Comparison of cranial sizes within the esfenodontes. Each red dot represents an event of gigantism in the lineage. Opisthodonts represent the most extreme event.
Priosphenodon belongs to the lineage of the eilenodontins, and these to the opisthodonts, a group of sphenodonts characterized by broad or square teeth and a medium to large size. Among its members are medium shapes, presumably omnivorous, such as Opisthias (skull: 42 mm), a form of the Upper Jurassic to Lower Cretaceous of North America and Europe, as well as two large radiations. One includes the eilenodontins (Toxolophosaurus, Eilenodon, Priosphenodon minimus and P. avelasi), the largest terrestrial sphenodonts, with skulls generally between 60 and 150 mm and body lengths exceeding one meter. The other is composed of the two known species of Kawasphenodon, also large (K. expectatus, skull: 110 mm and K. peligrorensis, skull: 38 mm) and whose forms exceed the limit K / P. The group would appear to have emerged in the Late Jurassic. However, the recent inclusion to the Sphenotitan group by Martínez et al. (2013) would take the group to the Upper Triassic and consider "Clevosaurus" latidens as its sister group. Whether the inclusion of Sphenotitan in the group is correct or not, it is evident that opisthodonts have been freely distributed throughout the Prenæic world before the Middle Jurassic, establishing later an American lineage (Eilenodon, Toxolophosaurus) and a South American lineage (Priosphenodon, Kawasphenodon). The relationship with them of the Triassic Pelecymala and the fragment reported for the Maastrichtian of Spain are still poorly resolved. The abundance of the remains of Priosphenodon and Sphenotitan suggest that these animals were gregarious and the great former limbs of Priosphenodon and Eilenodon, which were territorial.
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