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 ft) long, and about 4.9 meters (16 ft) 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.
Unlike most modern turtles, Archelon did not have a solid shell, but instead had a skeletal framework supporting a leathery or bony carapace. Other distinguishing features include a pointed tail, a narrow skull, a relatively narrow, high-vaulted shell, and a pronounced overbite. They probably had a very strong bite, and were optimized for feeding on pelagic mollusks such as squid. One theory concerning the one specimen exhibited by the National History Museum in Vienna is that it may have died while brumating on the ocean floor. However, brumation in reptiles is a response to cold weather and it is unlikely that the Western Interior Sea was ever that cold. This same specimen also suggests a century-long life span.
The live weight of an Archelon ischyros is estimated at more than 2,200 kg (4,900 lbs.)