Part 5

From B. taylori, B. brachygnathus differs in: incisor more slender, shorter, more prooedont; diastema shorter.

Remarks.--The molar teeth of B. brachygnathus, although worn, resemble those of B. taylori more than those of any known fossil species. Gidley (1922:124) stated that the absence of the divided anterior lobe of the first molar (anterior median fold) in brachygnathus was one of the chief characters separating brachygnathus from taylori. In taylori, the anterior median fold characteristically is only slightly developed, and in some specimens is absent. B. brachygnathus differs from taylori chiefly in prooedont incisors, which feature seems to preclude brachygnathus being ancestral to taylori. B. brachygnathus may have been a specialized divergence from B. minimus.

=Baiomys minimus= (Gidley)

Peromyscus minimus Gidley, U. S. Geol. Surv. Prof. Papers, 131:124, March 15, 1922.

Baiomys minimus, Hibbard, Amer. Midland Nat., 26:352, September, 1941; Gazin, Prof. U. S. Nat. Mus., 92(3155):488, 1942.

P. [eromyscus] minimus, Wilson, Carnegie Inst. Washington Publ., 473:33, May 21, 1936.

Type.--No. 10500, U. S. Nat. Mus.; left mandibular ramus bearing m1-m3 and incisor; 2 mi. S Benson, sec. 22, T. 17 S, R. 20 E, Late Pliocene (Blancan, Gazin, 1942:482), Cochise County, Arizona.

Referred material.--None.

Diagnosis.--Ramus small for the genus; molar teeth depressed; cingular ridges (ectolophids) of m1 and m2 well-developed; anterior median fold present (appearing larger owing to chip of enamel missing); external reentrant fold of m3 progresses half way across crown of tooth; diastema short; incisor moderately large, recurved; length of molar row, 2.6 mm.

Comparisons.--For comparisons with B. brachygnathus, B. kolbi, and B. sawrockensis, see accounts of those species. From B. rexroadi, B. minimus differs in: anterior median fold deeper; incisor longer, more recurved, less prooedont; molars slightly more depressed (though worn).

From B. musculus, B. minimus differs in: over-all size of jaw and molars smaller; incisors shorter; masseteric ridge more depressed.

From B. taylori, B. minimus differs in: anterior median fold slightly deeper; molar teeth more depressed; cingular ridges on m1 and m2 better developed; masseteric ridge more depressed.

Remarks.--Gidley (1922:124) stated that B. minimus differed considerably from B. taylori in that the coronoid portion of the ascending ramus diverges at a wider angle from the alveolar part of the jaw. Study of large samples of lower jaws of B. taylori reveals considerable individual variation in the angle formed between the coronoid part of the jaw and the alveolar part.

B. minimus, except for its small size, is like B. musculus and is considered to be ancestral to that species.

PHYLETIC TRENDS

It seems that the important trends in phyletic development in the pygmy mice have been from an ancestral stock (see Figure 3) that possessed relatively brachydont teeth having raised cingular ridges (ectolophids and mesolophids) and relatively short orthodont to prooedont incisors, to species having teeth more hypsodont on which cingular ridges were reduced, stylids were isolated or completely absent, and incisors were longer and more recurved or retrodont. Baiomys sawrockensis, or an unknown stock resembling it, might have been ancestral to the other known species. Of the four remaining fossil species, B. kolbi seems least likely to have been ancestral to the two living species, owing to its prooedont incisors, reduction of cingular ridges, loss of an anterior median fold in m1, and long mandibular tooth-row. B. kolbi may have been an early, specialized derivation from the ancestral stock. From his knowledge of the habitats of B. musculus, the larger species, and B. taylori, the smaller species, Hibbard (1952:203) suggests that B. kolbi, a large species, might have inhabited lowlands, and B. rexroadi, a small species, highlands. I have no evidence to dispute this suggestion except that B. musculus has more prominent cingular ridges (or at least vestiges of this lophid condition) than either B. kolbi or B. rexroadi. B. musculus (see page 610) is less of an open grassland inhabitant than is B. taylori. Therefore, both B. kolbi and B. rexroadi, because of their poorly developed cingular ridges, might be expected to have lived in a relatively open grassland habitat.

The relationship of B. rexroadi to fossil species other than B. kolbi is not clear. Superficially, the former resembles B. taylori, but, owing to the specialized development of the molars of rexroadi, it could hardly have been ancestral to either of the living species. The resemblance of B. rexroadi to B. taylori may result from each having occupied the same ecological niche in different periods. The incisors of B. rexroadi, however, are much shorter than those of B. taylori and suggest somewhat different food habits.

