Meeting Abstract
The morphological modifications that equid ungulates underwent during the Miocene climatic and floral changes represent a classic evolutionary story. During their radiation they showed increases in cursoriality, body size, hypsodonty, and occlusal enamel complexity. Enamel microstructure changes accompanied the dental modifications. Browser-frugivore Hyracotherium displays primitive horizontal Hunter-Schreger bands (HSB). The cladogenesis of Parahippus-grade mixed-feeding horses show derived modified radial enamel (MRE), a microstructure retained in subsequent lineages. The changes in diets led to greater ingestion of small, hard particulates in grazers and body size increases that promoted enamel degradation. We posit that MRE evolved to counter these wear and fracture promoting agents. Sampling from nine equid genera, we use instrumented Vicker’s microindentation to determine hardness and promote controlled fracture to study crack channeling. Hardness measurements show a gradient across the enamel, with increased hardness toward the outer enamel. However, there is no clear trend of increased hardness across taxa. On the other hand, we find evidence of increased crack channeling within the MRE. Perfuse indentation fracture follows intersprismatic rows and travels normal to the enamel-dentine junction. Fracture path in the HSB mid-enamel displays a bimodal distribution as cracks are channeled along prism boundaries. Outer radial enamel exhibits a reduced degree of fracture control. Results were further corroborated by neontologic data. MRE is a wear and fracture resistant microstructure that inhibits enamel damage by localizing damage and preventing microcrack coalescence that evolved in equids in response to dietary and loading changes. The development of biologically-inspired ceramics may result.
