Unique Tooth Morphology and Prismatic Enamel in Late Cretaceous Sphenodontians from Argentina

Mammals and reptiles have evolved divergent adaptations for processing abrasive foods. Mammals have occluding, diphyodont dentitions with taller teeth (hypsodonty), more complex occlusal surfaces, continuous tooth eruption, and forms of prismatic enamel that prolong the functional life of each tooth [1, 2]. The evolution of prismatic enamel in particular was a key innovation that made individual teeth more resilient to abrasion in early mammals [2-4]. In contrast, reptiles typically have thin, non-prismatic enamel, and shearing, polyphyodont dentitions with multi-cusped or serrated tooth crowns, multiple tooth rows, rapid tooth replacement rates, or batteries made of hundreds of teeth [5-9]. However, there are rare cases where reptiles have evolved alternative solutions to cope with abrasive diets. Here, we show that the combined effects of herbivory and an ancestral loss of tooth replacement in a lineage of extinct herbivorous sphenodontians, distant relatives of the modern tuatara (Sphenodon punctatus) [10], are associated with the evolution of wear-resistant and highly complex teeth. Priosphenodon avelasi, an extinct sphenodontian from the Cretaceous of Argentina, possesses a unique cone-in-cone dentition with overlapping generations of teeth forming a densely packed tooth file. Each tooth is anchored to its predecessor via a rearrangement of dental tissues that results in a novel enamel-to-bone tooth attachment. Furthermore, the compound occlusal surfaces, thickened enamel, and the first report of prismatic enamel in a sphenodontian are convergent strategies with those in some mammals, challenging the perceived simplicity of acrodont dentitions [11-15] and showcasing the reptilian capacity to produce complex and unusual dentitions.