United by chewing: Hunter-Schreger band-like pattern and wavy enamel in a fossil crocodile suggest functional convergence with mammals and dinosaurs

Tooth enamel of most mammals shows alternating light-dark bands, called Hunter-Schreger bands (HSB), in longitudinal sections caused by a decussating arrangement of bundles of prisms, the unit building blocks of mammalian enamel. HSB is thought to increase resistance to abrasive food and mitigate crack propagation and hence is considered a mammalian adaptation to high-efficiency mastication. Here we report for the first time the presence of HSB-like features in the tooth enamel of a non-mammalian amniote, Iharkutosuchus, an extinct herbivorous crocodile with strong heterodonty and a unique chewing mechanism. Lacking mammal-like decussating prisms as revealed by traditional visualization techniques, the enigmatic nature of this HSB-like enamel pattern in Iharkutosuchus was only explainable using X-ray diffraction computed tomography which showed its purely crystallographic origin. Wavy enamel, a well-known structure in herbivorous ornithopod dinosaurs with shearing-type mastication, is also demonstrated in Iharkutosuchus. The unexpected finding of both enamel features in this herbivorous crocodile speaks for their role in high-efficiency chewing. However, the profoundly different structural background of mammalian and crocodilian HSB demonstrated here and the phylogenetic distribution of both HSB and wavy enamel imply nanostructure-scale convergences highlighting the importance of mastication-related challenges in driving dentary evolution of amniotes.