Fracture Property Experimentation On Hadrosaurid Dinosaur Wavy Enamel Reveals Energy‐Robbing Crack Deflection and Channeling To Localize Damage: A Rare Case of Mammalian‐Like Dental Sophistication In Reptiles

Reptiles rarely approached the biomechanical sophistication of feeding or dietary diversity seen in mammals. Their teeth are typically non‐occluding, semi‐conical structures with simplistic parallel‐crystalite enamel surrounding an orthodentine core. Conversely, most mammals possess multi‐cusped teeth that are drawn across one another during mastication and self‐wear to their functional morphology. Mammalian enamel is complex ‐‐ a fiberglass‐like composite composed of bundles of hydroxyapatite crystals known as prims, surrounded by compliant proteinacous sheaths. Among the most sophisticated prism architectures is the modified radial enamel (MRE) of grazing ungulates whose coarse tooth surfaces enable grinding of tough, abrasive‐laden plant matter. MRE conveys exceptional fracture toughness and controlled fracture propagation minimizing damage to the brittle enamel crests. Hadrosaurid (duck‐billed dinosaurs) are notable in independently evolving self‐wearing grinding dentitions and complex wavy enamel (WE) composed of folded layers of hydroxyapatite crystals whose biomechanical import is unknown. We tested the hypothesis that WE served an analogous role to ungulate MRE by: 1) tribologically modeling the effects on hadrosaurid occlusal surfaces should a section of enamel become fractured; 2) introducing enamel fractures to the teeth of hadrosaurids, outgroups lacking WE, and horses and bison using Vickers‐tipped indenters; 3) testing for controlled‐crack propagation using Watson's Two‐Sample Test of Homogeneity; and 4) contrasting the results in comparative ecological contexts. Removal of enamel sections leads to aberrant self‐wear to hadrosaurid chewing pavements (= “wave mouth” in equine veterinary science) inhibiting functionality. Non‐WE enamels show isotropic fracturing and catastrophic damage to the enamel shells. WE and MRE three‐dimensionally limit damage in enamel crests through energy‐robbing crack deflection at kinks in the enamel fabrics or prism boundaries and channeling perpendicular to the enamel‐dentine junction. Hadrosaurid WE represents an alternative reptilian solution for damage resistance in grazing taxa, one that evolved millions of years earlier than in mammalian ungulates. Our findings add to a growing body of evidence that material properties are preserved in fossil teeth and can be used to explore dental form and function throughout the fossil record and to develop biologically‐inspired industrial materials. Support or Funding Information NSF EAR 0959029 awarded to GME