Stockoceros is an extinct pronghorn antelope which lived in the late Pleistocene, over 10,000 years ago. Pronghorns originated in North America in the middle Miocene, about 15 million years ago. Today there is a single genus and species, Antilocapra americana, but in the Pleistocene there were at least eight species and four genera. The specimen shown here belongs to the species Stockoceros onusrosagris, also known as Quentin’s pronghorn, from the late Pleistocene. The horns of pronghorns have a core of bone covered with a keratinous sheath which is shed annually. The horncore is often forked; in modern pronghorns, the fork is quite high on the horn and the forward-facing point is very small. In Stockoceros, the fork occurs near the base of the horn and both points are roughly equal in length; this gives the animal a four-horned appearance.

This specimen of Quentin’s pronghorn is one of 125 individuals discovered in Papago Springs Cave, which lies in the Coronado National Forest, about 5½ miles southeast of Sonoita, Arizona. Why so many pronghorns ended up in the cave is something of a mystery. Many of the skeletons were still articulated and there are no tooth marks on the bones, which suggests that the remains were not brought to the cave by predators. It is possible the animals may have been attracted by accumulated water or mineral salts; alternatively, they may have used the cave for warmth in winter, or to avoid heat and flies in the summer.