Opisthodontia is a clade of extinct sphenodontian rhynchocephalians. They first appeared in the late Triassic and survived at least as late as the Paleocene. 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).