Micromammal Teeth Reflect Megamammal Extinction Ecology

As humans radiated across the Americas, we restructured New World ecosystems by driving 100+ ecologically influential megafaunal species extinct in the terminal Pleistocene extinction (TPE) ~11,500 years ago. Using the exceptional fossil record at Hall\u27s Cave in Texas, we study the last 22,000 years of surviving micromammal community composition and ecology to characterize long-term ecosystem response to climate change and extinction. Small mammals like mice that survived mammoths\u27 extinction represent a near-ideal study system due to their well-understood ecology, numerical abundance across strata, small home ranges recording local conditions, and short generation times promoting observable evolutionary change in response to environmental perturbations. We employ several paleoecological proxies to reconstruct the TPE\u27s effect on body size, isotopic niche, and dental ecomorphology for several micromammal taxa. We estimated body size from dental measurements, we microCT-scanned fossils to infer ecomorphology, and we collected bone collagen carbon (δ13C) and nitrogen (δ15N) isotopes tracing chemical dietary niche across the Hall\u27s Cave record. Extracting each proxy from the same fossil, we evaluate ecological shifts in response to climate change since the last ice age and extinction of Pleistocene megafauna. Following Bergmann\u27s rule, we predict that micromammal body size was larger in colder, Pleistocene climates and smaller in warmer, Holocene conditions, but we also predict TPE-driven ecological niche shift. Several micromammal taxa (Microtus voles and Neotoma woodrats) appear to follow Bergmann\u27s rule and track climate pre/post-extinction. Others (Chaetodipus pocket mice, Onychomys grasshopper mice, Peromyscus deermice, and Reithrodontomys harvest mice) respond inversely across the extinction horizon, growing significantly larger immediately post-extinction during a warming period suggesting TPE-related selection pressures overwhelmed climate effects on body size. We also find TPE-coincident niche shifts, with Reithrodontomys and Peromyscus shifting to exploit more C4 grass resources post-extinction mirroring similar expansion in deer and bison. Post-extinction Onychomys nearly doubled in size (relative to modern and pre-extinction sizes) while shifting in isotopic niche space to occupy a solely carnivorous trophic level. Preliminary results also reveal greater resource partitioning pre- and immediately post-extinction than modern micromammal communities. Thus our work quantifies longitudinal extinction and climatic ecosystem effects to provoke policy action and inform current conservation