Stylemys: A Fossil Tortoise from the White River Badlands

by Broden William Dymond

Stylemys was a tortoise that lived about 33 million years ago, during the Oligocene. Stylemys lived on land, rather than land and water. Fossils of Stylemys have been found in rocks in North Dakota, nearby states, Asia, and Europe. Stylemys was first described by Joseph Leidy, a fossil expert, in 1851. He was one of the important scientists in America at the time. This tortoise fossil was found in the Brule rock formation in the White River Badlands of South Dakota. 

Fossilized carapace of Stylemys nebrascensis.
Found in the White River area of North Dakota.
Photo by B. Dymond. From the collection of S. W. Veatch. 

During this Oligocene, North Dakota was a treeless plain covered in shrubs, plants, and some types of grass. It rained sometimes and the temperature was mild.

Stylemys had a domed carapace (upper shell) and a plastron (lower shell) for protection. Their limbs were made for living on land. Stylemys was similar to Gopher tortoises that are living today and they probably ate plants. The legs of Stylemys were different from Gopher tortoises so they probably did not dig tunnels. Turtles and tortoises at this stage in evolution survived well and have not changed much since that time.

The fossil of this Stylemys is slightly crushed with scratch marks near the tail and seems to have the mark from a tooth. Here is what might have happened to this Stylemys:

‘’One day, the tortoise was just walking along. He ate some grass just a little bit ago. He heard something behind him. It was a prehistoric crocodile! When the tortoise saw the crocodile, it started to run. The crocodile was faster, and it caught up to the tortoise. It bit and scratched the tortoise, but the tortoise fell into the mud. It was stuck so the predator could not get to it. The crocodile wanted the tortoise so bad but he noticed a sand storm coming and ran for safety. The end.”   It was from this comic:

Artwork by the author.

About the author:   Broden Dymond is an active Pebble Pup in the Grand Traverse Area Rock and Mineral Club. He is 10 years old and is in the 4^th grade.