Twenty-one species of small mammals are recognized from the Observation Quarry fauna including four new species: the lipotyphlans Lanthanotherium observatum (Erinaceidae), Antesorex wilsoni (Soricidae), and Scalopoides hutchisoni (Talpidae), and the rodent Mioheteromys subterior (Heteromyidae). Due to more complete material, emended diagnoses are provided for the plesiosoricid Plesiosorex greeni Martin and Lim, 2004, and cricetid rodent Copemys lindsayi Sutton and Korth, 1995. Of the 21 species recognized, seven are unique to this faunal assemblage, six are restricted to the Hemingfordian, seven are known elsewhere only from the Barstovian or later, and one is known from both the Hemingfordian and Barstovian. The small mammals suggest that the Observation Quarry contains a transitional Hemingfordian-Barstovian fauna with a greater number of species from the latter, supporting an early Barstovian age.