COMMUNITY COMPOSITION AND ENVIRONMENTAL SETTING OF THE MCKAY RESERVOIR FAUNA (MIOCENE, OREGON)

The Dalles Group of Oregon’s Columbia Basin preserves a series of Late Miocene vertebrate fossil localities that have played an outsized role in the history of vertebrate paleoecology. Not only do these sites date to a period of climatic cooling, expanding grasslands, and migration from Asia and South America, but early paleoenvironmental work suggested that they represent a wide array of environments, making landscape-scale paleoecological analyses possible. Particularly important among the Dalles Group sites is McKay Reservoir near Pendleton, which preserves a vertebrate fauna dating to 5.5-6 Ma. Its significance lies in the diversity of taxa present, ranging in size from shrews to mastodonts. However, collection at McKay Reservoir took place largely prior to 1980, and many taxa described from there are either problematic or defunct, complicating any attempts at reconstructing biostratigraphic relationships, paleoenvironment, or community structure. Recent work has made possible the reidentification of previously problematic specimens and has led to the identification of several taxa not previously reported from the Dalles Group. Significant new occurrences include an oscine passerine, one of the oldest in North America. A ground sloth metapodial indicates the presence of another recent migrant to the continent. The metapodial is consistent in size and morphology with Pliometanastes, which would not only make it one of North America’s oldest sloths but the first of its genus reported from the Northwest. More enigmatic are the remains of a massive felid represented by copious postcranial material but no skulls or dentition. The absence of craniodental remains makes a precise identification difficult, but a size consistent with the largest specimens of pantherines and a morphology more typical of machairodontines suggests it may represent an undescribed taxon and one of the continent’s earliest big cats. These recent discoveries support the historical interpretation of McKay Reservoir as a biogeographic crossroads and bolster its significance as a natural paleoecological laboratory, but many questions remain. Ongoing field work will further add to our understanding of this rich Miocene paleocommunity and resolve long-standing uncertainty surrounding the physical setting of the McKay ecosystem.