Dvinosaurus is an extinct genus of amphibious temnospondyls localized to regions of western and central Russia during the middle and late Permian, approximately 265-254 million years ago. Its discovery was first noted in 1921 by Russian paleontologist Vladimir Prokhorovich Amalitskii in a posthumously published paper that documents the findings of a site in Russia's Arkhangelsk District. Its name is derived from the proximity of this site to the Northern Dvina River.
Dvinosaurus is thought to have been a gill-breathing, fully-aquatic tetrapod, characterized by a large, triangular head, short limbs, and a long powerful tail. A typical individual could grow to be approximately 40 in (100 cm) in length.
Within this genus, the number of documented species has varied over the years since its discovery. Prior to his untimely death, Amalitskii described three species, Dvinosaurus primus, Dvinosaurus secundus, and Dvinosaurus tertius. Upon further analysis, however, these three proposed species would be reclassified as solely D. primus as the latter two were found to be age-stages of the same species. Later in the century, Mikhail Shishkin would describe two different species of Dvinosaurus named Dvinosaurus egregius and Dvinosaurus purlensis based on specimens from a locality south of Amalitskii's original site. Finally, in 2004 a new species of Dvinosaurus named D. campbelli was described by Y.M. Gubin based on deposits from the Middle Volga Region, a locality centered around the Volga River, which runs through Western Russia before draining into the Caspian Sea.
History and description
History
Following his death in 1917, Amalitskii's notes and initial observations of Dvinosaurus were transferred to the Paleontological Institute of the Russian Academy of Sciences. Over the course of the next 8 years, Prof. Petr Sushkin carried out a thorough analysis of these notes in order to provide the first description of Dvinosaurus. Later in the century, at a site near the small town of Vyazniki in western Russia, many more Dvinosaurus specimens were identified and analyzed by B.P. Vjuschkov and Mikhail Shishkin, who classified two new species of Dvinosaurus and added to the depiction of the genus as whole in the process. What follows is a summary and general description of their combined findings.
General description and skull
As detailed by Amalitskii and subsequently Sushkin, Dvinosaurus is roughly 40 inches in length, although some species could be larger, up to 2.5 meters. It featured smooth skin suited for an aquatic environment. Its head is a large, rounded triangular shape with sizable orbits that account for nearly 1/4 of the length of the skull. Of note, Dvinosaurus’ skull lacks an otic notch, while also featuring a noticeably elongated occipital region. Like other temnospondyls, it features large interpterygoid vacuities and a number of tusks dispersed throughout its palate. Its teeth consist of both large, recurved fangs and smaller, conical teeth, the combination of which suggests that it fed on fish or similarly sized tetrapods.
Post-cranial elements
It is known that Dvinosaurus featured a highly ossified branchial skeleton just posterior to the skull that would have provided support for 4 pairs of gills. Although gills were once considered as external, later study shows that are more likely to be internal gills like fish have. Additionally, Dvinosaurus had a long vertebral column (pre-sacral vertebral count of 28) with short, thick ribs reminiscent of those found in Amphibia. Its vertebrae also featured hemal arches. Its vertebrae were rhachitomous with an enlarged intercentrum and small, paired pleurocentra. The bones of its forelimbs and hindlimbs are short, stout, and display several characteristics of aquatic organisms such as incomplete ossification and flattening of limb bones. Finally, prominent spinous processes found in the more caudal vertebrae indicate that Dvinosaurus possessed a powerful tail that, in combination with its short, muscular limbs, could propel it rapidly toward its prey.