The Giant Archelon Ischyros

Archelon fossil
Archelon fossil. Courtesy of Black Hills Institute of Geological Research, Inc., Hill City, SD

Believe it or not, the fossilized remains of the largest sea turtle that roamed the Earth was found in the grassy plains of South Dakota. This giant marine turtle, known as the Archelon ischyros, lived about 70 million years ago during the Cretaceous period. During this time, the colossal turtle swam gracefully through the muddy waters of a prehistoric seaway that divided North America. This shallow body of water was called the Western Interior Seaway, and it flowed from north-central Canada to the Gulf of Mexico. South Dakota was once a small portion of this prehistoric seaway's muddy bed.

The extinct marine giant may have lived along side the dinosaurs, but the prehistoric Archelon was clearly no dinosaur. This marine reptile belonged to the Order Chelonia and the Family Protostegidae and is believed to be a relative of the modern-day, endangered Leatherback Sea Turtle.

The prehistoric Archelon ischyros had paddle-like legs that served as flippers, so the creature could swim. It also had a fairly narrow and highly vaulted shell that was unlike that of most modern-day turtles in that it did not have solid shell bones. Researchers believe the Archelon had either a leathery covering or horny plates over the bony framework on its back.

One distinct characteristic of this gigantic marine turtle was its tremendous overbite. While Archelon's beak had no cutting edge, the turtle's heavy mandible, thick palate and hooked beak produced a deadly bite. Since the Archelon was a carnivore (meat-eater), this large, powerful jaw served the creature well. Its curved beak also kept food in its mouth while being crushed. The Archelon may have used its beak to dig for food, as well.

Additional characteristics of the Archelon include a long, narrow head and a pointed tail. While the average Archelon typically reached lengths of nine to 13 feet, the largest specimen found spanned 16 feet from snout to tail. Its shell alone measures seven feet, three inches.

In 1895, the first Archelon fossil was found by G. R. Wieland at the south fork of the Cheyenne River, 35 miles east of Black Hills, South Dakota. This specimen measures almost 11 feet from snout to tail, and when fully stretched out, it measures 15 feet from the flipper to flipper. This giant Archelon skeleton has been on display Yale University's Peabody Museum of Natural History in New Haven, Connecticut since 1907.

While fossilized remains of the giant Archelon ischyros have been found throughout the states of South Dakota, Kansas and Nebraska, the largest and most complete specimen ever found was unearthed from the soil in southwestern South Dakota, near the Cheyenne River at Buffalo Gap during the mid-1970s. This particular specimen was discovered by a private collector who later sold it to Siber and Siber of Switzerland. Siber and Siber then sold the Archelon skeleton to the Naturhistorisches Museum Wien (NMW) in Vienna, Austria, where it has been on exhibit since 1982. This ancient creature is referred to as the Vienna specimen of Archelon ischyros and, to date, was the largest turtle to have inhabited our planet.

With folded arms and head bowed when discovered, this giant sea turtle is said to have died while brumating (hibernating) on the seafloor. Much of the Vienna specimen's skeleton had been very well-preserved due to being embedded in a limestone concretion. The rest of the specimen, exposed to the elements for 70 million years, were weathered and crushed by compaction. For this reason, the snout to tail measurement of 15 feet, 1 inch was several inches shorter than the giant creature's actual measurements when it was alive and swimming in the Western Interior Seaway. It is estimated that the Vienna specimen of Archelon was approximately 16 feet from snout to tail, 13 feet wide, flipper to flipper and weighed 11,300 pounds.

Since Archelon fossils are extremely rare, representatives from the Black Hills Institute of Geological Research went to Austria to make a cast of the skeletal remains of the Vienna specimen to make identical replicas of the fossil for domestic and international museums. One such replica can be viewed at Reptile Gardens in Rapid City, South Dakota -- just 20 miles from its excavation site.

Representatives from the South Dakota School of Mines and Technology have found two Archelon skeletons in the shale banks near Pierre, South Dakota. One specimen was found in 1996 and the other was found in 1998. According to Dr. Jim Martin, a geology professor at the school who was on both digs, "We were able to recover about 70 percent of the skeleton that was found in 1996. The skeleton that was found in 1998 was much more complete." Dr. Martin said that both specimens are roughly the same size and estimated that from the tips of their flippers the creatures were 15 feet wide and the snout to tail measurement is about 12 feet. Both Archelon fossils are currently being prepared for display at the school's geology museum.

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The Giant Archelon Ischyros

The Cretaceous Period: Age of Reptiles

The Western Interior Seaway

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The Anatomy of a Turtle

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