This may look like a modern sun-bleached turtle shell, but it's actually the fossilized shell of Stylemys nebrascensis, an ancient tortoise from around 30 million years ago, during the Oligocene Epoch. The species name means “pillar turtle of Nebraska,” with "pillar" being a reference to their sturdy, column-like limbs.
Named by Dr. Joseph Leidy in 1852, this species is one of the earliest described fossil turtles from North America. Individuals have been found throughout the Badlands in Nebraska, Colorado, Wyoming, North Dakota, and South Dakota.
Due to their sturdiness, it is not unusual to only find the tortoise’s shell. A fused spinal column and rib cage make up the extra-dense bone of the shell. The horizontal ridges along the sides are the result of annuli (growth lines), where more keratin is deposited in layers at the base of the scutes (the individual plates that cover the shell) on top of the bone. These layers give the appearance of bands, similar to tree rings. Stylemys nebrascensis is likely related to modern-day gopher tortoises (Gopherus polyphemus), which roam the coastal plains of Louisiana, Mississippi, Georgia, Florida, and into South Carolina.
Our fossil individual was discovered in the Brule Formation in Nebraska, a particular layer of fine-grained sediments formed by streams and floodplains in a grassland environment. Over millions of years, the environment became arid, and erosion sculpted the earth into bluffs, pinnacles, and other geologic formations. This exposed early sedimentary layers like the Brule Formation and its fossils.
The River Museum is home to our very own gopher tortoise, who educates guests both on-site and as part of the outreach program!