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Castoroides ohioensis was a species of Giant Beaver, huge members of the family Castoridae (Rodentia), endemic to North America during the Pleistocene epoch (1.8 mya to 11,000 years ago).

These giant Ice Age beavers had a length up to 8 feet and an estimated weight of 130-220 pounds; some estimates went up to 485 pounds. It lived in North America during the Pleistocene epoch, and went extinct at the end of the last Ice Age, 12,000 years ago.

Fossils of the Giant Beaver are concentrated around the Midwestern United States in states near the Great Lakes, particularly Illinois and Indiana, but specimens are recorded from Alaska and Canada to Florida. Specimens from Florida have been placed in a separate subspecies, Castoroides ohioensis dilophidus, based on differences in premolar and molar features.

One of the important anatomical differences between the Giant Beaver and modern beaver species, besides size, is the structure of their teeth. Modern beavers have chisel-like incisor teeth for gnawing on wood. The teeth of the Giant Beaver are bigger and broader, and grew to about 6 inches long. In addition, the tail of the Giant Beaver must have been longer but narrower, and its hind legs shorter. Its great bulk might have restricted its movement on land (although large squat-legged hippopotamuses can move on land with little difficulty).

The first Giant Beaver fossils were discovered in 1837 in a peat bog in Ohio, hence its species epithet ohioensis. Nothing is known on whether or not the Giant Beaver built lodges like modern beavers. In Ohio, there have been claims of a possible Giant Beaver lodge four feet high and eight feet in diameter, formed from small saplings. The recent discovery of clear evidence for lodge building in the related genus Dipoides indicates that the Giant Beaver probably also built lodges.