Paper No. 6
Presentation Time: 9:00 AM-6:30 PM


ANDERSON, Deborah K., Division of Natural Sciences, St. Norbert College, 100 Grant Street, De Pere, WI 54115,

New rodent fossils from several Raven Ridge localities of the northeastern Uinta Basin, Green River Formation, document faunal changes during the Early Eocene Climatic Optimum, a significant global warming event. Previously, the only known record of Bridgerian (middle Eocene) rodents from the Uinta Basin was the fauna collected from Powder Wash (Dawson, 1968). Since then, thousands of specimens have been recovered from more than 100 new Raven Ridge localities, which span a time period from the early (Wasatchian) to middle Eocene (Wa3-5 - Br2). The newly described transitional Wasatchian-Bridgerian fauna includes taxa from Wa3-5 (Tuscahomys major, Paramys taurus, and Knightomys minor), Br1a (Sciuravus popi, Microparamys minutus, Knightomys sp., and K. huerfanensis) and Br2 (M. minutus, S. nitidus, P. delicatior, and Mysops sp.). These new specimens expand the temporal range of S. popi (Ui2-Br2) to include Br1a. Discovery of Tuscahomys in northeastern Utah expands the geographic range of this cylindrodontid; previously only known from the Tuscahoma Formation, Mississippi, (Dawson and Beard 2007) and the Willwood and Wasatch Formations of Wyoming (Rose et al., 2012; Anemone et al., 2012).

Consistent with findings at other Eocene sites, the Wasatchian and Br1a rodents from Raven Ridge are smaller and morphologically distinct when compared to their Br2 counterparts. For example, the upper molars of M. minutus (Br1a) lack accessory ridges in the central basin, a feature known to occur in members of this species from Br2. For P. taurus, lack of an entoconid crest and a wider lingual gorge distinguish Wa3-5 specimens from those known from Br2. In addition, species richness and diversity patterns are broadly similar to those found at other early and middle Eocene fossil localities. The Raven Ridge rodents provide evidence of taxonomic and ecological homogeneity during the Wasatchian-Bridgerian transition, thus, climate change likely influenced the faunal dynamics of the early to middle Eocene rodents of the Green River Formation.