A NEW MAMMALIAN FAUNA FROM FLORISSANT FOSSIL BEDS NATIONAL MONUMENT, COLORADO

The latest Eocene Florissant Formation is comprised of a series of lake shales, volcanics, and fluvial deposits exposed within Florissant Fossil Beds National Monument and its immediate vicinity. While the formation is well-known for its diverse and abundant plant and insect fossils, discoveries of fossil vertebrates have been relatively rare, consisting mainly of fish and a few birds. Fossil mammal discoveries have been scarce and rather serendipitous, the most notable being a skeleton of Herpetotherium from the lake shales and a Mesohippus jaw from the lower mudstone unit, described by others.

A systematic investigation in the summer of 2003 of the fluvial lower mudstone unit greatly increased the known mammalian diversity from the Florissant Formation. The discovery of a single large molar confirms the presence of eubrontotheres in the Park, which had been inferred previously only from a large atlas and tooth enamel fragments. Two artiodactyl taxa are represented by isolated lower molars, Leptomeryx cf L. speciosus, and a second yet to be identified. Five teeth belonging to the lagomorph genus Megalagus were recovered. Additionally, the rodents Eutypomys parvus, Prosciurus vetustus, Adjidaumo minimus, and a large species of Ischyromys occur in the Florissant mammalian fauna. Three insectivoran jaws were recovered, representing the soricid Domnina thompsoni, the geolabidid Centetodon magnus and a tiny talpid.

This mammalian fauna is consistent with the Chadronian North American Land Mammal Age (NALMA). This agrees with the 40Ar/39Ar age of 34.07 ± 0.10 Ma of the Florissant Formation, determined by others. The Florissant fauna is most similar to the middle Chadronian Pipestone Springs Local Fauna of Montana. It also closely resembles the middle Chadronian Calf Creek Local Fauna of Saskatchewan, and the fauna of the Chadronian section at Flagstaff Rim, Wyoming. The similarity to these particular faunas stems from a similar age, but also supports an emerging Rocky Mountain region faunal coherence.