Part 4

According to Simpson (1947), the occurrence of the same group of mammals on two different land masses is to be taken as prima facie evidence that migration has occurred. Keeping in mind then the present geographic distribution, unspecialized condition of the dentition of Eozapus, and its resemblance to the extinct Pliozapus known from North America but not from Asia, it may be that Eozapus descended from primitive stock of a North American jumping mouse that was forced to the periphery (across the Asiatic North American land bridge) by the more specialized zapidine stock.

Subsequently or perhaps during the migration of the pre-Eozapus stock the zapidine stock may have dispersed transcontinentally, occupying most of northern North America. The unprogressive Macrognathomys and Pliozapus line which remained in North America may have become extinct. Any such period of dispersal and climatic equilibrium ended when glaciers came to cover most of the northern part of the continent and the mammals living there were forced southward by the ice or remained in ice-free refugia within the glaciated area. Later, with melting and retreat of the ice, the jumping mice could have again spread enough to occupy the northern part of the continent. Such glaciation isolated segments of the population and aided their evolution into distinct species.

If it be assumed, as Matthew (1915) did and as Hooper (1952:200) later on the generic level did, that the region of origin and center of dispersal for a given group of animals is characterized by the presence of the most progressive forms, then southeastern Canada and the northeastern United States make up the area of origin and center of dispersal in relatively late time of the subfamily Zapodinae. This area is inhabited by Zapus hudsonius and Napaeozapus, the most progressive members of the subfamily.

As I visualize it, the evolution of the Zapodinae occurred in two stages: the first stage involved the movement of the primitive pre-Eozapus stock to Asia and the second stage involved the dispersal, isolation, and specialization in North America of the more progressive basic zapidine stock into the present genera Zapus and Napaeozapus.

Status of the genera Eozapus, Zapus, and Napaeozapus

The genus Zapus is one of three living genera in the subfamily Zapodinae. These genera Zapus and Napaeozapus from North America and Eozapus from China have been variously considered as subgenera of the genus Zapus (Preble, 1899) or as three separate genera (Ellerman, 1940).

FIGS. 4-7. Eozapus s. vicinus, adult, male, No. 240762 USNM, Lanchow, Kansu, China.

FIGS. 8-11. Zapus h. pallidus, adult, male, No. 240762 KU, 5-1/2 mi. N, 1-3/4 mi. E Lawrence, Douglas County, Kansas.

FIGS. 12-15. Napaeozapus i. insignis, adult, male, No. 41109 KU, Shutsburg Rd., at Roaring Creek, 600 ft., Franklin County, Massachusetts.]

FIGS. 16 and 19. Eozapus s. vicinus, adult (age group 3), male, No. 240762 USNM, Lanchow, Kansu, China.

FIGS. 17 and 20. Zapus h. alascensis, adult (age group 2), female, No. 29073 KU, E side Chilkat River, 9 mi. W and 4 mi. N Haines, Alaska.

FIGS. 18 and 21. Napaeozapus i. insignis, adult (age group 3), male, No. 41109 KU, Shutsburg Rd., at Roaring Creek, 600 ft., Franklin County, Massachusetts.

Note especially the variation in complexity of occlusal pattern, width of re-entrant folds, and degree of tubercularity.]

The remarkable similarity of the body form, post-cranial skeleton, mandibular rami, and general structure of the cranium of Zapus, Napaeozapus, and Eozapus indicate their relationship (see figs. 4-15); however, dissimilarity between the groups in the dentition (tooth number and occlusal pattern), bacula, and ear ossicles provides basis for considering them distinct genera. As pointed out earlier, Zapus and Napaeozapus appear to be more closely related and progressive and the Asiatic Eozapus somewhat removed and less progressive.

