Palaeocastor is an extinct genus of beavers that lived in the North American Badlands during the late Oligocene period to early Miocene, 29.5~18.5 million years ago. Palaeocastor was much smaller than modern beavers. There are several species including Palaeocastor fossor, Palaeocastor magnus, Palaeocastor wahlerti, and Palaeocastor peninsulatus. The animals first became known on grounds of their fossilized burrows, the "Devil's corkscrews."