Dietary ecology of ungulates from the La Brea tar pits in southern California: A multi-proxy approach

The La Brea Tar Pits are a highly fossiliferous late Pleistocene locality, with over 230 vertebrate species, and the potential to help clarify the ecology of Pleistocene megafauna prior to their extinction. Here, we analyze dental microwear textures, dental mesowear, and stable carbon isotopes to assess the dietary behavior of the three most abundant ungulates at La Brea. We examined Bison antiquus, Camelops hesternus, and Equus occidentalis teeth from pits spanning the latest Pleistocene, focusing on pit 77 (~ 35 Ka, glacial period), and pits 61/67 (~ 11.5 Ka, interglacial period). Dental microwear attributes demonstrate that B. antiquus and E. occidentalis consumed foods with similar textural properties to those preferred by mixed feeding and browsing bovids, as well as significantly different food textures than the diet of modern bison and horses. Camelops hesternus has attributes most similar to modern browsing bovids and giraffes, revealing that all taxa studied likely consumed at least some woody browse. Complexity values (Asfc) in B. antiquus and E. occidentalis are significantly greater at pit 77 than at pits 61/67 and modern populations, suggesting a dietary decrease in woody vegetation and shrubs from the glacial to interglacial period. Dental mesowear values further indicate that B. antiquus and C. hesternus consumed woody vegetation, whereas E. occidentalis grazed. The different dietary interpretation for E. occidentalis may reflect a discrepancy between its average diet inferred from mesowear and its most recent diet inferred from microwear. Lastly, stable carbon isotopes reveal diets composed mostly of C3 vegetation, with B. antiquus incorporating significantly more C4 vegetation than either of the other dominant herbivores during the accumulation of both assemblages. Collectively, these data suggest that the dietary ecology of the dominant La Brea herbivores was more nuanced than previously thought, shifting coincident with changing climates, and not necessarily predictable from morphology or any one proxy method.