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Mechanical and Fracture Properties Evolution of Gnathostome Dental Enamel and Orthodentine

During gnathostome diversification modifications to dental form and mineralized tissues (enamel, dentines, and cementum) facilitated the exploitation of novel food resources, leading to the occupation of an unprecedented variety of ecological trophic niches. Generally, it has been assumed that the intra‐tissue biomechanics of these constituents had little bearing on whole‐tooth functionality, aside from mammalian enamel in occluding dentitions. Many mammals, for example, possess teeth that self‐wear to a functional topography with a diversity of derived tissues—some which possess unique mechanical attributes to resist wear and fracture. Here we formally test the hypothesis that gnathostome dental tissue material properties were static prior to the cladogenesis of Mammalia. Hardness (a proxy for wear resistance) and elastic modulus (proxy for structural rigidity and relevant to whole tooth rigidity) were tested using two standardized material science techniques, microindentation and nanoindentation, as well as a novel approach for quantifying fracture propagation patterns from indentation cracks. These data were analyzed in an ecological context using modern phylogenetic comparative methods. The results show these material properties to be highly variable within and between major groups of gnathostomes, with enamel and orthodentine hardnesses ranging from ~2.1 gigapascals (GPa) to ~6.1 GPa and ~0.5 GPa to ~1.5 Gpa, respectively. Elastic modulus values measured spanned ~34.6 GPa to ~112.5 Gpa in enamel, and from ~15.6 GPa to ~33.4 GPa in orthodentine. Lissamphibia display the lowest hardness and elastic modulus values of both enamel and orthodentine while the higher hardness and elastic modulus values are found in specific taxa from chondrichthyans, mammals, and some squamates. Aside from enamel hardness, there is no significant relationship between most material properties and assigned dietary classes. An ancillary goal of this work is also to glean initial insights about how dental attributes for non‐mammalian and mammalian taxa more generally may contribute to whole‐tooth form, function, performance, and diet. Clade‐specific complex fracture patterns in the enamels of mammals and chondrichthyans, for example, show that gnathostome lineages independently evolved traits that function to control fracture and minimize damage associated with catastrophic failure. Overall, this study suggests that selection operated at the tissue level primarily in enamel to bring about shifts in whole‐tooth functionality across Gnathostomata.