Paleobiological Studies on the Early Eocene Equid Hyracotherium (Perissodactyla, Mammalia) from the Clarks Fork and Northwestern Bighorn Basins, Wyoming.

Abstract

The early Eocene equid Hyracotherium, the first North American horse, is an ideal taxon for investigating links between mammalian evolution and climatic/environmental change due its dense fossil record and the high-resolution stratigraphic and paleoenvironmental framework developed in the Clarks Fork and Bighorn Basins. Documentation of dental and post-cranial morphological time series is necessary to understand how these early horses evolved in response to changing environmental conditions during the early Eocene. A new, nearly complete skeleton of Hyracotherium grangeri (UM115547), including a well-preserved vertebral column, provides a foundation for understanding the post-cranial evolution of early horses. The posterior thorax and lumbus of UM115547 are divided into facultatively and obligately dorsostable regions, allowing for dynamic positioning of the center of mass during acceleration and maximizing energetic efficiency of vertebral stabilization during locomotion, respectively. The wide range of movement allowed at the hip and shoulder joints contrasts with the restricted parasagittal movement at distal limb articulations, representing an evolutionary compromise between energetic efficiency and maneuverability in the complicated terrain of Hyracotherium’s habitat. Dental wear can potentially obscure evolutionary patterns in dental morphology and limit sample sizes available for tests of evolutionary dynamics. Simulated morphological time series, in which teeth with various amounts of dental wear were sampled, show that dental wear imposes a time-independent source of variation that asymmetrically hinders the ability to detect directional trends. To mitigate this asymmetric bias, continuous, quantitative indices of dental wear were used to predict unworn dental morphologies to mitigate this asymmetric bias. Time series of Hyracotherium dental morphology over a ~1.5 million year interval exhibit heterogeneous evolutionary dynamics in size (i.e. noncontemporary directional change and stasis) in contrast to homogeneous evolutionary stasis in dental shape. Lower third molar (m3) lingual shape represents an exception in which a shift in optimal morphology occurs between two segments of evolutionary stasis. Coordinated shifts in evolutionary dynamics indicate three intervals of changing selective pressures requiring further investigation. An adaptive hypothesis involving decreasing atmospheric pCO2 and changing proportions of fruit and foliage in the diet of Hyracotherium is discussed to explain correlations between stable carbon isotope records and dental time series.