Artists reconstruction of Merycoidodon. By Nobu Tamura (http://spinops.blogspot.com)
This is a 2.15" long jaw section of Merycoidodon culbertsoni, an Oreodont from the White River Formation of Pennington County, South Dakota.
Oreodonts are an extinct mammal most closely related to camels and pigs. They have no close relatives living today. They were herbivorous, with short faces and fang-like canine teeth. About the size of a sheep, they roamed the plains of North America in huge numbers during the Oligocene period.