Chroniosaurus is a representative of a peculiar group of relic antracosauridae – chroniosuchia. Chroniosuchia is characterized by mobility contact of skull deck with a squamosum, weak sensor system of side line and with a shell covering the whole body from above and the front part of the animal’s tail. The shell consisted of some separate thoraxes situating in one line along the vertebra. The thoraxes had a difficult lock joint, their back side was covered with sculpture. Remains of chroniosaurus are very often in late Tatarian deposites of East Europe. Their single finds are pointed in Upper Permian of China.
Chroniosaurus had elongated three – corned skull with marked stretched preorbital part and to 25 sm. long. Two pairs of sculpture combs are good marked on the deck of the skull. In front of the eye-sockets there were quite large stretching longitudinal holes – preortital windows, which probably were the place of salt-out glands. Covering ornament of thoraxes of back shell was mainly knobby. Chroniosaurus had an active predatory way of life.
Only three finds of Chroniosaurus levis fragments are known from the Kotelnich locality (Sokolovogora Member).
Dvinosaurus had a parabolic flat skull with typical hole – furrow sculpture, well marked sensor system of side line (probably they served for perception of weak movement of water), a hole of the “third” eye and eye – sockets in the front part of it. Dvinosaurus had a skeleton of branchiate arches which probably had external branchias by life. It is supposed that like a modern acksolotel dvinosaurus in spite of its large body (to 1.5 m) was an neothenical (i.e. saving larval traits) amphibia and had only water way of life, hunting fishes and other water animals from ambushes.
On the Kotelnich locality remains of Dvinosaurus (skulls’ fragments) are known from the lens of grey – green sandstones (Sokolovogora Member). They belong to the same species (Dvinosaurus primus) as ones found by V. P. Amalitskim on the Malaya Severnaya Dvina river.
To labyrinthodonts belongs a fossil group of crocodile – like amphibians with difficult folded enamel on the teeth.
On the Kotelnich locality there are representatives of two main groups of Labyrinthodonts – Batrachomorpha (ancestors of modern frogs) and Antracosauridae (ancestors of reptiles).