Early Tertiary Rodents of North America

Abstract

The gnawing mammals of the present day exist in the greatest of profusion and diversity. In sheer population they exceed all other mammals combined, and this is nearly as true of number of species and genera. In the higher taxonomic categories below the rank of suborder, their great diversity is shown by the much greater number of families and subfamilies than in any other order of mammals. Almost all these gnawing animals are classified in a single order, the Rodentia (rodents). Those that are not, the Lagomorpha (hares, rabbits, pikas), though existing in considerable numbers, are among the most homogeneous of orders. They are at best only distantly related to the rodents. The following remarks apply only to the Rodentia. The great number of categories necessary for classifying existing rodents testifies to the tremendous evolutionary development which has taken place within the order. A second outstanding feature of the order is the astonishing amount of parallelism in tooth pattern between forms placed in different major groups. The same pattern in some instances is reproduced several times over. The repeated parallel developments in both living and fossil rodents suggest that all these animals have descended from a common ancestor which lived in the not too remote past. It is generally agreed that the greater the parallelism, the greater the genetic relationship, and this tenet applies not only to duplication of pattern, but necessarily also in some measure to the number of such duplications. Similar needs acting on distantly related groups should not be so effective in producing parallelisms as are those which act on closely related stocks.