High Quality Content by WIKIPEDIA articles! Archelon is a genus of extinct sea turtle, the largest that has ever been documented. The first specimen of Archelon (YPM 3000) was collected from the Pierre Shale of South Dakota by Dr. G.R. Wieland in 1895 and described by him the following year (Wieland, 1896). The largest Archelon fossil, found in the Pierre Shale of South Dakota in the 1970s, measures more than 4 meters (13.5 feet) long, and about 4.87 meters (16 feet) wide from flipper to flipper. It was a marine turtle, whose closest living relative in the present day is the Leatherback Sea Turtle. Archelon's fossils date to around 75-65 million years ago in the Late Cretaceous period, when a shallow sea covered most of central North America. Most of the known remains have been found in South Dakota and Wyoming. Though anatomically similar to the earlier species Protostega gigas, it was much larger.