Acleistorhinus (ah-kles-toe-RYE-nuss) is an extinct genus of parareptile known from the Early Permian (middle Kungurian stage) of Oklahoma.

The morphology of the lower temporal fenestra of the skull of Acleistorhinus bears a superficial resemblance to that seen in early synapsids, a result of convergent evolution.

Acleistorhinus was first discovered and named by Eleanor Daly in 1969 in the Hennessey Formation of South Grandfield, Tillman county, Oklahoma.

This would have offered a degree of binocular vision giving Acleistorhinus, a land-dwelling insectivore, depth perception necessary for hunting fast moving objects.

The genus Acleistorhinus belongs to the taxon parareptilia along with Millerettidae, Lanthanosuchidae to whom it is a sister taxa, Macroleter and Procolophonia.

A recent restudy, phylogenetic analysis, of Acleistorhinus indicates that this Early Permian amniote from North America, the oldest known member of parareptilia, is a sister taxon to Russian Lanchanosuchidae.

[7] In addition, the results support Laurin and Reisz (1995) hypothesis that Parareptilia is a monophyletic group, while differing in the number of synapomorphies diagnosing the clade.

†Millerettidae †Acleistorhinus †Lanthanosuchidae †Macroleter †Procolophonia Acleistorhinus was discovered by Daly in 1969, in the Early Permian outcrops of the Hennessey Formation, the locality of South Grandfield of southwestern Oklahoma.

[9] The Hennessey Formation is believed to be contemporaneous with the Richards Spur locality near Fort Sill, Oklahoma, as they both possess a mixed fauna, which is generally disarticulated and incomplete.