Castoroides

Castoroides, or giant beaver, is an extinct genus of enormous beavers that lived in North America during the Pleistocene. C. leiseyorum and its northern sister species Castoroides ohioensis, were the largest beavers to ever exist.

Description
Castoroides, also known as giant beavers, are much larger than the closely related average beaver, as it name implies. This knowledge makes it the largest rodent in North America during the last ice age and the largest known beaver. Their starting length was approximately 1.9 meters and they could grow as large as 2.2 meters. Weight of the giant beaver can vary from 90-125 kilograms, which is the equivalent to the range of 200-275 pounds.

Its hind feet were much larger than in modern beavers, but because soft tissues decay, it is not known whether its tail resembled the tails in modern beavers, and it can only be assumed that its feet were webbed like in modern species.[2] [2] One of the defining characteristics of the beavers is their incisors, which differ in function from that of regular beavers. These incisors were not efficient in cutting wood, therefore it can safely be assumed that the giant beaver did not construct dams. All of this considered, the skull structure of the giant beaver shows that it presumably participated in extended underwater activity, thanks to the ability to take in more oxygen into its lungs.[2]

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 15 cm (6 in) long.[3] In addition, the tail of the giant beaver must have been longer but narrower and its hind legs shorter.[1]

One other major difference between the giant beaver and the average beaver is that the size of its brain is proportionally smaller than its counterpart. From this information it can be gathered that giant beavers had inferior interactions in its environment as well as less complex patterns of thoughts and behavior.

Classification
There are two known species:
 * Castoroides leiseyorum (found in Florida only)
 * Castoroides ohioensis, synonym Castoroides nebrascensis (found throughout continental United States and Canada)

These two species of giant beaver (genus Castoroides) are not close relatives to modern beavers (genus Castor).[2]

This genus typifies the extinct subfamily Castoroidinae, which forms a North American lineage beginning with the Hemingfordian genus Monosaulax, followed by Eucastor, Dipoides, and Procastoroides, to finally culminate and go extinct with Castoroides.