Phylogenetics has predicted that parareptiles first evolved in the Carboniferous, parallel to eureptiles ("true reptiles").

[1][2] It was likely carnivorous, and could be characterized by a variety of skeletal features, including a relatively elongated body and large claws with powerful tendon attachment points.

This specimen hailed from Cape Egmont, in southwestern Prince Edward Island, Canada, in rock layers of the Edgmont Bay Formation that preserves fossils that date back to Gzhelian stage of the Carboniferous period.

[2] It was acquired by the Royal Ontario Museum (ROM) in 2004, and later rep.[2] Cape Breton University's Sean Modesto, a paleontologist and expert in ancient reptiles, was the lead author of a 2015 article published in the Proceedings of the Royal Society, The team, which included researchers from the ROM, University of Toronto and the Smithsonian Institution, described and named the new genus and species, the first parareptile known from Carboniferous period fossils.

Denticles reached almost as far back in the mouth as the large, robust quadrate bones which formed the upper part of the jaw joint.

The presacrals were hourglass-shaped when seen from above and tightly connected to each other by means of large, rounded zygapophyses (joint plates).

The knobs and joints forming the elbow were poorly developed, meaning that Erpetonyx may have had more flexible forelimbs than its contemporaries.

On the other hand, the unguals (claws) were long and sharply pointed, with robust flexor tubercules (tendon attachment bumps).

The femur (thigh bone) was twisted, with the knee joints offset at 45 degrees from the rest of the shaft.