Extinct muskox and other additions to the Late Pleistocene Riddell Local Fauna, Saskatoon, Canada

Fossils recently recovered from the Riddell Member of the Floral Formation, a richly fossiliferous intertill sand and gravel deposit in the Saskatoon area, include taxa previously unknown from the Riddell Local Fauna and confirm the presence of others. Bootherium bombifrons (= Symbos cavifrons), represented by a well-preserved but incomplete skull, is new. Details of its preserved morphology support concepts of developmental variability and sexual dimorphism in the extinct species. Also new is the beaver, Castor canadensis, represented by an incomplete ulna. Additional fossils of horses indicate that at least two species, Equus niobrarensis, as well as the previously identified E. conversidens, were present. A Rancholabrean age (probably Rancholabrean II) for the fauna is confirmed by the presence of Bootherium bombifrons, a muskox known only from Illinoian and younger time in North America, but lithologic and stratigraphic relationships of tills and ecological requirements of the fauna limit the Riddell Member to the Sangamonian. Disharmonious associations of small mammals and high megafaunal diversity are consistent with the emerging picture of Pleistocene ecosystems as highly co-evolved and heterogeneous and without modern analogs.