THE MEDIAL BLANCAN (LATE PLIOCENE) ARROYO DE LA PARIDA LOCAL FAUNA, SOCORRO COUNTY, NEW MEXICO

Vertebrate fossils were first reported in 1936 from Arroyo de la Parida in the Socorro basin, about 6 km NE of Socorro, Socorro County, New Mexico. The Arroyo de la Parida local fauna is derived from a >70-m-thick succession of sand and gravel that constitute the axial-river (ancestral Rio Grande) facies of the Palomas Formation. This facies interfingers with, and is overlain by piedmont deposits derived from the eastern-basin margin. No fossils have been recovered from the overlying piedmont interval. Ten species are represented: the land tortoise Hesperotestudo; the ground sloth Megalonyx cf. M. leptostomus; the 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. Five of these species are restricted to Blancan faunas, and the most biostratigraphically diagnostic is Rhynchotherium, which became extinct in the late Pliocene (~2.2 Ma). The lower jaws of R. falconeri from Arroyo de la Parida were collected near the top of the local section, 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, 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. The Arroyo de la Parida local fauna is thus interpreted to be medial Blancan (2.7-3.7 Ma) in age and is younger than the 4.9-Ma trachyandesite flow at San Acacia, which is exposed about 13 km to the north.