FIRST RECORD OF EQUUS OCCIDENTALIS FROM ORANGE COUNTY, CALIFORNIA, WITH IMPLICATIONS FOR THE LATE PLEISTOCENE DISTRIBUTION OF EQUUS IN THE AMERICAN SOUTHWEST

Horses (genus Equus) are common in Pleistocene faunas throughout North America. Despite this abundance, the number of species occurring remains unresolved. Molecular data from fossils has been employed to suggest no more than two species, one stout-limbed and the other stilt-legged, while morphology-based paleontological investigations propose multiple species.

In the late Pleistocene, two morphospecies of large stout-limbed horses have been documented from the American southwest: Equus occidentalis from California, west of the Sierra Nevadas and south-southwest of the Transverse Ranges, and Equus scotti from the central Mojave Desert. These species are readily distinguished dentally; E. scotti possesses lower incisor infundibula, while E. occidentalis lacks them. The presence of two large, stout-limbed equine morphs in the southwest, with no evident geographic overlap, suggests (in combination with fossils of small horses) that multiple horse species inhabited the region during the late Pleistocene. However, fully diagnostic remains of E. occidentalis and/or E. scotti are known from only five localities in the region. Assessing whether the hypothesized distribution of these two large horse morphs is real, or instead is an artifact of the dearth of diagnostic material, therefore requires additional fossils.

To test this hypothesis, we examined a previously undescribed late Pleistocene horse from San Juan Capistrano, Orange County, California. Remains include a skull, mandible, articulated right hind leg, and other postcrania recovered from nonmarine terrace deposits that also yielded remains of Bison, an index taxon for the late Pleistocene. Metric data confirm that this is a large species of horse; the metapodials are stout rather than slender, while the lower incisors lack infundibula. These characters demonstrate clear affinity with Equus occidentalis. Based upon size and morphology, this is the first confirmed record of E. occidentalis from Orange County.

The presence of Equus occidentalis in this region accords well with the hypothesized distribution of late Pleistocene large stout-limbed Equus in the American southwest. Our results indicate that multiple horse species inhabited North America during the late Pleistocene.