Teeth.--According to the complexity in dental pattern and in number and size of the cheek-teeth, these genera can be arranged in a structurally progressive series with Eozapus showing the least complexity and Napaeozapus the most (see figs. 16-21). There are three distinct molar patterns; one is simple (Eozapus) and the others (Zapus and Napaeozapus) are more complex. The complexity is greatest in Napaeozapus, which is characterized by numerous additional flexures in the enamel and dentine. The simplicity of the molars of Eozapus is evident in the tuberculate rather than flat-crowned occlusal surface; the wide, simple, re-entrant bays; the small (or sometimes absent) anteroconid; and the essentially quadritubercular nature of the teeth. The molars of Zapus and Napaeozapus are flat crowned; however, Zapus has wider and fewer re-entrant bays, a smaller anteroconid, and less complexity in the occlusal pattern. The characteristics of the molar teeth would tend to indicate a close relationship between Zapus and Napaeozapus and to place Eozapus as primitive.

The absence of P4 in Napaeozapus would lead one to suspect that this genus has evolved at a more rapid rate than the historically older Zapus and Eozapus which still retain this structure. The small size of P4, even in the primitive Eozapus, indicates that it has long been of little use to the mouse. An even greater reduction of P4 in the more complex dentition of Zapus argues for complete loss of this tooth as the next step in specialization, such as is seen in the more progressive Napaeozapus. The following parallel columns show selected differences between the occlusal patterns of the cheek-teeth of the three genera:

BACULUM.--The baculum (os penis) of Eozapus is known to me only from Vinogradov's (1925) figures of the dorsal and lateral aspects. The proximal end (base) is laterally expanded, and the shaft tapers gradually toward the distal end where it expands abruptly into the spade-shaped tip. In lateral aspect the bone is relatively thick; it is curved downward slightly from the proximal end to the base of the tip where it curves upward to a rounded point.

The baculum of Zapus differs from that of Eozapus as follows: base less expanded horizontally; shaft slenderer; distal end less spade-shaped except in Z. trinotatus. The tip is less expanded in Z. princeps and is still less so in Z. hudsonius. In Napaeozapus the tip is lanceolate, the base is narrow, and in lateral view the shaft is slender and curved (see figs. 22-31).

-----------------------+-----------------------+----------------------- Eozapus | Zapus | Napaeozapus -----------------------+-----------------------+----------------------- P4--Small | Smaller | Absent | | M1--Four wide labial | Four moderately | Three narrow labial re-entrant folds | narrow labial | re-entrant folds of equal length; | re-entrant folds | of unequal length, paracone and | of unequal length; | 1st long, 2d and 3d metacone largest | 1st and 3d longer | shorter; paracone cusps; anterior | than 2d, 4th | and metacone larger cingulum large. | shortest; paracone | than in Zapus and | smaller than in | Eozapus; anterior | Eozapus; metacone | cingulum absent. | largest cusp; | | anterior cingulum | | small. | | | | | M2--Four wide labial | Four moderately | Narrow labial re-entrant folds; | narrow labial | re-entrant folds, 2d short, others | re-entrant folds | variable in number, of equal length | of unequal length, | often as many as 6; but longer than | 1st and 3d long, | anterior and 2d; anterior and | 2d and 4th short; | posterior cingula posterior cingula | anterior and | small; occlusal large; occlusal | posterior cingula | pattern complex. pattern simple. | moderately large; | | occlusal pattern | | moderately complex. | | | | | M3--Three wide labial | Two moderately | Three narrow labial re-entrant folds | narrow labial | re-entrant folds of unequal length, | re-entrant folds | of unequal length, 1st short, 2d and | of equal length; | 1st long, 2d and 3d long; anterior | anterior and | 3d short; anterior and posterior | posterior cingula | and posterior cingula low, | moderately large; | cingula large; small; occlusal | occlusal pattern | occlusal pattern pattern simple. | moderately complex. | complex. | | | | m1--Anterior oblique | No anterior | No anterior re-entrant fold | re-entrant fold; 4 | re-entrant fold; separating equal | moderately narrow | narrow lingual sized protoconid | lingual re-entrant | re-entrant folds and metaconid | folds of equal | variable in cusps; 3 wide | length, 1st joining | number, often as lingual re-entrant | 1st labial | many as 4; folds of equal | re-entrant fold, | anteroconid well length; | 4th joining 2d | developed, encloses anteroconid absent;| labial re-entrant | 1 or 2 small lakes; occlusal pattern | fold; anteroconid | occlusal pattern simple; mesoconid | well developed, | complex; mesoconid present. | encloses | absent. | small lake; occlusal | | pattern moderately | | complex; | | mesoconid absent. | | | | | m2--Four wide lingual | Four moderately | Narrow lingual re-entrant folds | narrow lingual | re-entrant folds, of unequal length, | re-entrant folds, | variable in number, 1st short, other | 1st and 2d long, 3d | may be as many as 5; 3 equal and long; | and 4th short, 1st | anteroconid large, anteroconid | joins 1st labial | encloses moderately large; | re-entrant fold | complex folds from occlusal pattern | and 4th joins 2d | 1st labial re-entrant simple. | labial re-entrant | fold; occlusal | fold; anteroconid | pattern complex. | large; occlusal | | pattern moderately | | complex. | | | | | m3--Three wide lingual | Three moderately | Narrow lingual re-entrant folds | narrow lingual | re-entrant of near equal | re-entrant folds | folds variable in length; antero- | of unequal length, | number, as many as 3; conid absent; | 1st and 2d long, 3d | anteroconid present; occlusal pattern | short; anteroconid | occlusal pattern simple; 1 labial | absent; occlusal | complex; 2 labial re-entrant fold. | pattern moderately | re-entrant folds. | complex; 1 labial | | re-entrant fold. | -----------------------+-----------------------+------------------------

