Chapter 11

Abstract Studies of fossil remains of age-diagnostic land mammals have allowed recognition of assemblages of late Uintan, early and late Arikareean, and early Hemingfordian (late middle Eocene, early Oligocene to early Miocene) age in the Sespe Formation and equivalent marine formations of the northern Peninsular Ranges Provinces and western Transverse Ranges in Los Angeles, Orange, Santa Barbara, and Ventura counties, southern California. Recent fossil recovery efforts have resulted in the recognition of new land mammal assemblages in the Santa Ana Mountains and San Joaquin Hills of Orange County, the Santa Monica Mountains of Los Angeles County, and Simi Valley, Ventura County. The late Arikareean fauna appears to represent a new assemblage that has not been recognized previously in the fossil land mammal record of southern California. The presence of a late Uintan assemblage near the base of the undifferentiated Sespe and Vaqueros formations (S/V) in the northern Santa Ana Mountains of Orange County suggests that the base of the unit is similar in age to the base of the Sespe Formation in the Simi Valley area. The top of the S/V in the northern Santa Ana Mountains and San Joaquin Hills of Orange County and in the Santa Monica Mountains of Los Angeles County is approximately 10.4 million years younger than the top the Sespe Formation in the Simi Valley area, at South Mountain, and along Oak Ridge in Ventura County. In the northern Santa Ana Mountains and the Santa Monica Mountains, early Hemingfordian land mammal assemblages occur stratigraphically below late Hemingfordian land mammal assemblages in the overlying marine and continental Topanga Formation.