Acleistorhinus pteroticus
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Acleistorhinus Temporal range: Early Permian, 273.6–271.6 Ma |
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| Illustration of Acleistorhinus skull and full restoration. | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | †Parareptilia |
| Order: | †Procolophonomorpha |
| Family: | †Acleistorhinidae |
| Genus: |
†Acleistorhinus Daly, 1969 |
| Type species | |
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†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 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, the 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).
Acleistorhinus was first discovered and named by Eleanor Daly in 1969 in the Hennessey Formation of South Grandfield, Tillman county, Oklahoma. The name Acleistorhinus combines Greek rhin (ῥῑ́ν), meaning "nose," and akleistos, Greek for “unclosed.“
Although its total body length is unknown, an Acleistorhinus skull is about 3.5 centimetres long. From the dorsal side, the Acleistorhinus skull appears to have a triangular outline. The surface of the skull is generally smooth with a few small, shallow circular pits. Anteriorly, the snout is gently rounded. The elliptical external nares are each bordered by the maxilla. The tooth bearing portion of the premaxilla appears to be directed somewhat downward at its tip. Each premaxilla possess spaces for four teeth. The maxilla has a dorsal expansion immediately behind the nares forming the entire posterier border of the opening. This configuration resembles that of the procolophonids, turtles, and neopiapsids and results in the exclusion of the lacrimal from the posterior border of the nares. Slightly more than one third of the total length of the skull is contributed by the frontal. It is constricted anteriorly by the prefrontals, but otherwise expanded above the orbits. Generally, in early amniotes the largest element was the occiput of the supraoccipital. In Acleistorhinus the supraoccipital is rather plate-like. The reduction in the overall size of the supraoccipital allows for the development of large post-termporal fenestra, a characteristic of Reptilia.
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