PLEISTOCENE VERTEBRATES FROM THE DOEDEN LOCAL FAUNA (ILLINOIAN/SANGAMONIAN?), YELLOWSTONE RIVER VALLEY, EASTERN MONTANA

The Doeden gravel pit in Yellowstone River valley, Miles City, Montana, has yielded a vertebrate fauna including the ground sloths Megalonyx sp., cf. M. jeffersonii and Glossotherium (Paramylodon) harlani, mammoths (Mammuthus columbi), mastodon (Mammut americanum), horse (Equus spp.), camel (Camelops sp.), shrub-ox (Bootherium sp.), a cervid, an antilocaprid, and giant short-faced bear (Arctodus simus). This material, at the Museum of the Rockies (MOR), includes both grazers and browsers, suggesting a well-developed riparian forest within a more extensive grassland. Sloth specimens include limb bones (femur, two tibiae and claw) and two vertebrae. Arctodus is represented by the distal half of a rugged humerus. The Doeden Local Fauna (DLF) is younger than Lava Creek B tephra (ca. 600 ka) and, based on terrace correlations, older than the 160 to 124 ka dates on calcrete from terraces in the tributary Tongue River drainage. This wide timespan extends from Irvingtonian (Yarmouthian/Early Illinoian) to early Rancholabrean (later Illinoian/Sangamonian) times. Absence of Bison sp. from the extensive sample is consistent with an Irvingtonian age, if the arrival of Bison in the midcontinent marks the start of the Rancholabrean. If late Irvingtonian (Early Illinoian), the DLF was roughly coeval with Berends, Oklahoma; Sandahl and Adams, Kansas; and Angus and Mullen II, Nebraska. Mammuthus columbi appeared by the middle Irvingtonian, and Bootherium [=Symbos] by late Irvingtonian. The fauna could thus date as early as 500 ka. Glossotherium (Paramylodon) harlani ranged from Irvingtonian to Rancholabrean and the size of the Arctodus humerus suggests A. s. yukonensis, also Irvingtonian to Rancholabrean. Megalonyx jeffersonii appeared during Illinoian times, about 150 ka, which would make the fauna considerably younger but is still consistent with the geological evidence. If the DLF is significantly older, the ancestral species M. wheatleyi is a possibility.