Lagomorpha and Rodentia of the early Hemphillian (late Miocene) Mulholland localities, Contra Costa County, California, USA

Two University of California Museum of Paleontology localities, UCMP V3303 and V3611, from early Hemphillian deposits of Contra Costa County, California, USA, have long been recognized for their vertebrate fossil remains.The combined assemblages from these localities were previously referred to as the "Mulholland Fauna," but are now understood to occur in different stratigraphic units.Based on the lithology at both localities, the assemblages provide insights into rarely sampled riparian/lacustrine paleoenvironments during the late Miocene (~8.9-7.8Ma) of the West Coast of North America.To further clarify the diversity of small mammals in these paleoenvironments, we provide updated systematic accounts of the lagomorphs and rodents in these two assemblages.The following species are recorded in the assemblage from UCMP V3303 (bracketed between ~8.9-8.5 Ma): Hypolagus tedfordi; Pliosaccomys magnus; Bensonomys sp. and Pliotomodon primitivus.The following species are recorded in the assemblage from UCMP V3611 (bracketed between ~8.1-7.8Ma): Hypolagus fontinalis; Hypolagus tedfordi, Dipoides sp.; Pliosaccomys magnus; Postcopemys sp., cf.P. valensis; ?Copemys sp. or Postcopemys sp.; Pliotomodon primitivus; and Honeymys hoffmeisteri new species.The latter is the third species of Honeymys recently recognized.Honeymys represents a unique cricetid radiation, possibly related to later New World sigmodontines or European Collimys, during the late Miocene (medial Clarendonian through early Hemphillian, 11.7-8.0Ma).The occurrence of Bensonomys at UCMP V3303 represents the oldest record of the genus, extending its first appearance in the geologic record downward by at least ~0.5 Ma.