Abstract
A new biostratigraphic zonation for the middle Miocene Barstow Formation based on the mammalian megafauna is presented. Four biostratigraphic zones are outlined, two containing fossil assemblages of early Barstovian (Ba1) age, and two with late Barstovian (Ba2) assemblages. Recommendations are made for defining and characterizing the Barstovian North American Land Mammal Age (NALMA). The Ba1 biochron is defined based on the first occurrence of Plithocyon, and the base of the Ba2 biochron is revised based on the first appearance of the antilocaprid Ramoceros. The ursid Plithocyon remains a valid defining taxon for the base of the Barstovian NALMA. The use of both gomphotheriid and mammutid proboscideans to define the base of the Ba1 or Ba2 is abandoned due to the diachrony of first appearances across North America. The Ba1 biochron is additionally characterized based on the first appearance of the equid Scaphohippus. The base of the Ba2 biochron is revised based on the first appearance of Ramoceros. The Ba2 can also be characterized by the first appearance of the borophagine canid Protepicyon and the anchitherine equid Megahippus.
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