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.

First defined as a family by Apesteguia and Novas in 2003, Opisthodontia has generally been considered a well-supported clade, especially compared to other groups within Rhynchocephalia, which often shift in evolutionary position between studies.

Both opisthodonts and sphenodontids share the ability to move their jaw in a forward-to-back chewing motion, earning them the informal name "eupropalinal sphenodonts" as a result.

Other possibly herbivorous rhynchocephalians, such as Pelecymala and Ankylosphenodon, have sometimes been suggested to be very close relatives (or basal members) of Opisthodontia in some studies, although these hypotheses are not well supported.

This same analysis found that Opisthias and Kawasphenodon formed a clade at the base of Opisthodontia, with Sphenotitan being a basal eilenodont.

[6] Opisthodonts were numerous at times, with Opisthias having a practically worldwide distribution in the Jurassic and Priosphenodon being known from multiple specimens from the early Cretaceous of Argentina.

The oldest opisthodonts include Sphenotitan and Fraserosphenodon, from the Norian and Rhaetian (late Triassic) of Argentina and England, respectively.