Perissodactyla from the Sespe of the Las Posas Hills, California

Abstract

The concentration of fossil vertebrate remains at the Kew Quarry site in the Sespe deposits of the Las Posas Hills, California, furnishes one of the most varied early Tertiary mammalian faunas as yet recorded from western North America. Related in age to the John Day and upper White River, this assemblage offers for the first time an adequate basis for comparing the late Oligocene mammalian history of the Pacific Coast marine province with that determined in the Tertiary continental provinces to the east of the Cascade Range and of the Rocky Mountains. In order to reach a fuller understanding of the stage of evolution of the Kew Quarry fauna the structural features of individual members need now to be considered and the relationships of these forms determined in greater detail than was attempted in the original statement. Brief descriptions and comparisons will be given, therefore, in this and in subsequent papers. Among the more commonly occurring mammals in the Kew Quarry fauna are the rhinoceroses of the genus Subhyracodon (Caenopus). This type is represented by many parts of skeletons in the collections of the California Institute of Technology. In contrast, the horses are known at present by a single individual. The material, though scanty, is clearly referable to the genus Miohippus. While furnishing an opportunity to compare the stage of development of the Equidae in the Sespe with that found in the John Day beds of eastern Oregon and in the White River of the western Great Plains, added interest attaches to this specimen as the first documentary evidence of the occurrence of the genus in the Pacific Coast region of North America.

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