FIGS. 22 and 27. Eozapus setchuanus (after Vinogradov, 1925:585).

FIGS. 23 and 28. Zapus t. trinotatus, adult, No. 94596 MVZ, 1-1/4 mi. ENE Amboy, 350 ft., Clark County, Washington.

FIGS. 24 and 29. Zapus p. princeps, adult, No. 20870 KU, 3 mi. S Ward, Boulder County, Colorado.

FIGS. 25 and 30. Zapus h. pallidus, adult, No. 22954 KU, 4 mi. N, 1-3/4 mi. E Lawrence, Douglas County, Kansas.

FIGS. 26 and 31. Napaeozapus i. insignis, adult, No. 41110 KU, Shutsburg Rd., at Roaring Creek, 600 ft., Franklin County, Massachusetts.]

EAR OSSICLES.--The auditory ossicles are of three types which differ only slightly. These ossicles possibly are more conservative than some other structures because the ossicles are not so much affected by the molding influence of the environment.

Instances of variation in the auditory region in mammals in general are small, even at the family level; therefore, these differences in the subfamily Zapodinae are offered as additional support for recognizing Eozapus, Zapus, and Napaeozapus as distinct genera. The distinctive features are chiefly in the malleus and incus; the stapes, however, differs slightly and, therefore, it too is described (see figs. 32-34).

In Eozapus the head of the malleus is narrow, oblong, and rounded dorsally and attaches to the body by a long, slender, abruptly recurved neck. The body is weakly pointed ventrally and rounded dorsally. A beaklike manubrium malleus composed of anterior projecting external and internal spines extends from the body to the tympanum. The incus has a dorsally rounded body with an anterior downward snoutlike projection with which the malleus articulates. The short limb of the incus is broad basally and narrows somewhat distally. The long limb is narrow and its articulating lenticular process is a flat circular structure. The limbs of the stapes are wide-spread and heavy. The neck is short and wide with a large circular articulating surface.

In Zapus the head of the malleus is angular with an anterior projecting point and is flattened in dorsal aspect. The neck is slender, elongate, and gently curved away from the long limb of the incus. The body is pointed dorsally and rounded ventrally, the reverse of the condition in Eozapus. There is a beaklike manubrium malleus composed of internal and external anteriorly projecting spines extending from the body to the tympanum as in Eozapus. The incus has a rounded body with a long angular limb articulating via a small lenticular process with the stapes. The short limb is narrow but does not taper distally as in Eozapus. The limbs of the stapes are relatively narrow, weak, and gently curved. The neck is longer and more slender than that of Eozapus.