LARGE MAMMALS FROM THE HEMPHILLIAN OF MCKAY RESERVOIR, OREGON

McKay Reservoir is a fossiliferous locality in Umatilla County, Oregon. The McKay Formation outcrops at the site and has yielded a fauna dating to the Early Late Hemphillian North American Land Mammal Age (6.7-5.9 Ma, late Miocene). This site is particularly well known for its fossils of small mammals as well as an abundance of large mammal postcrania that have formed the basis of multiple pioneering studies in paleoecology. Fossils were first collected from the reservoir by J. Arnold Shotwell in the mid-1950s, but prior to this summer the site had not been revisited since 1980. Our work took place during July, when water levels at the reservoir were low enough to expose a large number of specimens in float, including the large mammal fossils outlined here. It has long been recognized that the rhinocerotid Teleoceras is especially abundant in the McKay fauna, which our work confirms. Many of the Teleoceras specimens collected show signs of pathology, particularly osteoarthritis. Rhinocerotids dominate the perissodactyl sample at McKay Reservoir, but isolated cheek teeth and a cervical vertebra indicate the presence of equids as well. Artiodactyls are also less common than rhinos, but fossils from the large camelid Megatylopus (not previously described from the site) and the tayassuid Platygonus brachirostris were recovered during this summer’s field work. The largest mammals at the site are proboscideans, including the mammutid Mammut, another taxon that had not previously been reported from McKay Reservoir. A single medial phalanx of the ground sloth Megalonyx represents the first fossil of a xenarthran recovered from the site. Several large carnivoran taxa were also uncovered, including the borophagine canid Epicyon and a giant felid represented only by postcrania. While Shotwell identified this felid as Machairodus, it is more likely attributable to Nimravides. These additions to the McKay Reservoir fauna and the increased sample of common taxa such as Teleoceras further illuminate the ecology of a remarkable Miocene ecosystem.