It is known from many well-preserved specimens found in the famous Fossil Butte deposits of the Green River Formation in Wyoming, US.

[2] It was a close relative of the modern alligator gar (A. spatula), and one of two Atractosteus species known from Fossil Butte alongside the even larger A. atrox.

It can be differentiated from the sympatric A. atrox by its lower number of lateral line scales and vertebrae, and reaching only 1 metre (3.3 ft) in length.

[1] A. simplex was a highly predatory fish, with one fossil specimen preserving a young Diplomystus in its jaws.

As gars are not known to engage in coprophagy, this is thought to likely represent an accidental ingestion or a consequence of taphonomy.