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Throughout vertebrate evolution, a number of lineages evolved dental occlusion, whereby the contact faces of the teeth self-wear to their functional morphology. It has been shown that in mammals, increases in dental complexity accompany such changes. These presumably allowed for modifications in biomechanical form, function and performance relevant to dietary ecology. Recently, it was shown that a lineage of reptiles, the Hadrosauridae, evolved some of the most architecturally sophisticated teeth known, in association with their acquisition of a grinding dental unit. Independently, another lineage of ornithischian dinosaurs, the Ceratopsia, evolved dental occlusion in the form of slicing cheek teeth. Here, I have tested the hypothesis that ceratopsian teeth increased in complexity in association with their evolution of shearing. Transverse and occlusal plane histological sections were made using cheek teeth from representative Ornithischia spanning the transformation series leading to the evolution of slicing in ceratopsians. The sections were viewed with dissecting and polarizing light microscopy. The microstructure was described and phylogenetically character-mapped in association with whole tooth and wear facet morphological attributes. My results show that ceratopsian teeth are considerably more complex than those of the outgroup ornithischians in possessing four distinct tissues: enamel, orthodentine, coronal cementum, and vasodentine. Coronal cementum evolved in association with a cohesive dental battery and a shearing masticatory system in the common ancestor of Leptoceratops + Triceratops. Vasodentine appeared in the common ancestor of Protoceratops + Triceratops with the advent of high-angled slicing. These findings represent the second demonstration of complex dental architecture outside of Mammalia, and show that some reptiles rivaled, if not exceeded, most mammals in dental complexity. It also supports the hypothesis that complex histological attributes in teeth appear in association with precise dental occlusion.
