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.

Etymology

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.

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:

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 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:

Mesenosaurus efremovi

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). Though M. efremovi and M. romeri share many distinct features, there are four main morphological differences between these specimens that deem a taxonomic distinction at the species level (differences insufficient for distinction above species level): M. efremovi is also larger than the largest known specimen of M. romeri.

Phylogeny

Below is a cladogram modified from the analysis of Benson, after the exclusion of Basicranodon:

Paleoenvironment

There is a 20 Myr gap between these two species, which exceeds the temporal range of most extinct tetrapods. Though this gap is questionable, the radioisotopic dating of speleothems, recovered for Richards Spur, used to identify this 20 Myr gap is a reliable technique that has been used to identify other large gaps/long temporal ranges. This gap is significant, as it indicates evolutionary stasis (persisting throughout different environments across Pangea as well as faunal turnovers throughout the Permian-specifically Olson’s gap). This evolutionary stasis may be attributed to a conserved niche occupation throughout their temporal and geographical ranges. Further research on postcranial features is required in order to determine if this hypothesis is correct, as we would need to observe a similar degree of stasis throughout the entire skeleton.

Paleobiology

Mesenosaurus was a small mobile creature, capable of climbing rocks/trees. Their small size allowed them to occupy and thrive in niches as small faunivores while therapsids dominated most terrestrial environments. It was not until the late Permian when small diapsids appeared and provided competition, leading to a decline in Mesenosaurus. Mesenosaurus represents a guild of highly agile subordinate predators in their communities due to their large, slender, curved teeth, which could cause severe wounds when piercing its prey. Mesenosaurus was also characterised by extremely rapid rates of tooth development and greatly reduced tooth longevity compared to almost every other land-dwelling amniote. They possessed unbent and flattened unguals, suggesting they were diggers. It is uncertain whether their digging unguals were adapted for burrowing or solely digging for food. Many varanopids were arboreal, however the well-developed olecranon(bony prominence of the elbow) of Mesenosaurus indicates the presence of triceps and anconeus muscle, both of which would provide powerful forearm extension. This forearm extension strength combined with its somewhat small/medium body size supports the idea of a burrowing lifestyle. It is also proposed that facultative bipedalism occurred in Mesenosaurus. This is based on the presence of a rearward shift of center of body mass (slender trunks, elongated hindlimbs, and short forelimbs) that is necessary for facultative bipedalism.