Paramys

Abstract: Representatives of the order Rodentia first appear in the late Paleocene (Tiffanian) deposits of the Fort Union Series. The known material is, however, exceedingly fragmentary, for only a single lower molar tooth and some incisors have been found. The specimens have been referred to the genus Paramys (sensu lato). They demonstrate that the characteristic rodent incisor and molar were already developed at the time. These teeth suggest that most of the other characteristic rodent features were also present, although it is, of course, possible that some primitive features absent in other rodents were still retained. Paramys (sensu lato) is relatively well known from the early Eocene (early Wasatchian) deposits of the Willwood formation, and similar rodents are known from the early Eocene of Europe. In their comparable parts, these later rodents do not differ in any important respect from the late Paleocene species. All rodents known from the late Paleocene and early Eocene pertain to the superfamily Aplodontoidea and to the subfamily Paramyinae (or family Paramyidae of other authors), the most primitive group of rodents. These early paramyines are nearly structurally antecedent to all later rodents, if indeed they are not their actual ancestors. Primitive as they are, they already have acquired the formally diagnostic characters of the order insofar as skeletal structures are concerned; and these characters, fully developed, show no transition to those of the Insectivora (sensu stricto), or to any known primitive placental. Several early insectivore or primate groups (mixodectids, apatemyids, Phenacolemur) exhibit some degree of convergence in the incisors and lower jaws toward a rodent type of structure. Osborn years ago placed the mixodectids in a distinct suborder of ancestral rodents, the Proglires (Osborn, 1902, p. 203). Further detailed examination has failed to reveal any evidence for such a relationship. Structures common to the mixodectids (presumptive insectivores) and ro'dents are shown also in several other groups which have no special relationship to the Rodentia. The apatemyids and Phenacolemur are regarded currently as probable primates. It seems that in the early Cenozoic several placental groups had similar adaptations of the front teeth with attendant, correlated modifications of the jaws. Perhaps these groups indicate a series of competing forms which finally passed to extinction as the rodents slowly rose to ascendancy. Many workers have accepted the conclusions of W. D. Matthew (1910), and have followed his thesis that the paramyines are actually ancestral to all later rodents, and that rodent diversification in the higher categories has been a postPaleocene event. Others, particularly Miller and Gidley (1918), have at least implied that the known stratigraphic record is faulty, and that all major groups of rodents (approximately the superfamilies of the Miller and Gidley classification of 1918) were in existence by early Eocene time. Advocates of Miller and Gidley's view perhaps tend to think of the origin of the order as occurring at an earlier time than do those advocating an Eocene diversification. According to various suggestions, the time of origin may" range from far back in the Cretaceous to early Paleocene. If we accept one of these dates, particularly a Cretaceous one, and agree

Abstract: Introduction The structural characters and phylogenetic relationships of Tertiary aplodont rodents have been discussed by several authors. Although the earliest known aplodonts occur in the John Day formation, it has been advocated, especially by Matthew, 1 that the origin of the group is to be found in the family Ischyromyidae as represented by the genera Prosciurus and Paramys . Schlosser, 2 however, has suggested the derivation of the John Day genus from Sciurodon of the European Phosphorites. The new genus of rodent described in this paper is clearly ancestral to Allomys ( Meniscomys ) and Haplomys . Occurring in the upper Eocene stage of the Sespe deposits of southern California, it is definitely recorded as a considerably earlier type than the forms from the late Oligocene or the early Miocene. The stratigraphic position of the new aplodont in the Sespe is distinctly lower in the section than the late Eocene faunal stage now being described 3 . . .

Abstract: Abstract The morphology of the external surface of the petrosal and its junction with the basioccipital in Paramys copei is described. Comparison with an outgroup of the Cretaceous Vincelestes and Recent Didelphis and Solenodon reveals that the auditory region retains many primitive features. The Eocene paramyines from North America show only slight differences. Sciuravus is set apart by the lack of a ventral petrosal sinus canal between the petrosal and basioccipital and by the facial nerve and stapedial artery sharing a common foramen in the petrosal. European theridomyids, too, are not as primitive as Paramys but share with it and Sciuravus a ridge on the promontorium that separates the transpromontorial continuation of the internal carotid artery from the origin of the tensor tympani muscle. Twelve characters of the auditory region are analyzed and summarized in a data matrix for use in future studies. Relative primitiveness of the auditory region in all of these Eocene rodents suggests that derived characters in later rodent taxa may represent intraordinal relationships but only homoplasy with nonrodents.