Lets face it Beavers are weird!

Beavers belong to a large diverse group of mammals called the Rodents.
Unlike the other members Rodents (mice and rats), beavers are rather unique and specialized in their habit. First they have those ever growing incisors that they use to cut down trees like an axe. Second, they build dams and create unique ponds in which they swim and live in. Lastly beavers have that characteristic flat tail. So, did all these specialization occur just once in the long fossil record of beavers, or did various beaver lineages specialize for a similar habit has the climate changed in the later half of the Cenozoic. Natalia Rybczynski at the Canadian Museum of Nature, just published a study that suggests that a single beaver lineage (Castorinae) made such a change in habit early in the fossil record. The diverse sister group Palaeocastorinae known only from the fossil record, specialized for a dramatically different environment. Palaeocastorines specialized in "tooth-digging" and lived in burrows. Somewhere in the late Oligocene to early Miocene the two groups split into a terrestrial burrowing group and aquatic swimming group. During the Miocene the burrowing group became extinct, leaving only the castorines with their strange ways.

Rybczynski, N. 2007. Castorid Phylogenetics: Implications for the Evolution of Swimming and Tree-Exploitation in Beavers. Journal of Mammalian Evolution. 10.1007/s10914-006-9017-3.