Meeting Abstract
Biological hard materials show considerable promise for inspiring next generation high-performance ceramics and composites. Complex architectures in materials such as nacre and tooth enamel display high mineral volume fractions, yet possess exceptional fracture resistance instead of brittle behavior. Previous research has evaluated material properties of such tissues in only one or a few taxa, with little consideration of their phylogenetic history. Here, I develop a novel evolutionary framework for evaluating the material properties of hard biological materials and employ it to identify adaptation in response to new loading scenarios and mechanical demands. As a case study, changes in material response of complex enamel microstructures in mammalian dental enamel are investigated across two classic vertebrate dietary transitions: the evolution of 1) grazing in equid ungulates; and 2) durophagy in hyenids. Berkovitch nanoindentation and Vicker’s microindentation are used to measure enamel elastic modulus and hardness in living and fossil ingroup and outgroup taxa. A new method is created for quantifying preferred fracture propagation orientation within the tissues. In contrast to isotropic fracture in ancestral radial enamel, modified radial enamel in advanced equid genera channels fractures along interprismatic rows, inhibiting enamel spalling on enamel crests. Fracture response in hyenid zig-zag Hunter-Schreger bands (HSB) is complex, with crack arrest at HSB boundaries preserving whole tooth function; ancestral radial enamel does not contain structures for crack arrest. However, elastic modulus and hardness are similar across microstructures. Using this approach, microstructural adaptation can be linked to material response, providing new avenues for materials development.
