Opisthodontia (reptile)
Opisthodontia is a clade of extinct sphenodontian rhynchocephalians.[1] They first appeared in the late Triassic[2] and survived at least as late as the Paleocene.[3] While most rhynchocephalians, such as the modern tuatara, were carnivores or insectivores, opisthodonts were herbivorous. Advanced opisthodonts of the subfamily Eilenodontinae were among the largest rhynchocephalians known. Opisthodontia is defined as all rhynchocephalians closer to Priosphenodon than to Sphenodon (tuatara).
Like other sphenodonts, opisthodonts had acrodont teeth which grew directly from the bone. They had one row of teeth on the lower jaw and two rows on the roof of the mouth. When processing food, their mandibular teeth would have slid between the outer (maxillary) teeth and inner (palatine) teeth.[4] Some opisthodonts, such as Sphenotitan, also had clusters of small teeth on the pterygoid at the center of the mouth roof. Opisthodont teeth were wide, numerous, and tightly-packed for grinding and shredding tough plant matter. Although wide shredding teeth are also known in a few other sphenodontians, such as Clevosaurus and Pelecymala, the most diverse and long-lasting group of herbivorous rhynchocephalians were the opisthodonts.[2] Some more generalized opisthodonts, such as Opisthias, were probably more capable of omnivory than the advanced eilenodonts.[3]
The mandibular teeth of opisthodonts were characteristically edged with forward-pointing flanges. Although Sphenotitan only had flanges on the medial (inside) edge, other eilenodonts had flanges on the lateral (outside) edge as well, making their teeth anteriorly concave and overlapping. The maxillary teeth, on the other hand, had long flanges aligned in a posteromedial-anterolateral direction.[5]
Opisthodonts lacked dental regionalization, meaning that all of their teeth had the same form and they did not have caniform or hatchling teeth like other sphenodonts.[2] Despite having had thicker enamel to resist wearing down their teeth, opisthodonts have often been found with their teeth significantly worn away, especially towards the front of the jaw.[4] The tip of the lower jaw is completely toothless in opisthodonts. Their jaws were also deep (particularly in eilenodonts) to counter stresses which would have occurred during food processing.[4]
In addition, most opisthodonts also had dental ridges (or crests) on their mandibular teeth. The most advanced opisthodonts belong to the subfamily (or tribe) Eilenodontinae (or Eilenodontini). Most eilenodonts were large, stockily built members of the clade, with massive jaws, low and overlapping teeth, and a large hooked "beak"(a rhynchocephalian feature). Their skulls were reminiscent of rodent skulls, leading some to presume that they were gregarious burrowers which fed on tough vegetation.[5] The largest known terrestrial rhynchocephalian was an eilenodont, Priosphenodon avelasi.
