Mesenosaurus is an extinct genus of amniote. It belongs 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.
Etymology
Famous Russian paleontologist, Ivan Efremov, established Mesenosaurus as a genus, and named it after its roots with the (literal translation is “lizard from Mezen”).
Mesenosaurus efremovi was named in honor of Ivan Efremov, who erected the genus.
Description
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.
Skull
The cranial features that characterize Mesenosaurus are:
Slender premaxilla forms a narrow, rectangular snout (from dorsal and ventral views) Dorsal process of premaxilla is long and forms anterior half of dorsal margin of external naris Expanded narial shelf that extends almost to the snout tip Palatal process of premaxilla is long with long median suture Well-developed depression on the lateral surface of the nasal that extends posteriorly from the narial border to nearly the anterior end of the prefrontal Slight lateral swelling of the maxilla at the level of the caniniform tooth Short posterior process of the maxilla fails to reach the level of the postorbital bar First premaxillary tooth smaller than the second and third teeth. (There is a maximum of five teeth on the premaxilla. The first tooth is smaller than the second and third, but larger than the last two. These five teeth are closely spaced, strongly recurved, sharply pointed, and have a sharp, cutting edge along the distal half of the posterior edge.) Single, median vomerine tooth row Postorbital cheek region of skull unusually broad and low, with nearly vertical posterior margin Posterior edge of transverse flange of the pterygoid is angled slightly anterolaterally from basal articulation. Stapes slender, short, and rod-like, with modestly developed footplate and distally expanded quadrate process. (The stapes of Mesenosaurus more closely resembles those in early therapsids of the Permian.) A very prominent, nodular ornamentation is present on the orbital margins of the prefrontal, postorbital, and jugal. (This characteristic is strikingly pronounced in well-preserved skulls of Mesenosaurus, so much so that Efremov originally considered it to be an autapomorphy of Mesenosaurus. However, careful examination and analysis have shown that this feature is present in other varanopids.)
Discovery
Mesenosaurus romeri
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 reptile 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: