New Mexico Geological Society Annual Spring Meeting — Abstracts

A Pleistocene site containing skeletons of large mammals preserved in fissure-fill deposits was discovered in 2005 at the White Mesa Mine near San Ysidro in Sandoval County, New Mexico. The fissures occur in gypsum of the Middle Jurassic Todilto Formation. Structural analysis suggests the fissures opened to the surface during the Pleistocene in response to extension associated with the Rio Grande rift. Bones were found 12 m below the land surface in four different fissures within a 30 m radius. The most fossiliferous fissure, containing skeletons of a bison and two camels, was 20 cm wide and contained bones over a length of about 3.5 m and a depth of about 30 cm. There are four species of large mammals in these fissure deposits, each known from at least one partial to nearly complete articulated skeleton: giant llama Camelops hesternus, extinct bison Bison antiquus, mule deer Odocoileus hemionus, and the rare stilt-legged horse Equus francisci. Camelops is represented by two partial skeletons, a juvenile with unworn teeth and an old individual with heavily worn teeth. There are nearly complete skeletons of a juvenile B. antiquus and a female O. hemionus, and a front limb of E. francisci. This site differs from Pleistocene cave faunas in the predominance of articulated skeletons and the lack of small vertebrates. An AMS (accelerator mass spectrometer) radiocarbon date on a Camelops bone is pending; however, the presence of Bison antiquus establishes a late Rancholabrean age (late Pleistocene, <100 ka).