[7] The first known Protostega specimen (YPM 1408) was collected on July 4 by the 1871 Yale College Scientific Expedition, close to Fort Wallace and about 5 months before Cope arrived in Kansas.

[8] It was not named until 1872, when E. D. Cope found and collected the first identified specimen of Protostega gigas in the Kansas chalk in 1871.

[11] During the Mid-to-Late Cretaceous period the Western Interior Seaway covered the majority of North America and would connect to the Boreal and Tethyan oceans at times.

Cope described other greatly modified bones in his specimen including an extremely long coracoid process that reached all the way to the pelvis and a humerus that resembled a Dermochelys,[18] creating better movement of their limbs.

[1] Another fossilized specimen showed that a bony extension, that would have been viewed as a beak, was lacking in the Protostega genus.

[18] The quadrato-jugal was triangular with a posterior edge that was concave, with the entire bone being convex from distal view.

[19] Cope concluded that these animals were most likely omnivores and consumed a diet of hard shelled crustacean creatures, due to the long symphysis of its lower jaw.

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, the presence of a presplenial bone in the jaw, a lack 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.