Protostega
Protostega is an extinct genus of sea turtle containing a single species, Protostega gigas. Its fossil remains have been found in the Smoky Hill Chalk formation of western Kansas, time-equivalent beds of the Mooreville Chalk Formation of Alabama and Campanian beds of the Rybushka Formation. Fossil specimens of this species were first collected in 1871, and named by Edward Drinker Cope in 1872. With a total length of, it is the second-largest sea turtle that ever lived, second only to the giant Archelon, and one of the three largest turtles of all time alongside Archelon and Gigantatypus.
Discovery and history
The first known Protostega specimen 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. However, the fossil that they found was never described or named. 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. A variety of bones were found in yellow Cretaceous chalk from a bluff near Butte Creek.Paleoenvironment
The Late Cretaceous was marked by high temperatures, with large epicontinental seaways. 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. Within these regions are where the fossil of Protostega gigas have been found.Description
Protostega is known to have reached up to in length. A specimen from the upper Taylor Marl is even larger, at in carapace length and in total length. Despite lacking its head and three limbs, it is well-preserved. Cope's Protostega gigas discovery revealed that their shell had a reduction of ossification that helped these huge animals with streamlining in the water and weight reduction. The carapace was greatly reduced and the disk only extending less than halfway towards the distal ends of the ribs. 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, creating better movement of their limbs.Edward Cope described Protostega gigas as having a large jugal that reached to the quadrate along with a thickened pterygoid that reached to the mandibular articulating surface of the quadrate. The fossil featured a reduction in the posterior portion of the vomer where the palatines meet medially. Another fossilized specimen showed that a bony extension, that would have been viewed as a beak, was lacking in the Protostega genus. The premaxillary beak was much shorter than that of Archelon. In front of the orbital region the skull was elongated with a broadly-roofed temporal region. The jaws of the fossil showed a large crushing surface. The quadrato-jugal was triangular with a posterior edge that was concave, with the entire bone being convex from distal view. The squamosal appeared to have a concave formation on the surface at the upper end of the quadrate. In Cope's fossil the mandible was preserved almost perfectly and from this he recorded that the jaw was very similar to the Cheloniidae and the dentary had a broad for above downward with a concave surface, marked by deep pits in the dentary.
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. Protostega also likely fed on seaweed and jellyfish or scavenged on floating carcasses as well, like modern turtles.