Pliocene rodents of western North America

Knowledge concerning the extinct mammal faunas of North America has increased enormously since the beginning of the present century. Certain groups of mammals are known in great detail and fairly satisfactory "trees" showing their phylogenetic relationships have been erected. Other groups are not so well known, but the general course of their history has been traced and little remains but to fill in the gaps and to make minor corrections. On the other hand, a few groups have proved more or less refractory, and the order Rodentia may be regarded as one of the more important of these. Not only are the broader phases of rodent evolution, such as the differentiation of modern families from ancestral stocks, not at all well known, but it is commonly assumed that rodent types are remarkably stable, the same genus, and even species, having an extremely long range in geologic time as compared with those grouped under other orders. Hence the rodents at present are regarded by many as an order offering but little from an evolutionary standpoint, and as barred by their stability from any use in detailed stratigraphy and correlation