Helodermatid Lizard from the Miocene of Florida, the Evolution of the Dentary in Helodermatidae, and Comments on Dentary Morphology in Varanoidea

Abstract

A helodermatid dentary and osteoderms from the Miocene Thomas Farm locality in Gilchrist County, Florida, demonstrate the broad historical distribution of Helodermatidae in North America during the Tertiary. A comparative approach, taking into account variation in extant Heloderma, reveals that the dentary is intermediate in several characters between the plesiomorphically long, low, mediolaterally narrow dentary of Eurheloderma gallicum and the taller, wider dentary of extant Heloderma. An eminence upon the intramandibular septum in the Thomas Farm helodermatid and some extant Varanoidea is part of a previously unrecognized anatomical structure, a foramen formed by dentary and postdentary elements and normally covered by the splenial. The teeth of the Florida helodermatid are intermediate between the more numerous, shorter teeth of E. gallicum and the fewer, taller teeth of extant Heloderma. Certain dental structures whose evolution is here elucidated are part of the novel helodermatid venom delivery system. The evolution of the dentary and teeth in Varanoidea as a whole is a more complex and informative story than was previously appreciated. No living varanoid displays unmodified the primitive varanoid conical tooth morphology. Extant Heloderma show a dentition modified in association with venom delivery, whereas Varanidae show labiolingual compression of the tooth crown. Varanus in particular is highly modified and should not be used as a proxy for the varanoid ancestor in studies of the dentigerous elements and dentition. The informativeness of isolated squamate dentaries is evidenced by the derivation of 15 new phylogenetic characters from the anatomy of helodermatid dentaries and dentition.