ASSESSING THE RELIABILITY OF STABLE ISOTOPES IN FOSSIL BONE: A UNIQUE CASE STUDY OF PREHISTORIC LUNG PATHOLOGY

Mammals at Ashfall Fossil Beds State Historical Park (NE Nebraska) offer a unique opportunity to test for the preservation of primary isotopic signatures in fossilized materials. At this site, large herbivores such as rhinoceroses, horses and camels are buried in volcanic ash sourced from an eruption of the Bruneau-Jarbridge caldera (Idaho/Nevada) ca. 11.8 million years ago. Most fossils from Ashfall display pathologic bone symptomatic of a lung disease (hypertrophic osteopathy or HPOA) likely related to inhalation of volcanic ash. In this study, we compare the stable oxygen isotopic composition (δ¹⁸O) of pathologic bone with that of normal cortical bone to determine if elevated body temperatures associated with HPOA can be reconstructed.

Our results show consistently lower δ¹⁸O values in pathologic bone, suggesting it may have formed at a higher body temperature. While the direction of offset between normal and pathologic bone (Δ¹⁸O_N-P) is consistent, the magnitude of the offset is variable and sometimes larger than can be explained solely by elevated body temperature. A change in body water δ¹⁸O related to physiology and/or HPOA could be an additional factor influencing Δ¹⁸O_N-P. For example, an increase in drinking water contribution to the body water reservoir could help to explain the observed δ¹⁸O offset for animals whose behavioral response to disease includes increased water consumption. This interpretation is supported by the high number of individuals concentrated in and around a shallow paleo-water body at Ashfall. Isotopic exchange during diagenesis may be another variable affecting the magnitude of Δ¹⁸O_N-P, and can be examined further through intra-individual δ¹⁸O comparisons of different skeletal tissues (e.g., enamel and bone, phosphate and carbonate) or through δ¹⁸O comparisons between different members of the Ashfall fauna.