Acleistorhinidae (meaning "open nostrils") is a family of Early Permian-aged parareptiles. Presently, the clade consists of only two taxa, Colobomycter and Acleistorhinus, both collected from the Permian of Oklahoma. Sister taxa include Nyctiphruretidae and Sclerosauridae. Acleistorhinidae is notable for being the oldest-known parareptilian clade. The family is diagnosed by the presence two synapomorphies:
- The largest tooth is located far anteriorly on the maxilla
- Cranial ornamentation consists of sparse and shallow circular dimples.
Two specimens of acleistorhinids have recently been described from the Richards Spur fissure-fill locality in Oklahoma that provide compelling evidence of the diet of acleistorhinids. These specimens, called OMNH 73362 and OMNH 73364, represent a new and currently unnamed species of acleistorhinid. Fragments of arthropod cuticles are present in between the many palatal teeth of both skulls. The fragments in OMNH 73362 are thought to be the segments of an antenna, while the fragments in OMNH 73364 are thought to be part of a cercus. In acleistorhinids, the marginal teeth, which are small and recurved, are suggestive of an insectivorous diet, as they probably were used for gripping and piercing arthropod cuticle. The denticulated palate, with three pairs of tooth fields and smaller teeth in between the fields, is seen as an adaptation for holding food in the oral cavity. The teeth, which possess cutting edges, may also have been suitable for a carnivorous diet in which vertebrate flesh may have been consumed. It is possible that acleistorhinids would have preyed on tetrapods that were small enough to swallow whole. It is likely that one acleistorhinid, Colobomycter pholeter, specialized either on invertebrates with hard cuticles or on small tetrapods.
Colobomycter phoeleter (Vaughn 1958, Modesto and Reisz 2008), Lower Permian ~278 mya, was originally considered a caseid pelycosaur, like Eothyris. Later, Modesto and Reisz (2008) considered it a "parareptile" close to Acleistorhinus. It is neither. The present analysis nests Colobomycter at the base of the Placodontia. It was derived from a sister to Claudiosaurus and succeeded by Paraplacodus. Only the front half of the skull is known. A reconstruction traced from the Modesto and Reisz (2008) paper (left) differs from their published reconstruction (right), which adds postorbital bones and tooth tips among other detail differences. Vaughn (1958) reported impressions of the posterior maxilla and additional skull roof bones. Acleistorhinus is dissimilar and does not have giant premaxillary teeth with polyplocodont infolding. Placodonts do. Unfortunately Modesto and Reisz (2008) chose not to include placodonts or Claudiosaurus in their study. Hence the result was a "by default" nesting wth Acleistorhinus. Here the giant premaxillary tooth is reconstructed as procumbent. The naris is relatively large. If otherwise similar to Claudiosaurus and Paraplacodus, Colobmycter would have had a relatively small skull, an elongated neck and torso with relatively small limbs suitable for aquatic propulsion.
Acleistorhinus pteroticus (Daly 1969) Early Permian, ~3.5 cm skull length was consiered by DeBraga (2001, 2003) to be a Parareptile related to Lanthanosuchus, which is a mismatch. Here Acleistorhinus is derived from a sister to the RC14 specimen of Milleretta and phylogenetically preceded Eunotosaurus. The skull of Acleistorhinus was further downturned anteriorly. The maxilla ascended to cut off the lacrimla from the naris and to contact the prefrontal. The jugal separated from the squamosal creating a lateral temporal fenestra. The supratemporal was enlarged. The second half of the skull was expanded laterally, rotating the orbits anteriorly for better binocular vision. Since both Eunotosaurus and Milleretta had expanded ribs, it is likely that Acleistorhinus did so too.