Samantha, HopkinsEleanor, Froehlich2021-07-272021-07-272021https://hdl.handle.net/1794/264091 page.Family Aplodontidae is made of a number of extant and extinct species that include the modern mountain beaver and show a wide range of morphological diversity. I identified a fossil aplodontid jaw from the John Day Formation of Oregon for the final project of the ERTH 434: Vertebrate Paleontology course. The fossil, designated as JODA 2942, is a partial left mandible containing a broken incisor, a deciduous fourth premolar, and both the first and second molars. The specimen is from the Turtle Cove Member of the John Day Formation and comes from a locality north of Dayville, Oregon. This specimen dates to approximately 26 to 24 million years in age. Through comparisons to fossil rodents, I was able to determine that this specimen is a juvenile Rudiomys mcgrewi. This is significant as the type specimen of R. mcgrewi is very fragmentary and the only published occurrence of this genus, making each additional specimen valuable. Additionally, individuals from subfamily Meniscomyinae have wear patterns that change throughout the animal’s life. As this specimen is very young, we are able to see the practically unworn morphology. This identification increases the known diversity of aplodontids in the Turtle Cove Member and extends the known temporal range of R. mcgrewi.application/pdfen-USCC0paleontologyevolutionbiologyrodentdescriptionPresentation