The histology of rhynchosaur (Diapsida, Archosauromorpha) ankylothecodonty

The study of the connection between the teeth and the jaw is important for understanding the palaeobiology of vertebrates, but inconsistent terminology and incomplete sampling have made it difficult to assess the evolutionary significance of some of the related characters. Among archosauromorphs, tooth attachment in dinosaurs and crocodylians is nearly identical to that of mammals in featuring a ligamentous connection (gomphosis), whereas closely related forms appear to have teeth fused to the jaws (ankylosis), as in most other amniotes. Hence, studying tooth attachment of stem-archosaurs is pivotal to characterize the main shifts in tooth attachment seen in the lineage. Here, we analyze the tooth attachment of rhynchosaurs - a group of quadrupedal herbivorous archosauromorphs that played a key role as primary consumers in many Triassic communities. Their dentition consists of multiple rows of marginal teeth with posterolingual addition of teeth during growth, but their tooth attachment has not been documented in a modern context. Histological data from three rhynchosaur specimens from the Middle Triassic Manda Beds of Tanzania show that, although ankylosed, rhynchosaur teeth are surrounded by an extensive network of Sharpey's fibers, layers of cementum, and well-defined zones of alveolar bone. What has been previously described as "spongy bone of attachment" in fact encompasses the same attachment tissues present in mammals, dinosaurs, and crocodylians, albeit completely mineralized in mature teeth. Analysis of different stages of tooth development shows that ankylosis occurs by the growth of alveolar bone towards the cellular cementum, which eventually mineralizes the soft ligament. This suggests that the tissues conflated as "bone of attachment"-alveolar bone, periodontal ligament, and cellular cementum-are homologous across Archosauromorpha. Our data add to a growing body of evidence that heterochronic changes to the timing and extents of mineralization, not convergent evolution to mammal-like attachment tissues, led to the independent evolution of gomphosis across many amniote lineages, including archosauromorphs.