EVOLUTION IN YODERIMYINAE (EOMYIDAE, RODENTIA), WITH NEW MATERIAL FROM THE WHITE RIVER FORMATION (CHADRONIAN) AT FLAGSTAFF RIM, WYOMING

Three species of Yoderimyinae (Eomyidae: Rodentia) are recognized from the lower part of the White River Formation (early to medial Chadroni an) in the Flagstaff Rim area, Wyoming. The new material allows an imp roved diagnosis for the subfamily. The enamel microstructure of Yoderi myinae supports its inclusion in the Eomyidae. A new genus, Zemiodonto mys, is established for Yoderimys burkei Black, and new material, incl uding upper dentition, is referred to this species. This genus differs from Yoderimys in having higher crowned and more lophodont teeth and in lacking P3. A second new genus, Litoyoderimys, is established for Y oderimys lustrorum Wood, and a new species, L. auogoleus, is referred to the genus. This genus has lower crowned, more cuspate teeth than Yo derimys. Through early and medial Chadronian time, evolution in yoderi myines includes the following morphologic transformations: increase in size; increase in crown height and lophodonty of cheek teeth; reducti on of P3 (from double-rooted, to single-rooted, to absent); increase i n relative size of P4 and p4; and increased longitudinal torsion of th e mandible.