PLESIPPUS IDAHOENSIS FROM THE EARLY PLEISTOCENE TULARE FORMATION, KERN COUNTY, CALIFORNIA

Outcrops of the early Pleistocene Tulare Formation in the Elk Hills, Kern County, California, are abundantly fossiliferous, and have yielded remains of a variety of terrestrial mammals and other vertebrates. The large mammal assemblage is dominated by fossils of horses, including teeth as well as postcrania. The majority of these fossils represent a large equid, here assigned to the early Pleistocene species Plesippus idahoensis (Merriam), 1918. A smaller plesippine horse is also present from the formation, but these remains are too incomplete to permit identification to species.

Plesippus idahoensis was originally named from two isolated teeth from the early Pleistocene Froman Ferry fauna, recovered from the Glenns Ferry Formation in southwestern Idaho. Subsequent discoveries of large horses in Idaho and Oregon have enabled a better understanding of the species. As presently defined, P. idahoensis is defined by large body size, frequent retention of the P1, moderately developed protocones with small anterior projections, “V”-shaped linguaflexids, and molar ectoflexids that penetrate the isthmus. Large horse fossils from the Tulare Formation in the Elk Hills are consistent with this definition.

The age of the fossils from the Tulare Formation is constrained biostratigraphically. The associated mammalian microfauna includes the rabbit Hypolagus furlongi and the rodents Sigmodon minor, Paraneotoma taylori, and Mimomys sp., indicating that the Elk Hills assemblage dates to the later Blancan North American Land Mammal Age (earliest Pleistocene). However, the presence of remains of Microtus californicus suggests that a younger faunal component is also present.

Previous unpublished records of horse fossils from the Tulare Formation include Equus calobatus and E. occidentalis. Examination of these fossils confirms that in fact the specimens are not diagnostic to species. At present, we recognize only two equids from the Tulare Formation, Plesippus idahoensis and a smaller indeterminate plesippine. P. idahoensis has also been reported from the late Pliocene – early Pleistocene San Timoteo Badlands, and may also have been present in similarly-aged deposits from Anza-Borrego. The species therefore appears to have enjoyed a broad geographic distribution in southern California in the earliest Pleistocene.