The fossil record of the genus Zapus is scanty. All of the known fossils of it are lower jaws of Pleistocene Age. The Recent species Z. hudsonius was recorded by Cope (1871:86) in the Port Kennedy Cave fauna (pre-Wisconsinian) of Pennsylvania. Gidley and Gazin (1938:67) reported a single mandibular ramus bearing m1-m3 recovered from the Cumberland Cave (pre-Wisconsinian) of Maryland. The teeth are not typical of modern Zapus in that m1 and m2 are shorter crowned and m1 has a longer anterior lobe. Gidley and Gazin, nevertheless, considered their material insufficient for establishing a new species.
Two extinct species have been described: Zapus burti Hibbard (1941:215) from the Crooked Creek formation (= Meade formation of the State Geological Survey of Kansas) mid-Pleistocene of Kansas and Zapus rinkeri Hibbard (1951:351) from the Rexroad formation (= Blanco formation of the State Geological Survey of Kansas) of Blancan Age of Kansas. Both species resemble Zapus hudsonius, but differ from it in broader crowned more brachydont cheek-teeth. Z. rinkeri differs from Z. burti and Z. hudsonius by a more robust ramus, broader molars, and three instead of two internal re-entrant valleys posterior to the anterior loop on m1. The three species Z. rinkeri, Z. burti, and Z. hudsonius are in a structurally, as well as a geologically, progressive series. The trend in dentition is from broad, brachydont cheek-teeth to narrow, semi-hypsodont cheek-teeth.
RELATIONSHIPS, DISTRIBUTION, AND SPECIATION
Relationships in the Subfamily Zapodinae
The subfamily Zapodinae is known from Pliocene and Pleistocene deposits of North America and now occurs over much of northern North America and in Szechuan and Kansu, China. The living species occur among grasses and low herbs in damp or marshy places both in forested areas and in plains areas.
The early Pliocene Macrognathomys nanus Hall (1930:305), originally described as a Cricetid, is actually a Zapodid as shown by the structure of the mandibular ramus, shape of the incisors, and occlusal pattern of the cheek-teeth.
If Macrognathomys can be considered a member of the subfamily Zapodinae (possibly it is a sicistine) then it represents the oldest known member of this subfamily. Judging from the published illustrations, Macrognathomys seems to be structurally ancestral to the Mid Pliocene Pliozapus solus Wilson; the labial re-entrant folds are wider and shorter and on m2 and m3 fewer. The difference in stage of wear of the teeth in Macrognathomys and Pliozapus is a handicap in comparing the two genera but they are distinct. Wilson (1936:32) points out that Pliozapus clearly falls in the Zapodinae and stands in an ancestral position with respect to the structurally progressive series Eozapus, Zapus, and Napaeozapus. Nevertheless, Pliozapus cannot be considered as directly ancestral to Eozapus because of the progressive features in the dentition of Pliozapus. Wilson (1937:52) remarked that if Pliozapus is ancestral to Zapus and Napaeozapus, considerable evolution must have taken place in the height of crown and in the development of the complexity of the tooth pattern. In contrast to Wilson’s opinion, Stehlin and Schaub (1951:313) placed Pliozapus and Eozapus in the subfamily Sicistinae because certain elements in the occlusal pattern of the cheek-teeth are similar. I disagree with those authors and hold with Wilson; I consider Pliozapus and Eozapus in the subfamily Zapodinae. In dental pattern Pliozapus, as Wilson (1936:32) pointed out, resembles the Recent Eurasiatic sicistid, Sicista more than do Zapus or Napaeozapus. Nevertheless, from Sicista Wilson distinguishes Pliozapus and relates it to the subfamily Zapodinae by: "more oblique direction of protoconid-hypoconid ridge, anterior termination of this ridge at buccal portion of protoconid rather than between protoconid and metaconid as in Sicista; cusps more compressed into lophs; cheek-teeth somewhat broader; greater development of metastylid; greater development of hypoconulid ridge, … absence of anteroconid…."
Eozapus is more closely related to Pliozapus than to either Zapus or Napaeozapus (Wilson, 1936:32) but all four genera are in the subfamily Zapodinae. Stehlin and Schaub (op. cit.:158 and 311) relate Eozapus to the subfamily Sicistinae on the basis of similarity in the occlusal pattern of the cheek-teeth of Eozapus and various sicistines. Stehlin and Schaub do not consider other structures such as the elongate hind limbs, the shape of malleus and incus, and the shape of the baculum, in which there is close resemblance to the Zapodinae. It is these structural similarities as well as those, pointed out by Wilson (loc. cit.), in dentition that leads me to place Eozapus in the subfamily Zapodinae. The early Pleistocene Zapus rinkeri Hibbard shows that the Zapus stage of development had already been achieved perhaps as early as the late Pliocene. Hibbard (1951:352) thought that Zapus rinkeri was not structurally intermediate between Pliozapus and any Recent species of Zapus; although the teeth of Z. rinkeri have the broader, shallower, re-entrant folds of Pliozapus, these teeth are higher crowned and have an occlusal pattern resembling that of the Recent species of Zapus. The middle Pleistocene species, Zapus burti Hibbard, progressed essentially to the structural level of the Recent Zapus hudsonius, but the molars were more brachydont, broader crowned, and their enamel folds less crowded. Pleistocene material of pre-Wisconsin age obtained from cave deposits in Pennsylvania and Maryland is most nearly like Zapus hudsonius. One such cave deposit in Maryland contained an example of the Recent genus Napaeozapus, indicating that its history dates from at least middle Pleistocene time.
The Asiatic Recent Genus, Eozapus, has not progressed much beyond the Pliocene stage in zapidine evolution if Pliozapus be taken as a standard; the North American Recent Genus Zapus essentially achieved its present form by early Pleistocene times, and the Recent Genus Napaeozapus achieved its more progressive structure by middle Pleistocene times.