Archelon is an extinct marine turtle from the Late Cretaceous, and is the largest turtle ever to have been documented, with the biggest specimen measuring 5 m (16 ft) from head to tail and 2,200 kg (4,900 lb) in body mass.[5] It is known only from the Dakota Pierre Shale and has one species, A. ischyros. In the past, the genus also contained A. marshii and A. copei, though these have been reassigned to Protostega and Microstega, respectively. The genus was named in 1895 by American paleontologist George Reber Wieland based on a skeleton from South Dakota, who placed it into the extinct family Protostegidae. The leatherback sea turtle (Dermochelys coriacea) was once thought to be its closest living relative, but now, Protostegidae is thought to be a completely separate lineage from any living sea turtle.
Archelon had a leathery carapace instead of the hard shell seen in sea turtles. The carapace may have featured a row of small ridges, each peaking at 2.5 or 5 cm (1 or 2 in) in height. It had an especially hooked beak and its jaws were adept at crushing, so it probably ate hard-shelled crustaceans and mollusks while slowly moving over the seafloor. However, its beak may have been adapted for shearing flesh, and Archelon was likely able to produce powerful strokes necessary for open-ocean travel. It inhabited the northern Western Interior Seaway, a mild to cool area dominated by plesiosaurs, hesperornithiform seabirds, and mosasaurs. It may have gone extinct due to the shrinking of the seaway, increased egg and hatchling predation, and cooling climate.
The holotype specimen, YPM 3000, was collected from the Late Campanian-age Pierre Shale of South Dakota along the Cheyenne River in Custer County by American paleontologist George Reber Wieland in 1895, and described by him the following year based on a mostly complete skeleton excluding the skull. He named it Archelon ischyros,[6] genus name from the Ancient Greek ἀρχη- (arkhe-) 'first/early',[7] χελώνη (chelone) 'turtle',[8] and species name from ἰσχυρός (ischyros) 'mighty' or 'powerful'.[9] Wieland placed it into the family Protostegidae, which included at the time the smaller Protostega and Protosphargis.[6] The latter is now in the family Cheloniidae.[10] A second specimen, a skull, was discovered in 1897 in the same region.[11]
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.[11] However, in 1909, Wieland reclassified it as Protostega marshii. In 1902, a third, mostly complete specimen was collected also along the Cheyenne River.[12] 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.[13] 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,[12] was moved to the genus Archelon as A. copei.[13] In 1998, A. copei was moved to the new genus Microstega as M. copei.[14] 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.[15]
In 2002, a fifth specimen, a partial skeleton, was discovered from the Pierre Shale of North Dakota along the Sheyenne River near Cooperstown.[3]