THE IMPLICATIONS OF THE ABSENCE OF HORSES (EQUUS) FROM A LATE PLEISTOCENE DEPOSIT AT LAKE ELSINORE, RIVERSIDE COUNTY, CALIFORNIA

Horses (Equus spp.) are a ubiquitous component of the Late Pleistocene megafauna of Southern California. The genus disappeared from California at or near the end of the Pleistocene along with most other megafaunal taxa. However, the order of disappearance of these taxa in Southern California has not been well documented.

Lake Elsinore is a naturally occurring lake in western Riverside County, California that has been inundated at various times extending back to the late Pleistocene. During the 2017-2019 excavations for the Summerly Homes Development project in Lake Elsinore, paleontological mitigation company VCS Environmental exposed Pleistocene lake deposits. These excavations yielded numerous fossils, including mammalian megafauna, microvertebrates, invertebrates, and wood. The recovered wood was radiometrically dated and yielded an age of 14,610 +/- 50 years before present (YBP).

Sixty-two identifiable individual bones belonging to large mammals were recovered from the site, with Camelops and Bison comprising the greatest percentages of the bones. Bison accounted for 50% of the material, 48.4% was attributed to Camelops, and the remainder to Mammut. Other material included small mammals, reptiles, birds, amphibians, fish, and mollusks, with all taxa consistent with other Late Pleistocene assemblages from Southern California. As the mitigation report noted, conspicuously absent are any fossils belonging to Equus.

The absence of Equus makes this site unusual among other Southern Californian Pleistocene fossiliferous deposits, at which the large herbivorous mammal faunal is between 15% and 39% Equus. Its absence from Lake Elsinore is likely not due to a sampling bias because of the number of well-preserved fossils recovered from the site. Its absence is also likely not due to habitat preference, as horse fossils are found in abundance across many different sites, from the Mojave desert to Rancho La Brea.

The absence of Equus could be perhaps attributed to its extirpation prior to the extinction of Mammut and Camelops and the extirpation of Bison from Southern California. The first evidence of human settlement in this region does not appear until around 12,000 YBP in the Mojave desert and around 8000-9000 YBP in the nearby Diamond Valley. The possibility of Equus extirpation prior to 14,610 +/- 50 YBP may indicate the beginning of megafaunal extinctions before the settlement of humans.