Mesenosaurus

The name “mesenosaurus” itself means “lizard from Mezen” ^[6] , since the skulls and skeletons of these animals were found in clay red limestones in a place located near the Mezen river in the Arkhangelsk region ^[4] .

The first description of these animals was made by the famous Russian science fiction writer and paleontologist I. A. Efremov using the skull from the Kiselikha deposit ^[5] (it should be noted that the genus was originally described only by the partially preserved skull ^[6] ), and he described them as pelicosaurs , considering the first "real" reptile ^[5] . Then they were considered diapsids for a long time, and in 1956 the paleontologist Romer singled them out as a separate family of mesenosaurids ( lat. Mesenosauridae ), belonging to the order of millerosaurs ( lat. Millerosauria ), and their similarity with primitive representatives of pelicosaurs was considered superficial ^[7] . Moreover, it is very curious that the vagueness of the relationship of the mesenosaurus remained until the 1980s , for example, in the book “Living Past of the Earth” by M. F. Ivakhnenko and V. A. Korabelnikov, it is referred to eosuchia - possible ancestors of archosaurs ^{[8] [5 ] ]} . After finding additional specimens, the mesenosaurus was re-studied and finally assigned to the group of small-sized varanopids - small predatory pelicosaurs whose remains are found in deposits from the Late Carboniferous to the Late Permian of North America , Europe and Africa , but recently, however, the position of this group itself is unclear ^{[5] [3]} .

A recent study of mesenosaurus skulls, published in 2001 , confirmed that it belongs to the eupelicosauridae synapsids of the Varanopidae family. A comparison of the mesenosaurus with other members of the family made it possible to single out the new subfamily Myctersaurinae, which now includes the genera ^[1] :

• Mesenosaurus ;
• Mycterosaurus from Late Perm, USA;
• Heleosaurus from the Late Permian of South Africa ^[9] .

The fossil remains of these reptiles contains a number of deposits in the Mezen river basin of the Arkhangelsk region ^[4] : Kiselikha (typical), Ust-Peza, Ust-Nyaft, Glyadnaya Shchelya, Dorogaya Gora, Kozmogorodsky, Peza-1, Petrova Shchelya, Mezensky district; Ust-Vashka, Leshukonsky, Nisogora and Leshukonsky district ^[3] , and they make up to 15% of all finds in the collections of red marls of Mezen, and quite often there are full skeletons ^{[10] [11]} .

The remains of mesenosaurs were found in sediments of the Middle Permian (this era lasted 272-250 million years ago ^[4] ), 268-265 million years old ^[2] (some sources also report that mesenosaurs lived on our planet and in the late Carboniferous era , in total, the time period is estimated at about 300-260 million years ago ^[12] ). To be absolutely precise, the remains of the mesenosaurus come from the Krasnoshchelsky suite and the undivided Upper-Kazan sub-tier - the Urzhum tier, belonging to the Biarmi division, which corresponds exactly to the Middle Permian ^[3] .

In 2004, the skeleton of another varanopeid was discovered - pezii ( Latin Pyozia ), a reptile slightly inferior to the mesenosaurus in size, the skull length of which was about 4 centimeters (although a young individual was found). Her teeth, unlike the mesenosaurus, were reduced in the backward direction. She could also eat insects ^[5] . Along with pezia, the mesenosaurus is so far the only known representative of science of the Varanopeids, whose remains were found in Eastern Europe ^[3] .

Mesenosaurs was a small reptile that resembled a lizard ^[13] . The length of their body reached 50 centimeters ^[3] (in some sources - 40 centimeters ^[6] , sometimes the value is 30 centimeters ^[13] ), one of the skeletons exhibited in the museum is 37 centimeters ^[3] . They had a gracile (i.e. thin and fragile) skeleton, long limbs with elongated fingers, which were equipped with well-developed claw phalanges .

