The official state fossil of West Virginia is Megalonyx jeffersonii, the remains of an extinct species of ground sloth. During the Ice Age, the creature inhabited woodlands, knocking over trees and digging up vegetation with its giant claws. The ground sloth stood more than eight feet tall and weighed more than 1,000 pounds.

In the 1790s, workers mining for saltpeter in a Monroe County cave found several large bones. A Virginia man named John Stewart sent the bones to Thomas Jefferson, who presented them at a meeting of the American Philosophical Society in Philadelphia in 1797. Jefferson believed that the bones were the remains of a prehistoric lion and called the creature Megalonyx, meaning "great claw." In 1799, a physician named Caspar Wistar published a paper that described the fossil and credited Jefferson with its discovery.

The original Megalonyx fossil is housed in the Academy of Natural Sciences in Philadelphia. Other Megalonyx fossils have been found in two Monroe County caves and caves in Pendleton County and Greenbrier County. Such fossils have also been found in others parts of the United States.

Replicas of the Megalonyx jeffersonii are on display at the West Virginia Geological and Economic Survey Museum in Morgantown and at the Interstate 64 welcome center near Lewisburg. Ray Garton, the curator of the museum, helped craft the legislation that made the Megalonyx jeffersonii the official state fossil. The legislation was first introduced in 2007 and passed in 2008.