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 and dark bands, called Hunter-Schreger bands (HSB), in longitudinal sections caused by decussating bundles of prisms, the unit building blocks of mammalian enamel. HSB are thought to increase resistance to abrasive food and mitigate crack propagation and hence are considered a mammalian adaptation to high-efficiency mastication. Using traditional microscopy techniques as well as X-ray diffraction computed tomography (XRD-CT), 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. XRD-CT showed that the enigmatic HSB-like pattern in Iharkutosuchus enamel, which lacks mammal-like decussating prisms, has a purely crystallographic origin. Iharkutosuchus teeth also exhibit wavy enamel, a well-known structure in herbivorous ornithopod dinosaurs with shearing-type mastication. 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 indicate nanostructure-scale convergences, highlighting the importance of mastication-related challenges in driving dental evolution of amniotes.