Locomotion on the edge: Structural properties of the third metacarpal in Thoroughbred and Quarter Horse racehorses and feral Assateague Island ponies

The osseous fusion patterns in an equine limb: A theoretical deconstruction of the evolutionary mechanisms

Adaptations in equine limbs, including fusion of its bones, such as the metapodials II and IV (splint bones) with metapodial III (cannon bone), have been debated. It is argued that cursorial adaptations in equine limbs with a progressive reduction in the number of digits lighten up the limbs and facilitate fast running. Similar fusions in other bones, such as the radius and ulna, tend to exhibit similar traits. For example, the fusion often occurs in a disto-proximal direction while leaving sharp ridges. This article proposes a model to understand the biomechanical advantages that a fused symmetric system of cannon and splint bones offers and tries to investigate osseous fusion from an evolutionary perspective. It demonstrated the uniqueness of this system from the context of symmetry and the presence of an array of parallel axes of rotation in equine limbs that amplifies the kinetic energy by maximizing the moment of inertia. The model highlighted the robustness of such an evolutionary mechanism in place and elucidated why it is a preferable construct. Furthermore, the discourse proposes a zipping action to describe the osseous fusion before making predictions on where the equine limb adaptations are perhaps heading.

Investigating the reliability of metapodials as taxonomic Indicators for Beringian horses

The metapodials of extinct horses have long been regarded as one of the most useful skeletal elements to determine taxonomic identity. However, recent research on both extant and extinct horses has revealed the possibility for plasticity in metapodial morphology, leading to notable variability within taxa. This calls into question the reliability of metapodials in species identification, particularly for species identified from fragmentary remains. Here, we use ten measurements of metapodials from 203 specimens of four Pleistocene horse species from eastern Beringia to test whether there are significant differences in metapodial morphology that support the presence of multiple species. We then reconstruct the body masses for every specimen to assess the range in body size within each species and determine whether species differ significantly from one another in mean body mass. We find that that taxonomic groups are based largely on the overall size of the metapodial, and that all metapodial measurements are highly autocorrelated. We also find that mean body mass differs significantly among most, but not all, species. We suggest that metapodial measurements are unreliable taxonomic indicators for Beringian horses given evidence for plasticity in metapodial morphology and their clear reflection of differences in body mass. We recommend future studies use more reliable indicators of taxonomy to identify Beringian horse species, particularly from localities from which fossils of several species have been recovered.