Linear enamel hypoplasia as a proxy for environmental stress in fossil apes

Abstract

The Anthropocene, our current geological era, is defined by increased anthropogenic environmental alteration that is reducing great ape populations globally. As this occurs, there becomes an increased need to not only understand how great apes are being impacted by the changing environment, but how they will respond to this change. The focus of this research is to use the dental developmental defect Linear Enamel Hypoplasia in the Late-Miocene great ape <i>Hispanopithecus crusafonti</i> to reconstruct Late-Miocene seasonality. These patterns are then analyzed in relation to previous research on Late-Miocene climate to understand how <i>H. crusafonti</i> responded to increasingly variable seasonality. The Late-Miocene, similarly to the Anthropocene, faced climatic upheaval that resulted in the extinction of many great ape species. Seasonal bouts of physiological stress are expressed through enamel depressions in fossil teeth. Through microscopic analysis and reconstruction of the duration of annual stress intervals, the irregularity of Late-Miocene seasonality is identified. These findings are applied to a comparison with the extant great ape, <i>Pongo</i>, who expresses many of the same specialized characteristics as <i>H. crusafonti</i>; features like morphology, diet, and ecology are influenced by their similar warm and tropical forest habitats and are therefore reflected in both species. It was determined that the primary factors influencing the decline of both fossil and extant great apes is the frequency of stress events and great ape specialization, which limits adaptability during seasonal and environmental variability. This constrains them and bases their survival on specific environmental conditions that are not conducive to the rapidly changing climate of the Anthropocene. This is likely what led to the extinction of <i>H. crusafonti</i> and what is currently severely decreasing orangutan and great ape populations.