Cope wrote about the characteristics that distinctly separated this particular species from the two controversial groups. The differences he described were that the fossil had a reduced or lacking amount of dermal ossification on the back, the articulation of the pterygoid and quadrates, presplenial bone in the jaw was present, no articular process on the back side of the nuchal, simple formation of the radial process on the humerus and a peculiar bent formation of the xiphiplastra. He concluded that genus Protostega and species Protostega gigas was an intermediate form of the two groups Dermochelys and Chelonidae.
Cope described the characteristics that distinguished this species from two controversial groups, including a reduced or absent dermal ossification on the back, articulation of the pterygoid and quadrates, presence of a presplenial bone in the jaw, absence of an articular process on the back side of the nuchal, simple formation of the radial process on the humerus, and a peculiar bent formation of the xiphiplastra. He concluded that genus *Protostega* and species *Protostega gigas* was an intermediate form between *Dermochelys* and *Chelonidae*.