A systematic review of the mammalian megafauna of the middle Miocene Barstow Formation, Mojave Desert, California

The Barstow Formation (middle Miocene) crops out in the Mud Hills region of the Mojave Desert, San Bernardino County, California. The formation contains a rich and diverse mammalian fauna of late Hemingfordian (He2) (roughly 17.5--16.0 Ma) and Barstovian (roughly 16.0--12.5 Ma) age. The Barstovian component consists of three distinct faunal units, the Green Hills Division Fauna (Ba1), the Second Division Fauna (Ba1), and the Barstow Fauna (Ba2). Despite being designated the type fauna for the Barstovian Land Mammal Age, the mammalian fossils from the Barstow Formation have not been systematically reviewed in almost ninety years.;In addition to a comprehensive review of the megafaunal components of the Barstovian portion of the Barstow Formation, three new taxa are described. The first is a new genus of hypsodont equid, Scaphohippus n. sp. Two new species of camelids are described, Paramiolabis minutus and Michenia mudhillsensis n. sp. Additionally, a number of taxonomic revisions are made to the borophagine canids, amphicyonids, felids, dromomerycids, and antiolacprids, including synonymy of several species and descriptions of extended stratigraphic ranges.;The three Barstovian faunal components are compared with equivalent faunas throughout the western United States using Simpson coefficients and Jaccard's indices. The Green Hills Division and Second Division faunas compare most favorably with the Lower Snake Creek Fauna (Ba1) of Nebraska. The Barstow Fauna compares most favorably with the fauna from the Valentine Formation (Ba2) of Nebraska. Examination of faunal composition within the Barstow Formation suggests that the most significant faunal turnover occurs between the Green Hills Division and Second Division Faunas, or within the Ba1 interval. A similar faunal turnover is not reported in any other Ba1 fauna.;Examination of paleoecological data reveals intriguing information about ecological conditions during the medial Miocene of North America. Classic interpretations suggest that the initial spread of grasses had a key impact on the evolution of mammals. However, examination of paleobotanical data suggests that grasses were not an abundant part of the flora. Tectonic factors and changes in floral assemblages exclusive of grasses were more likely causes of the high levels of mammalian diversity seen in Barstovian faunas.