Mammalian Fauna of the Middle Eocene Kishenehn Formation, Middle Fork of the Flathead River, Montana

ABSTRACT The Kishenehn Formation is a unit of sedimentary rocks exposed largely in the cutbanks of the Flathead River and its tributaries in and around Glacier National Park in northwestern Montana and adjacent British Columbia. From these rocks along the Flathead's Middle Fork, which range from very fine-grained oil shales to and including pebble-cobble conglomerates, vertebrate and molluscan faunas of middle Eocene age have been collected over nearly 40 years by a combination of prospecting, with access by raft, to screen washing sediment from the most fossiliferous exposures. The mammalian fauna from the formation includes at least twenty-six taxa, ranging in size from tiny rodents and insectivores to a very large brontothere. A radiometic date of ca. 46.2 Ma was obtained from below the fossiliferous deposits. The age of the fauna, presumably within one or two million years younger than the radiometric date, is reinforced by the presence of the Uintan index taxon Amynodon Marsh, 1877, as well as the co-occurrence of an eomyid rodent referred to Metanoiamys Chiment and Korth, 1996, and the sciuravid rodent Pauromys Troxell, 1923. The fauna has a scarcity of Carnivora, which may reflect the real faunal composition. It is striking for its total absence of selenodont artiodactyls. Whether the latter should be attributed to geologic age, which is presumably early after these animals developed in North America or Asia, or is environmentally significant, must remain conjectural. Somewhat widely distributed in the North American west, earlier Uintan and/or Shoshonean mammalian faunas remain difficult to correlate, due at least in part to distinctive endemism or environmental uniqueness. The fauna also has indications of interchange between North American and Asian components at that time. Of the twenty-six mammalian taxa recognized in the fauna, one is established as new, the rodent Microparamys solis, new species.