A second specimen, a skull, was discovered in 1897 in the same region. In 1900, Wieland described a second species, A. marshii, from remains collected in 1898 by American paleontologist Othniel Charles Marsh, to whom the species name refers, on the basis that the shell underside (plastron) was thicker and the humeri were straighter. However, in 1909, Wieland reclassified it as Protostega marshii. In 1902, a third, mostly complete specimen was collected also along the Cheyenne River. In 1953, Swiss paleontologist Rainer Zangerl split Protostegidae into two families: Chelospharginae and Protosteginae; to the former was assigned Chelosphargis and Calcarichelys, and the latter Archelon and Protostega. In the same study, the Kansas Protostega copei, which was first described by Wieland in 1909 and named in honor of Edward Drinker Cope who first erected the family Protostegidae, was moved to the genus Archelon as A. copei. In 1998, A. copei was moved to the new genus Microstega as M. copei. In 1992, a fourth and the largest specimen to date, nicknamed "Brigitta", was discovered in Oglala Lakota County, South Dakota and resides in the Natural History Museum Vienna. In 2002, a fifth specimen, a partial skeleton, was discovered from the Pierre Shale of North Dakota along the Sheyenne River near Cooperstown. Evolution The sister group of Protostegidae had, in the past, been considered to be Dermochelyidae, and thus their closest living relative would have been the dermochelyid leatherback sea turtle (Dermochelys coriacea). However, phylogenetic studies conclude that protostegids represent a completely separate, ancient and (basal) lineage that originated in the Late Jurassic, removing the family from the superfamily Chelonioidea (which includes all sea turtles). In this model, Archelon does not share a marine ancestor with any sea turtle. Description The holotype measures from head to tail, with the head measuring , the neck , the thoracic vertebrae , the sacrum , and the tail . The largest specimen, Brigitta, measures around from head to tail and from flipper to flipper, and, in life, weighed around . Skulls of Archelon measured at up to . Archelon had a distinctly elongated and narrow head. It had a defined hooked beak which was probably covered in a sheath in life, reminiscent of the beaks of birds of prey. However, in the back, the cutting edge of the beak is dull compared to such animals. Much of the length of the head derives from the elongated