ISOTOPICALLY RECONSTRUCTING LATE PLEISTOCENE LANDSCAPE USE BY AN AMERICAN MASTODON

Evaluating landscape use and regional mobility from paleontological records has been hindered by a weak toolkit. Oxygen (δ¹⁸O) and strontium (⁸⁷Sr/⁸⁶Sr) isotope analyses may alleviate this constraint for terrestrial species. Oxygen isotopes track precipitation and show predictable patterns of variation across continents. Sr isotope ratios vary across landscapes as a function of bedrock composition and age. Organismal tissues incorporate this geographical fingerprint with little fractionation and reflect resources available during formation. A recently developed continental-scale model of Sr offers new opportunities to reconstruct movement patterns at fine spatial resolution. Here, we pair this new analytical resource with a serially sampled male mastodon tusk (late Pleistocene, NE Indiana; ISM 71.3.261) to establish the first high-resolution reconstruction of mobility and dispersal for a fossil individual. Using expectations from modern elephant ecology, we also test for geographic changes between when the bull was young (and part of a matriarchal herd) and mature (when he likely lived in greater isolation). We sampled five years from his early life and the three years just prior to death. Samples were milled in ~0.5 mm increments (averaging eight samples per year) and analyzed for O and Sr isotopes. Oxygen isotope data reveal strong cyclic patterns reflecting seasonal variability. The overall range in δ¹⁸O (~3‰) is essentially constant through life, which helps constrain models of regional movement. Sr isotope ratios from early in life display less evidence of annual cycles, but become highly cyclic with maturity. Sr isotopes also differ significantly between younger and older samples, indicating a geographic shift. Modeling indicates that the mature bull’s primary home range was likely in the Indiana/Ohio/Kentucky area, which aligns with where he was found. Earlier in life, he spent significant time in the Minnesota/Iowa area. This regional shift is greater than movements reported for modern elephants and offers perspective into the mobility of proboscideans when unconstrained by fragmented anthropogenic landscapes. Reconstructing individual mobility is now a realistic goal for paleobiology and offers new insight for understanding extinct species, particularly from the late Quaternary.