and is similar in age to the Belen Fauna from the "Sierra Ladrones Formation" in the southern Albuquerque basin and the Cuchillo Negro Creek Local Fauna from the Palomas Formation in the Engle basin near Truth or Consequences.