LATE PLEISTOCENE BISON ANTIQUUS FROM ORCAS ISLAND, WASHINGTON, AND EVIDENCE FOR AN EARLY POSTGLACIAL LAND MAMMAL DISPERSAL CORRIDOR FROM THE MAINLAND TO VANCOUVER ISLAND

We report a skull and partial skeleton of Bison antiquus from the base of a peat bog, Ayer Pond, on Orcas Island, Puget Sound, Washington. The specimen, dated to 11 760±70 ¹⁴C yr BP (Beta-216160), lay in lacustrine sands above an unconformity marking the emergent Everson Glaciomarine Drift surface (>12.0 ka). Other Orcas Island and Vancouver Island bison finds in similar bog settings indicate an established population and suggest a late-glacial land mammal dispersal corridor between the mainland, San Juan Islands, and Vancouver Island, partly involving a briefly emergent glaciomarine landscape, with smaller water barriers than today. Relative sea level curves indicate the onset of emergent conditions by about 12.0 ¹⁴C ka, lasting for at most a few millennia. Rich in marine-derived organic material, this landscape was colonized rapidly by terrestrial plants and animals. An early unstable tundra-like or herbaceous meadow community and succeeding open pine woodland, documented in nearby pollen sequences predating 11.0 ka, would have been favorable, though not optimal, for bison. However, expansion of closed mixed-conifer forests after 11.0 ka likely contributed to their extirpation. Interpretation of vegetation chronosequences for these islands must take into account the probable role of large mammals in importing seeds and in impacting succession through grazing, browsing, and trampling. Evidence for possible butchering by humans adds interest to the Ayer Pond discovery in view of its pre-Clovis age.