Archelon ischyros, chief or lead turtle, swam the shallow seas of the late Cretaceous, a contemporary of dinosaurs. The bone plates of the carapace are reduced to struts possibly covered with thick skin similar to a modern leather-back turtle. Large front flippers and strong vertical strokes were possibly used to propel itself through the water in a similar manner to the aqua-flight of penguins. This turtle would have been twice as long as the leather-back, about the size of a Volkswagen bus, reaching 4 meters in some individuals.

This is a cast of the discovery made by the Watson family in the Pierre Shale near Buffalo Gap, South Dakota. It was displayed under the canvas tent set up at the Animal Kingdom, Disney World, Orlando Florida, July 2000.