Possible evidence of a venom apparatus in a Middle Jurassic Sphenodontian from the Huizachal red beds of Tamaulipas, México

ABSTRACT

A new sphenodontid is described from the Middle Jurassic red beds in Huizachal Canyon, Tamaulipas, northern Mexico. The new taxon is characterized by the presence of two fang-like acrodont teeth at the anterior end of the lower jaw similar to those of the upper Jurassic sphenodontian Theretairus antiquus from the Morrison Formation. However, the new sphenodontian has a single well established groove on the anteromedial surface of each fang, here interpreted as a venom conducting groove convergent with those of opisthoglyphous colubrids and helodermatid lizards. The presence of a complex venom apparatus is supported by the reduction of the postcoronoid portion of the lower jaw to only 30% of the total jaw length and the anterior displacement of the jaw articulation by an unusual, medially inflected condyle. The shortening of the postcoronoid portion of the jaw reduces the estimated jaw moment-arm, trading bite strength for speed; simultaneously the mouth gape is increased by an additional 100% compared to other sphenodontians. Both modifications are necessary in an akinetic skull to facilitate venom injection to the victims. The phylogenetic relationships of the new sphenodontian are uncertain although the presence of propalinal jaw action and caniniform teeth suggests close relationship with Theretairus, Cynosphenodon, and Sphenodon. The transformation series leading to the presence of one or two caniniform teeth can be reconstructed either as an independent origin for both conditions or as an ordered transformation from one to two caniniform teeth. The new sphenodontian adds to the growing list of unusual forms and corroborates the view that Rhynchocephalia were once a very diverse group.