Castoroides and Mastodon

So far the largest known rodent to have existed, Castoroides ohioensis the giant beaver probably lived its life more like an enormous Muskrat than the engineering beavers we know today.

Specimens of this animal have been found from Florida to the Yukon territories, but primarily in areas just south of the present day great lakes.

With lengths in the six to eight foot range including tail and an estimated weight of 500 pounds this beast definitely left its mark upon the land...or marsh.

Although referred to as the "giant beaver" Castoroides which means "beaver like" was not a direct ancestor of modern Castor, just a close cousin.

 There are anatomical differences between the two. Castoroides had much shorter limb proportions which made maneuvering its bulk on dry land slow and awkward. So unlike today's beaver and Muskrats it probably did not migrate over land to new marshland territories very effectively, which may give one clue to its extinction. The tail was also longer and shows evidence of being only slightly flattened like a modern beaver. The most striking differences are in the skull of Castoroides. Like todays beaver it is very deep, but the similarities end when looking at the teeth. Castoroides teeth were longer and blunter at the tips with prominent ridges in the enamel. Perhaps the ridges helped to buttress the long teeth against lateral forces when doing whatever it was that Castoroides did with its teeth when feeding. The molars of castoroides also had a simpler grinding pattern than modern beaver. Some paleontologists think that castoroides fed mainly on softer types of vegetation such as cattails, rushes, and water Lillies like a muskrat. Grooves in the skull where the nerves passed through also seem to indicate that castoroides had very poor vision, but an excellent sense of smell that surpassed today's beaver.

during the final stages of the Pleistocene Castoroides and modern beaver coexisted in the southern parts of its range. 

It seems that about 9,000 years ago the vast glacial melt water ponds, lakes and marshes began to dry up and castoroides, unable to manipulate its environment like modern beavers do, or to move its bulk overland effectively disappeared as well. Who knows what role man had in the disappearance of the giant beaver, there are no records left by ancient hunters that show any representations of castoroides.