The mountain type of this, seen here, is the most primitive living rodent
I've only ever seen maybe three or four in all my years doing... [DOC] [TLE] Rodent - definition of rodent by The Free Dictionary [PAR] Rodents make up about half the living species of mammals, and include rats, 
mice, beavers, squirrels, ... squirrel - a kind of arboreal rodent having a long 
bushy tail ... Aplodontia rufa, mountain beaver, sewellel - bulky nocturnal 
burrowing rodent of uplands of the Pacific coast of North America; the most 
primitive living rodent.