A TAXONOMIC REVISION AND CHARACTERIZATION OF SCIURAVUS NITIDUS AND S. UNDANS (RODENTIA; SCIURAVIDAE), EARLY EOCENE RODENTS FROM THE BRIDGER FORMATION, GREEN RIVER BASIN, WYOMING

New specimens from the Bridger Formation allow for a critical review of the alpha taxonomy and evolution of the two most abundant species of sciuravid rodents from the Early Eocene of the Green River Basin, Sciuravus nitidus and S. undans. Previously, the two species were considered to be synonymous, based on studies of the upper and/or lower dentitions. Specimens with nearly complete associated upper and lower dentitions are studied for the first time and they reveal clear distinctions between the two species in the molar crown patterns, incisor shape, and mandible morphology. Sciuravus nitidus can be distinguished from S. undans based on several crown pattern characters: double mesostyle on M1-2 vs. single in S. undans; lingual aspect of M1-2 protocone square vs. rounded in S. undans; M1-2 metacone connects with protocone via metaloph with wear; single metastylid m1-2 vs. double in S. undans; distinct anterior cingulum and metaconid only slightly anterior to protoconid in p4; m1-2 posterolophid ends at center of hypolophid vs. reaching posterior aspect of entoconid as in S. undans. The lower incisor of S. nitidus is tapered posteriorly, broader and flattened anteriorly with a shallow sulcus while that of S. undans, which is similar in shape, has a smaller AP/TR ratio and lacks a sulcus. The mandibule of S. nitidus is very slender with a deep masseteric fossa bounded by a gently sloping dorsal ridge. A thicker mandible, shallower masseteric fossa and sharply angled dorsal ridge characterize S. undans. Patterns of dental evolution in the sciuravids contribute to our knowledge about the early Bridgerian expansion in mammalian diversity. Discovery and description of specimens including associated upper and lower dentition clarifies the alpha taxonomy for two species of Sciuravus, which is one of several rodent genera to show increased species diversity coincident with the late early Eocene Climatic Optimum (EECO, 53-50 MA).