STABLE ISOTOPE ECOLOGY AND DIETARY MODELING OF MAMMALIAN MEGAFAUNA FROM THE MIDDLE IRVINGTONIAN FAIRMEAD LANDFILL LOCALITY, CHOWCHILLA, CA

Previous paleodietary studies of California megafauna have focused primarily on the coastal late Pleistocene Rancho La Brea tar seeps, while Irvingtonian sites remain understudied. We report δ¹³C and δ¹⁸O values measured from tooth enamel of mammalian megafauna from the mid Irvingtonian (.78-.55 Ma) Fairmead Landfill site, near Chowchilla. The assemblage is dominated by large herbivores, with Equus, Camelops, Mammuthus and Paramylodon the most prominent. Ten carnivore taxa are also present including Canis dirus, Canis latrans, Smilodon, Homotherium, and Miracinonyx.

We used stable isotope values of carnivore and herbivore tooth enamel to investigate paleodietary and paleoecological patterns. δ¹³C values of tooth enamel are controlled by diet and can be used to investigate resource usage and partitioning among and between taxa. In large mammals δ¹⁸O ratios are controlled by drinking water, and by proxy, climate. Ontogenetic serial tooth δ¹⁸O and δ¹³C reveal seasonal trends in resource usage, from which we can infer patterns of forage availability.

Fairmead Landfill δ¹³C values are consistent with an environment dominated by C₃ vegetation. Platygonus and Camelops had the lowest and highest δ¹³C, respectively. Camelops exhibited the most variable δ¹³C among herbivores, though several other taxa are represented by single individuals, thus lacking dietary ranges. Among carnivores, Homotherium had the lowest δ¹³C values (-13.5‰) and Canis latrans the highest (-10.7‰). In addition to population level analysis (ANOVA, t-tests) we also used the Bayesian mixing model Stable Isotope Analysis in R (SIAR) to model carnivore diets against potential herbivore prey. Both canids and felids show generalist dietary patterns, however felids on average consumed Mammuthus than Canis dirus.

We serially sampled teeth from four taxa to investigate seasonality and temporal resource usage among megafauna. Equus, Camelops and Tetrameryx show little variation in δ¹³C suggesting little turnover in Fairmead’s floral community, on a seasonal scale. We also report a serial sampled Homotherium canine and a calculated growth rate of 3.4 mm/year; a growth rate intermediate between Smilodon and modern pantherine felids.