Mesenosaurus is an extinct genus of synapsid belonging to the family Varanopidae. This genus includes two species: the type species Mesenosaurus romeri from the middle Permian (upper Kazanian) Mezen River Basin of northern Russia, and Mesenosaurus efremovi from the early Permian (Artinskian) Richards Spur locality (Oklahoma, United States). M. romeri’s stratigraphic range is the middle to late Guadalupian while M. efremovi’s stratigraphic range is the Cisuralian.

Famous Russian paleontologist, Ivan Efremov, established Mesenosaurus as a genus. The genus name means “lizard from Mezen” while the specific epithet is given in honor of Alfred Romer.

Mesenosaurus efremovi was named in honor of Ivan Efremov, who erected the genus.

Mesenosaurus are small sized varanopid synapsids. They are characterized by mainly cranial features. Many of the postcranial features of this genus have not been analyzed fully due to a lack of fossil evidence.

The cranial features that characterize Mesenosaurus are:

The Mezen River basin is located in northern Russia and has extensive exposures of Middle Permian sediments along the edges of affluent rivers of the Mezen River. These sediments have produced many skeletal remains of diverse amniotes, but most importantly, a partial skull of “a small synapsid of varanopseid affinities”, Mesenosaurus romeri. M. romeri was discovered in 1938 by Ivan Efremov and became the first species of Mesenosaurus, due to its lack of cranial similarities to others. It was also the first synapsid described from the Russian area to be considered a “good pelycosaur”, as it possessed upper jaw teeth that were consistent with other known pelycosaurs (slender, recurved, enlarged incisors, single caniniform tooth). Romer and Price hypothesized that M. romeri belonged to Varanopseidae. This hypothesis was confirmed in 2001 based on the following synapomorphies:

The second species of this genus is Mesenosaurus efremovi. Its nearly complete skull and mandible was discovered at Richards Spur locality within a series of infilled karst fissures in the Ordovician Arbuckle limestone in Oklahoma, which is one of the most plentiful sites for early Permian tetrapod fossils.

In terms of classifying M. efremovi, it shares distinct cranial features with mycterosaurines (stem based group that includes Mycterosaurus longiceps and all varanopseids related more closely to it than to Varanodon agilis), such as the “exclusion of the lacrimal from the external naris and an anteroposteriorly broad dorsal lamina of the maxilla that underlies the nasal and contacts the prefrontal”. However, M. efremovi shares more features with M. romeri from Russia. Some of these shared features include relative size and shape of the temporal fenestra, lateral swelling of the maxilla in the caniniform region and five premaxillary tooth positions (not reported in other mycterosaurines).