The fossilized remains of a four-foot reptile known as Champsosaurus were excavated from a hillside in the South Unit in October 1995 by Dr. John Hoganson, paleontologist with the North Dakota Geological survey, and his assistant, Jonathan Campbell. The ancient crocodile-like reptile once inhabited the fresh waters in what is now western North Dakota about 55 million years ago during the Paleocene epoch. This was a time when the climate was subtropical, similar to that of present day Florida. .

The long, narrow snout and large powerful back legs would have enabled the Champsosaurus to feed on fish, snails, mollusks and turtles. It is believed that this aggressive predator, that attained lengths up to about 10 feet, spent much of its time submerged in water waiting for prey and could lunge from the bottom using its powerful back legs. A turtle shell with bite marks also found during this survey provides a vignette of life from this era.

During the paleontological inventory, more than two hundred fossil sites were identified and mapped from the rock layers known as the Sentinel Butte and Bullion Creek formations. The sites include two other partial Champsosaur skeletons plus numerous freshwater mollusk remains, turtles (snapping and soft-shelled), parts of crocodile and alligator, as well as plant fossils. The most common kind of fossil found was petrified wood, including large tree stumps measuring seven to eight feet in diameter. It is believed that most of these trees were conifers, such as cypress and sequoia.

The paleontological investigation enhanced our view of the park's geologic story. By identifying what creatures and plants existed here, one can get an impression of what life was like millions of years ago. With a clearer picture of the past, we gain a better perspective on this continuously changing environment.