SMALL MAMMALS CAN TELL BIG STORIES: MESOCARNIVORE INSIGHTS ON RANCHO LA BREA

While North American mammal fossil history is dominated by large carnivores and prey, most carnivores today are small (“mesocarnivores”): a transition that followed a combination of climatic change, megafaunal extinctions, and human disturbance at the end of the last Ice Age. Here, we evaluate response to these intersecting major impacts in the late Pleistocene to Holocene mesocarnivore fauna at the Rancho La Brea (RLB) asphalt seeps in Los Angeles, California, USA. Excluding coyotes, RLB preserves five mesocarnivore species each represented by >=10 individuals: American badger (Taxidea taxus), bobcat (Lynx rufus), grey fox (Urocyon cinereoargenteus), striped skunk (Mephitis mephitis), and long-tailed weasel (Mustela frenata). Fossilized in 34 asphaltic deposits spanning 55,000 – 200 years ago, these five species continue to inhabit the area today. In multiple ecologically relevant linear traits, RLB specimens differ significantly from historic (<100 years old) representatives of the same species from the same geographic area. Fossil mesocarnivores were generally larger—decreasing in body size through time by as much as 27%—and exhibited greater tendencies for carnivory, perhaps due to competition presented by megafaunal predators. Species-specific differences in postcranial morphology highlight shifts in locomotor ability, potentially reflecting environmental changes. A preliminary radiocarbon chronology indicates that not all species inhabited RLB at the same time; rather, almost all specimens of bobcats and grey foxes date from before the Last Glacial Maximum, while badgers seem to have been a latest Pleistocene addition. Further, mesocarnivore dates do not always align with megafaunal dates from the same deposits, suggesting differential entrapment likely stemming from differential body size and natural history. Preliminary results of stable isotope analyses of carbon and nitrogen appear to show resource-use shifts into the modern-day, although these shifts may be driven by taxonomic turnover rather than within-species dietary change. Leveraging biogeochemical and ecomorphological techniques, this multi-proxy approach provides complementary metrics for diet and resource use, illuminating the roots of today’s mesocarnivore fauna in the last Ice Age.