SMALL MAMMAL PALEOECOLOGICAL RESPONSE TO THE TERMINAL PLEISTOCENE MEGAFAUNAL EXTINCTION

The terminal Pleistocene extinction (TPE) of 100+ megafaunal species restructured New World ecosystems. We study the long-term ecological consequences of the TPE using a fine-grained age model and the exceptional small mammal fossil record at Hall’s Cave in central Texas. We employ multiple paleoecological proxies to characterize the TPE’s effects on body size, isotopic niche, and dental ecomorphology for 10 ecologically diverse small mammal taxa. We estimated body size from dental measurements, we microCT-scanned fossils to infer ecomorphology, and we calculated Bayesian isotopic niche ellipses from bone collagen carbon (δ¹³C) and nitrogen (δ¹⁵N) from 16 time bins spanning the present to ~22,000 cal BP. Extracting each proxy from the same individual specimens for each taxon and time bin, we evaluate ecological shifts and will ultimately assess paleoecology proxies alone and in combination. Current results reveal especially strong ecological differences relative to modern counterparts and pre- vs. post-extinction. In contrast to moderns, fossil Peromyscus deer mice occupied narrow C₃-dominant primary consumer niches, whereas both Reithrodontomys harvest mice and Sigmodon cotton rats shifted to more exclusively exploit C₄ isospace post-TPE. Post-extinction Onychomys grasshopper mice occupied solely secondary consumer niches in mixed C₃/C₄ isospace. Fossil Onychomys across all well-sampled Holocene and Pleistocene time bins averaged at least 25% larger (~30g) than modern museum specimen means (~23g), and body size positively correlated with higher δ¹⁵N values (7.4-12.5‰). Mean Onychomys mass spiked immediately post-TPE (9,692-11,010 cal BP), with some 45+g individuals significantly exceeding maximum modern mass (39g). Fossil Chaetodipus also averaged larger than moderns across all well-sampled bins and peaked in median and maximum mass (~25 and 38g) during the same period. Opposing predictions from Bergmann’s rule, these increases in body size occur during warming after the Younger Dryas and may represent response to extinction of Pleistocene megafauna. To characterize and constrain potential ecological response, we augment stable isotope and body size analyses with microCT-based ecomorphological proxies and by increasing small mammal community coverage across the Hall’s Cave record.