Pliocene Rodents of Western North America

This thesis embraces a review of Pliocene rodent faunas and their evolution as well as the description of several new Pliocene rodent faunas which contain forms of interest to paleontology. In the course of the work detailed faunal studies were carried on covering rodent faunas from (1) Kern River beds, California; (2) Pliocene beds of Smiths Valley, Nevada; (3) Owyhee Pliocene of Rome, Oregon; (4) Pliocene beds in the Coso Mountains, California; and (5) beds exposed near Grand View and Hager­man, Idaho. Examination, in whole or in part, of the fol­lowing faunas was also made: (1) Pliocene fauna from Bart­lett Mountain near Drewsey, Oregon; (2) Fish Lake Valley fauna, Nevada; (3) Rattlesnake, Oregon; (4) Thousand Creek, Nevada; and (5) Curtis horizon of the San Pedro Valley beds, Arizona. Other Pliocene rodent faunas were studied only through the published accounts of these faunas. As a result of the studies it was found that Pliocene rodent faunas fall into several major faunal stages distinguished by the general nature of the faunas as well as by the presence of characteristic genera and species. The chief characteristics of these stages are as follows: I. Lower Pliocene (1) great predominance of sciuromorphs over myomorphs (2) high percentage of extinct genera (3) no strikingly new or introduced types (4) presence of the genus Eucastor II. Middle Pliocene (1) great predominance of sciuromorphs over myomorphs (2) genera about half extinct and half living (3) introduction of Castor in later faunas (4) first appearance of Microtinae (5) presence of "gigantic" Peromyscine types (6) presence of the genus Dipoides (7) last appearance of the mylagaulid rodents III. Upper Pliocene (1) sharp faunal break with the middle Pliocene as evidenced in: (a) decided increase in the myomorph population (b) decided decrease in the number of extinct genera (c) first appearance of many modern types (2) no Mylagaulidae (3) presence of the genus Mimomys, as well as the relative abundance of microtines (4) no Dipoides, or at present any representative of the Eucastor-Dipoides line (5) absence? of Lepus, except perhaps in the final faunal stages (6) first appearance of "typical" Citellus (7) all species probably extinct The rodent record is still very incomplete and this fact together with the short duration of Pliocene time has served to limit the amount of observable evolution in the Rodentia during this epoch. However, the study has shown, contrary to the usual belief, that in certain groups evolu­tion has proceeded at a fairly rapid rate. Moreover, it appears that post-Pliocene evolution in the rodent group is quite marked, in certain families at least.