A review of tooth implantation among rhynchocephalians (lepidosauria)

Abstract

Acrodont dental implantation is widely considered an important character for referring fossil material to Rhynchocephalia. Under its purest definition, acrodonty involves teeth being attached to the crest of the marginal bones without roots. A similar mode of tooth attachment is known in a variety of other reptile groups including some squamates and procolophonids. There is a lack of consensus on the definition of acrodont, how best to characterize tooth implantation, and the relationship between implantation and tooth replacement. Rhynchocephalians already are known to demonstrate variation in their mode of tooth attachment. Unambiguous acrodonty associated with little or no tooth replacement has been associated with Sphenodon, but it appears to have been the most widespread condition for much of the Mesozoic. A form of pleurodonty, where teeth are attached to the inside of the jaw bone with shallow roots, appears to be the plesiomorphic condition for both Lepidosauria and Rhynchocephalia. Jaws with anterior pleurodont teeth and posterior acrodont teeth appear to have been common for early rhynchocephalians in the Triassic, and Ankylosphenodon from the early Cretaceous of Mexico demonstrates that at least some later rhynchocephalians possessed continually replacing dentition, but identification of this trait requires inspection of internal anatomy. When cross-sections of teeth are unavailable or the lingual view of jaws is obscured, one cannot be 100% confident of acrodont implantation, and “acrodonty” should not be used as a single character to refer incomplete jaw material to Rhynchocephalia. Tooth implantation is a component that was highly variable in a once-diverse reptile group.