CONSTRAINING THE RANCHOLABREAN NORTH AMERICAN LAND MAMMAL AGE IN THE SOUTHWESTERN UNITED STATES

The Rancholabrean North American Land Mammal Age, named for the mammal assemblage from the Rancho La Brea asphalt deposits in Los Angeles, California, is characterized by the presence of the Eurasian immigrant genus Bison south of 55ºN latitude. Recent reviews suggest Bison arrived in the coterminous United States less than ~220 ka before present, but estimates vary.

Land mammal ages are defined in faunal terms, and faunas do not manifest themselves across the entirety of a continent in a geologic instant. Rather, index species employed to define mammal ages can appear in different geographic regions at different times, depending upon dispersal patterns of the species in question. Land mammal ages can thus be considered time-transgressive rather than synchronous. In the case of the Rancholabrean, this diachronous nature may be quantifiable due to the abundance of Pleistocene localities and the accuracy of radiometric dating techniques.

Such quantification requires precision in identification and dating of fossil remains from late Pleistocene localities. In the American southwest, many published “Rancholabrean” faunas entirely lack the index taxon for this time period, Bison. These published accounts considered land mammal ages to be temporally rather than faunally delimited. Other southwestern “Rancholabrean” assemblages reported to contain remains of Bison actually lack any such fossils; reports of Bison from inland southern California and from multiple sites in the Mojave Desert were based upon misidentified elements. Some Mojave Desert records, if confirmed, might have helped define the beginning of the Rancholabrean, but the corrected identifications reject this possibility. Records of early Bison from coastal southern California, also potentially useful biochronologically, were recovered from sites with complex stratigraphy, and often lack reliable stratigraphic context. At present, these records cannot be used to constrain the Rancholabrean.

Recent studies in inland southern California and the Mojave Desert by the San Bernardino County Museum emphasize the importance of both precise identifications and detailed contextual data when interpreting the age and nature of late Pleistocene faunas. These studies will help better determine the extent of the Rancholabrean, both regionally and continentally.