The structure of the mesenosaurus was very primitive and peculiar, which caused difficulties in determining its kinship ^[13] . The skull of this lizard had a length of 5-6 centimeters, it was very narrow and elongated ^[7] , and was at least twice as high as that of the already mentioned relative of the archaeologist . The head was small and pointed ^[13] . The muzzle was also narrowed, but only slightly ^[12] . The bones of the roof of the skull had tuberous osteoderms . The temporal window had an elongated oval shape, the size of no less than the orbits, which, in turn, were large, oval, with raised edges ^[7] . The eyes of the mesenosaurus were large, the parietal eye was especially large ^[5] . The temporal bones were shaped like protruding back horns. The parietal opening of the mesenosaurus was round, located at the posterior edge of the skull roof, and the parietal bones in front were deeply wedged between the frontal ^[1] . The pterygoid bones had very strong processes and carried six to seven teeth directed forward ^[7] . The teeth were cutting, of different sizes ^[5] , in particular, those that were on the palate , were somewhat enlarged and were located in strips on high ridges ^[3] . There were also long teeth in the shape of fangs ^[5] . And on parasphenoid teeth were absent ^[7] . The incisive intermaxillary bone extended beyond the nostrils ^[12] .

The jaw joint at the mesenosaurus was far behind the occipital, the lower part was rounded in cross section. The jaws of this reptile had sharp, strongly bent back teeth, which had a well-developed cutting edge with serrations along the posterior edge. In front, 2-3 teeth of the maxillary bone were canine-like enlarged. A total of 24 teeth were located on the upper jaw ^{[7] [11]} .

Among the anatomical features of the mesenosaurus, an elongated neck, short dorsal ribs , a wide sacral bone , and a very slender tail should also be distinguished. In this reptile, the scapula and coracoid , unlike many other animals, were not connected. The ulnar and tibia were straighter and located closer to each other ^[12] . The body itself had an elongated shape, the tail was long ^[13] .

It was a medium-sized mobile creature, capable of climbing rocks and trees ^{[3] [4] [13]} . It probably fed on small vertebrates and insects ^[5] .

The so-called “Mezensky faunistic complex” (which, incidentally, included not only the mesaenosaurus and pezius, but also the caseid ) could exist simultaneously with the Ocher fauna , but under different conditions, for example, in the zone of thickets of calamite plants on the swampy shores of large reservoirs ^[3] .

• Efremov I. A. Some new Permian reptiles of the USSR. - 1938 .-- S. 775.
• Efremov I.A. A brief review of the fauna of the Permian and Triassic Tetrapoda of the USSR. - 1941.- S. 100.
• Ivakhnenko MF Late Paleozoic faunistic tetrapod complex from sediments of the river basin Mezen. - 1990.
• Ivakhnenko M.F. Tetrapods of the East European Placard - Late Paleozoic territorial-natural complex . - Perm, 2001 .-- S. 79-80. - 200 p. - (Proceedings of the Paleontological Institute of the Russian Academy of Sciences, Volume 283). - 1000 copies. - ISBN 5-88345-064-4 .
• Ivakhnenko M.F., Korabelnikov V.A. Living past of the Earth. - 1987.
• Ivakhnenko M.F., Kurzanov S.M. Mezenosaurus - a primitive archosaur. - 1978. - S. 152-153.
• Ivakhnenko M.F. 1 // Fossil reptiles and birds / Kurochkin E.N. - 2008. - P. 96-98.
• Ivakhnenko M. F. et al. Mezenosaurus - a primitive archosaur. - 1997 .-- S. 28.
• Orlov Yu. A. Fundamentals of paleontology. Amphibians, reptiles and birds. - 1964. - S. 441, 445-446.
• Efremov IA Die Mesen-Fauna der permischen Reptilien. - 1940 .-- S. 399.
• Efremov IA Kurze Ubersicht uber die Formen der Perm- und Trias-Tetrapoden-Fauna der UdSSR. - 1940 .-- S. 378.
• Huene F. von. Palaontologie und Phylogenie der niederen Tetrapoden. - 1956 .-- S. 253.
• Olson E.S. Late Permian terrestrial vertebrates, USA and USSR. - 1962. - P. 88.
• Reisz RR, Berman DS The skull of Mesenosaurus romeri , a small varanopseid (Synapsida: Eupelycosauria) from the Upper Permian of the Mezen River basin, northern Russia // Annals of the Carnegie Museum. - 2001. - Vol. 70. - Vol. 2 . - P. 113-132.
• Romer AC, Price L. Review of the Pelycosauria. - 1940. - P. 282.

• Mesenosaurus (English) on fossils.valdosta.edu
• MESENOSAURUS (English) - Gondwana Studios