Meeting Abstract
Little is known about the evolutionary history of vertebrate venom delivery systems owing to a sparse record of fossils preserving aspects of this morphology. Previously, only four Mesozoic reptile records existed: Sinornithosaurus millenii (Early Cretaceous: China), Sphenovipera jimmysjoyi (Early Jurassic: Mexico), Uatchitidon kroehleri (Late Triassic: Virginia, USA), and Uatchitidon schneideri (Late Triassic: North Carolina and Arizona, USA). Here we present a new, likely venomous, reptile collected from a microvertebrate-bearing horizon (~214 Ma; Norian age) in the Upper Triassic Chinle Formation of Arizona. This new taxon is represented by a fragmentary dentary bearing three conical, labiolingually-compressed teeth with mesially curved tips and ankylothecodont attachment. Each tooth bears a lingual and labial groove that extends from the base of the tooth to the apex. Computed tomographic data reveal foramina in the bone tissue at the base of each groove on either side that connect to internal cavities within the dentary. We interpret this morphology as an envenomation apparatus of venom glands within the dentary connected to venom-conducting grooves on the teeth, strongly convergent on the morphology of the extant squamate Heloderma. This new taxon is one of the oldest venomous reptiles and one of the only examples of a complete envenomation apparatus in the fossil record. The occurrence of the new taxon and records of U. schneideri in microvertebrate assemblages in the Upper Triassic of North America show that venomous reptiles were a common component of the vertebrate ecosystem in equatorial Pangaea.
