NEW EOCENE FOSSIL PERISSODACTYLS FROM THOUSAND LAKE MOUNTAIN, CENTRAL UTAH

In 2005, Utah Geological Survey (UGS) geologists were mapping a coarse clastic rock unit of unknown age on Thousand Lake Mountain in Wayne County, Utah. This unit is composed of unconsolidated sand and coarse gravel of presumed fluvial origin, and is unconformably underlain by upper Cretaceous rocks and capped by Oligocene volcanics. Previous workers hypothesized that these rocks correlate with the Paleocene Flagstaff Fm. During their investigation, the UGS geologists found a right dentary fragment of a perissodactyl preserving three molars. Based on the molar morphology, we identify this jaw as belonging to the basal rhinoceratid genus Teletaceras. Molar dimensions fall below the range of T. radynskyi from the Eocene Clarno Fm. of Oregon. At over 10,000 feet in elevation, this is the highest-altitude vertebrate fossil found in Utah, and the first report of this genus in Utah.

During a subsequent visit to the area in 2006, we discovered a partial skull of a brontothere that preserves most of the anterior portion of the skull including the palate and a damaged, but complete, set of upper teeth. Based on the presence of two upper incisors and details of the molar morphology, we tentatively identified this specimen as Duchesneodus uintensis. Together, the two fossils indicated a Duchesnean Land Mammal Age (42-37 Ma) for these rocks. A Duschesnean age for these rocks shows that they are not related to the Paleocene Flagstaff Fm. A reassessment of these specimens confirms our generic identification but we have noted features, including a distinct diastema in the brontothere, that suggest these may be different and possibly new species. Although we’re confident in an Eocene age for these rocks, an exact age determination will have to await further study.

Another specimen of Duchesneodus was reported from the Eocene Green River Fm. 100 kilometers north of this site in Sanpete County. The Green River Fm. in central Utah appears to be younger than it is in northeastern Utah and Wyoming where it lies below the Uinta and Duchesne River Fms. The rocks on Thousand Lake Mountain may be correlative with the upper Eocene Crazy Hollow Fm. that interfingers with the Green River Fm. in central Utah. The discovery of identifiable vertebrate fossils has provided important age data in addition to providing significant paleontological information.