Skull of Acleistorhinus. A, dorsal view. B, palatal view. C, occipital view. D, left lateral view. Reproduced with permission from deBraga (in press).
Acleistorhinus shares with other anapsids a jaw joint located anterior to the occiput (the posterior end of the skull, which articulates with the neck) whereas the jaw joint is even with the occiput in most other early amniotes. Therefore, its jaw, as in most other anapsids, is short, and its bite probably produced a relatively large amount of force. Exactly what it used this forceful bite for is unknown.
Acleistorhinus shares a few synapomorphies (shared derived characteristics) with lanthanosuchids (a group of strange Upper Permian anapsids from Russia) that show that it is closely related to them. These synapomorphies, among others, include a reduction in size of the postparietal (a bone at the back of the skull) and a long basicranial articulation (an ancient articulation between the braincase and the palate that has fused in extant turtles) (deBraga, in press).
Daly E. 1969. A new procolophonoid reptile from the Lower Permian of Oklahoma. Journal of Paleontology 43: 676-687.
deBraga, M. In press. The Early Permian reptile Acleistorhinus pteroticus and its phylogenetic position. Journal of Vertebrate Paleontology 28 pages, 4 figures.