Ennatosaurus is an extinct genus of caseid synapsid that lived during the Middle Permian (late Roadian - early Wordian) in northern European Russia.

[3][4][5] Ennatosaurus has the typical caseid skull with a short snout tilted forward and very large external nares.

However, the existence of some bones of subadult and adult individuals suggests that Ennatosaurus did indeed have a proportionally smaller body than that of other derived caseids.

Ennatosaurus also differs from all other caseid in its temporal fenestrae significantly larger than the nostrils and orbits, its palate with a narrower parasphenoid, and its upper dentition more reduced in number.

The vertebral centra of all regions of the body are characterized by the presence of two well-developed ventrolateral pits, deep and elongated anteroposteriorly.

The neural spines of the vertebrae show a diamond-shaped section along its entire length, a condition similar to that observed in Ruthenosaurus.

The vertebrae of the "lumbar region" are characterized by the absence of fused or co-ossified ribs (a characteristic to be taken with caution given the juvenile condition of the specimens).

The femur is very characteristic, its proximal articular surface is much wider dorsoventrally than anteroposteriorly and an elevated and robust bony crest extends from the narrowest part of the shaft to the top of the posterior condyle with which it merges to form a single support structure.

The tibia is distinguished by its flattened shaft with a subelliptic and non-circular cross section, as is the case in most caseids which have not undergone diagenetic deformation.

The holotype and the majority of the referred specimens were discovered in 1955 in the Nijneoustinskaia Formation (Karpogorskaia Member), on the banks of the Pinega River, and come from the locality of Moroznitsa near the town of Karpoga in the Pinezhsky District.

[4] However, the Nisogora locality, which has yielded Ennatosaurus remains, contains a more diverse fauna containing the parareptiles Nyctiphruretus acudens, Macroleter poezicus and Lanthaniscus efremovi, the Varanopidae Mesenosaurus romeri, and the juvenile therapsid of uncertain affinity Niaftasuchus zekkeli.

[10] All these species (except Ennatosaurus) are also known in at least eight other localities of the Arkhangelsk Oblast which have yielded additionally the parareptiles Lanthanolania ivachnenkoi, Bashkyroleter mesensis and Nycteroleter ineptus, the Varanopidae Pyozia mesensis, and several basal therapsids : the Nikkasauridae Nikkasaurus tatarinovi, Reiszia gubini and R. tippula, the biarmosuchian Alrausuchus tagax,[10] and a yet undescribed basal anteosaurid dinocephalian.

[5] In the first phylogenetic analysis of Caseidae published in 2008, Ennatosaurus was identified as the sister group of a clade containing Cotylorhynchus romeri and Angelosaurus dolani.

However, the close relationship between Angelosaurus dolani and Ennatosaurus tecton may be distorted by the extreme incompleteness of the material of the North American species.