Forty-three species of mammals are known at present from the McKittrick tar seeps, in addition to a larger number of bird species and a smaller number of plant types. In the McKittrick fossil assemblage Recent or still living forms are more abundant than extinct types. Since at Rancho La Brea the reverse is generally true, it appears that McKittrick is a somewhat later accumulation. The interval does not appear to be greater, however, than that separating a glacial and interglacial epoch.Many lines of evidence indicate that Rancho La Brea dates from the late rather than early Pleistocene, and there seems to be good reason for believing that this deposit is of Sangamon age. The McKittrick assemblage thus appears to be referable to the Wisconsin, or last glacial epoch.
Of the 49 mammalian species known at Rancho La Brea only 21 are found also at McKittrick. In view of the rather marked specific differences still existing between the faunas of the Los Angeles Basin and San Joaquin Valley, it seems reasonable to assume that a large part of the difference between the two faunas is due to ecology rather than to a time factor. In addition, environmental conditions surrounding the tar seeps at the two localities do not seen to have been exactly alike and some of the faunal differences may be due to this cause.
Judging from evidence of the rodents and plants, the late Pleistocene climate of the San Joaquin Valley was not greatly different from conditions still prevailing in the area. A possible explanation is that the Coast Ranges then as now prevented free passage of moisture-laden winds over the area.
