FOSSIL MAMMALS FROM THE LATE EOCENE BRENNAN BASIN MEMBER OF THE DUCHESNE RIVER FORMATION IN UTAH

Directly overlaying the fossiliferous yellow-gray siltstone dominated Uinta Formation, the Brennan Basin Member of the Duchesne River Formation consists of sandstone and conglomeratic lenses, inter-bedded by reddish brown siltstone. Fossil mammals from this unit are rare, but stratigraphically important in the understanding of mammalian evolution across a poorly sampled period in the fossil record of Utah. During this interval of time, the Carnivora diversified into felids and canids, while the Primates disappeared from the fossil record in Utah. Pig-like entelodonts and anthracotheriids first appear, while the rhino-like brontotheres came to dominance. Most fossil mammals from the Duchesne River Formation are associated with discoveries made in 1929 and 1930 from a large death assemblage of twenty individuals of the horn-less brontothere Duchesneodus. From this quarry came several other fossil mammals including the large carnivorous mesonychid Hessolestes, the advanced creodont Hyaenodon, and early camel Poabromylus. Fairly well preserved specimens of the bizarre clawed agriochoerid artiodactyl Diplobunops are known elsewhere in the formation, in addition to the transitional early horse Duchesnehippus only known from a single lower jaw. More abundant fossilized remains were discovered near Randlett, Utah near the boundary with the lower Uinta Formation. This stratigraphically lower fauna includes the early rhino Uintaceras, the sheep-sized Protoreodon, the primitive tapiroid Dilophodon and amynodont Megalamynodon. The rodents Pareumys and Mytonomys as well as the early lagomorph Mytonolagus are also preserved from the Randlett Horizon. In 2009, fossils from a separate location in the lower Brennan Basin Member were discovered south of Vernal Utah. The new fauna includes the hyracodont Epitriplopus, the protoceratid Leptotragulus, the small carnivorian Miacis, and the cylindrodont rodent Pareumys, as well as an alligatoroid provisionally identified as Procaimanoidea. While not extensively diverse, the new fauna helps expand what little knowledge there is of fossil mammals from the lower Brennan Basin Member of the Duchesne River Formation. Study of the sparse mammalian fauna from the late Eocene of Utah continues to highlight the formation’s importance.