On June 19, 1920, workers on the Michigan Central Railroad yards nears Niles unearthed the skull of a giant beaver. The giant beaver was a prehistoric beast that roamed North America along with mammoths and mastodons. It could grow to approximately eight feet in length and weigh nearly a quarter of a ton. The giant beaver became extinct during the last Ice Age 10,000 years ago.
Source: Michigan Every Day
For more information, see Castoroides wikipedia entry