| Acleistorhinus Temporal range: Early Permian,
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| Illustration of Acleistorhinus skull | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | †Parareptilia |
| Order: | †Procolophonomorpha |
| Family: | †Acleistorhinidae |
| Genus: | †Acleistorhinus Daly, 1969 |
| Type species | |
| †Acleistorhinus pteroticus Daly, 1969
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Acleistorhinus (ah-kles-toe-RYE-nuss) is an extinct genus of parareptile known from the Early Permian (middle Kungurian stage) of Oklahoma.[1] It is notable for being the earliest known anapsid reptile yet discovered. 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. Only a single species, A. pteroticus, is known, and it is classified in the Family Acleistorhinidae, along with Colobomycter (also from the Early Permian of Oklahoma).
- ^ Daly, E. (1969). "A new Procolophonomorpha Procolophonoid reptile from the Early Permian of Oklahoma". Journal of Paleontology. 43 (3): 676–687.