Fig. 10. Probable distribution of S. veraepacis, S. longirostris, and the S. ornatus group (stipple) and of their Wisconsinan ancestors (lines). Heavy line indicates limits of Wisconsinan glaciation.
In fact, we do find species closely related to Sorex vagrans in just such places today ([fig. 10]). Probably Sorex ornatus, including members of the ornatus group such as S. trigonirostris, S. sinuosus, S. willeti, S. tenellus, and S. nanus, and also S. veraepacis, arose by separation from the ancestral vagrans stock in Sangamonian time. Probably the eastern S. longirostris arose in a like manner. The ancestor of S. ornatus may have been isolated in southwestern California during Sangamonian time, spread north and south during the Wisconsinan age, and afterward given rise to S. trigonirostris and the modern S. ornatus complex of California and Baja California. In at least one place reproductive isolation between ornatus and the invading S. vagrans has broken down (Rudd, 1953); the place is a salt marsh along San Pablo Bay, where a hybrid population between S. vagrans and S. sinuosus, an ornatus derivative, has formed. Sorex tenellus may have been isolated in the Sierra Nevada in the Sangamonian interval, moved into the valleys east of the mountains during the Wisconsinan age, and become restricted to its present range since the retreat of the last ice. Sorex nanus may have occurred in the Black Hills and isolated mountains of Arizona and New Mexico during the Sangamonian interval and remained in these general areas during the Wisconsinan age. Its present range is peripheral to the main body of the Rockies and the Colorado Plateau.
The eastern species Sorex longirostris has many similarities with shrews of the ornatus-vagrans stock. S. l. longirostris is close in many ways to S. nanus. Indeed, the differences between the species S. nanus, S. ornatus, and S. longirostris seem to me to be of the same magnitude and indicate a similar period of differentiation from a common ancestor. The ancestor of S. longirostris may have gained access to the eastern United States in the Illinoian Age via the northern Great Plains south of the glacial boundary ([fig. 7]). The ancestor of Sorex veraepacis of southern Mexico probably reached that area in Illinoian time as part of the ancestral vagrans stock and probably attained its differentiation during the Sangamonian interval.
All the kinds of shrews so far discussed, including the S. vagrans complex, might thus be thought of as having had a common ancestor in the Illinoian Age. This entire group of shrews has the third unicuspid smaller than the fourth, a pigmented ridge from the apex to the cingulum of each upper unicuspid, and, in most individuals, lacks a post-mandibular foramen in the lower jaw (Findley, 1953:636-637). The pigment is not always prominent in S. longirostris.
Two other species of North American shrews, Sorex palustris, the water shrew, and Sorex bendiri, the marsh shrew, show these three characters to a greater or lesser degree, and it seems that these two species and the vagrans-ornatus-veraepacis group had a common ancestor, probably before Illinoian time for reasons stated beyond. I judge, however, that far from being subgenerically distinct as they have been considered to be, S. palustris and S. bendiri are actually closely related species of the same subgenus and may have differentiated from one another because of separation into eastern (palustris) and western (bendiri) segments in the Sangamonian interval, much as has been postulated concerning the eastern and western stocks of Sorex vagrans. Indeed, Jackson (1928:192) has noted that in the Pacific northwest the characters of the two kinds approach one another and become differences of degree only.
The widespread species Sorex cinereus resembles all the foregoing species in the ridges on the unicuspid teeth and in the lack of a post-mandibular foramen, but differs from those other species in having the third upper unicuspid larger than the fourth. The subspecies S. cinereus ohionensis, however, often has the sizes of these teeth reversed. With S. cinereus I include S. preblei (eastern Oregon) and S. lyelli (Sierra Nevada), both obviously closely related to cinereus as Jackson (1928:37) recognized when he included them in the cinereus group. Sorex milleri (Coahuila and central western Nuevo Leon) seems to me to resemble S. cinereus more than it does other species of North American Sorex, and I judge that it also belongs to the cinereus group. Sorex cinereus and its close relatives seem more closely related to the species which have thus far been discussed than they do to such other North American species as S. arcticus, S. fumeus, S. trowbridgii, S. merriami, and the members of the S. saussurei group; most of these five species last mentioned possess a post-mandibular foramen, lack pigmented unicuspid ridges, and have the third unicuspid larger than the fourth. Because of the morphological resemblances mentioned above, it seems likely to me that S. cinereus and the vagrans-ornatus-veraepacis-palustris complex had a common ancestor in early Pleistocene time. Sorex cinereus has recently been considered to be conspecific with the Old World S. caecutiens Laxmann (Van den Brink, 1953) which name, being the older, would apply to the circumpolar species.
Hibbard (1944:719) recovered S. cinereus and a species of Neosorex (a name formerly applied to the water shrew) from the Pleistocene (late Kansan) Cudahy Fauna. This indicates that the ancestors of the modern S. cinereus and of the water shrew had diverged from one another before that time. Brown (1908:172) recorded S. cinereus and S. obscurus from the Conard Fissure in Arkansas. These materials were deposited probably at a later time than was the Cudahy Fauna. The S. obscurus from Conard Fissure probably represents the ancestral S. vagrans stock which I think reached eastern United States in Illinoian time and gave rise to S. longirostris. The Conard Fissure material was deposited at a time (Illinoian?) when northern faunas extended farther south than they do today.
All of the species mentioned as having structural characters in common with S. vagrans seem to have arisen from a common ancestor which had already differentiated from the ancestor of such species as S. arcticus, S. saussurei, and others. Consequently all are here included in a single subgenus. The oldest generic name applied to a shrew of this group, other than the name Sorex, is Otisorex DeKay, 1842, type species Otisorex platyrhinus DeKay, a synonym of Sorex cinereus. The subgenus can be characterized as follows.
Subgenus Otisorex DeKay
1842. Otisorex DeKay, Zoology of New York, pt. 1, Mammalia, p. 22, and pl. 5, fig. 1. Type, Otisorex platyrhinus DeKay (= Sorex cinereus Kerr).