B. minimus seemingly is more closely related to B. sawrockensis and B. musculus than to the other described species. The development of the cingular ridges leads one to suspect that B. minimus was the ancestor of B. musculus. B. minimus may have been derived from a sawrockensis-like stock and probably gave rise to B. musculus.

Hershkovitz (1955:643-644) suggests that "... primitive brachydont, buno-mesolophodont cricetines have survived ... in forested parts of the range," whereas "... the progressive branch of cricetines with mesoloph absent or vestigal, has become increasingly specialized for life in open country and a diet of grasses." Species of the genus Baiomys can be divided into two morphological groups. One group, composed of B. sawrockensis, B. minimus, and B. musculus, includes those species, the teeth of which were relatively brachydont and had prominently developed cingular ridges (ectolophids or mesolophids) or, at least, showed some development of these ridges. B. sawrockensis probably lived in semi-wooded to shrubby habitats. According to Hibbard (1953:409), "The Saw Rock Canyon fauna lived in that area at a time when conditions were comparable to the conditions at the time the Rexroad fauna lived." The conditions in which the Rexroad fauna lived are discussed by Hibbard (1941:95). Presumably, there were at least some well-wooded situations, and the climate was warm. B. sawrockensis probably inhabited denser vegetation than did B. minimus or than does B. musculus. The teeth of the second group (B. kolbi, B. rexroadi, B. brachygnathus, and B. taylori) lack cingular ridges or have them much reduced and have more hypsodont molars. The three fossil species probably inhabited relatively open grassland. This assumption is based largely on the known habitat of B. taylori (see page 632).

The suggested grouping, based on supposed similarities in niches inhabited by the extinct species, does not necessarily indicate degree of relationship. B. taylori probably was not derived from an ancestor like B. rexroadi or B. kolbi, although, in certain characters, the three species resemble one another. B. kolbi and B. rexroadi were already specialized in Blancan times, probably for living on grassland. B. taylori shows only a slight advance in specialization of molar structures compared to either of the aforementioned species but is slightly smaller and does have longer and more recurved incisors. If only morphological criteria of lower jaws were considered, without recourse to other data derived from the study of many samples of populations of the living species, time alone might account for the differences among B. taylori, B. rexroadi, and B. kolbi. The available evidence (see page 658) suggests, however, that B. taylori was derived from the B. sawrockensis-B. minimus-B. musculus line.

Baiomys seems to have undergone little basic evolutionary and morphological change since Late Pliocene time. According to Simpson (1945:207), hesperomine rodents as a group have undergone little basic evolution, and "The rapid evolution of new genera was more a matter of segregation of characters in a group with a great variation than of the origin of significantly new characters." Perhaps, the living southern pygmy mouse retains many basic characteristics of one of the early North American cricetine-like stocks that emigrated to South America near the end of the Pliocene epoch. There is much to suggest close relationship of the pygmy mice to certain species of South American hesperomine rodents of the genus Calomys.

NON-GEOGRAPHIC VARIATION

Non-geographic variation in pygmy mice (variation in a single population resulting from age, individual, seasonal, and secondary sexual differences) has been but little studied in the past. Mearns (1907:381) figured progressive stages of wear on the teeth of B. taylori; Osgood (1909:252) and Blair (1941:380) referred to changes in dentition, weights, and pelages.

The largest samples available for this study were 47 B. taylori from the vicinity of Altamira (6 mi. N, 6 mi. W; 5 mi. N, 5 mi. W; 1 mi. S), Tamaulipas, and 44 B. musculus from El Salvador (1 mi. S Los Planes, and 1 mi. NW San Salvador--two localities 3 miles apart).

VARIATION WITH AGE

Specimens of both species were segregated into five categories: Juveniles, young, subadults, adults, and old adults. Juvenal and young pygmy mice are readily separable from the other three categories; subadults are less easily distinguished from adults. In order to obtain an accurate understanding of geographic variation in these mice, only adults should be used in making taxonomic comparisons.

Juveniles.--Nestling mice yet unweaned; sutures in cranium incompletely closed; bony parts of skull fragile; M3 and m3 not erupted or only partly erupted and not protruding above margins of alveoli.

At birth, juveniles are pink, without pelage except for the mystacial vibrissae and a few hairs about the eye. Blair (op. cit.:381) recorded changes with age in color of the skin of new-born and suckling pygmy mice. Data obtained by me from three litters born in captivity agree with his findings. Pygmy mice are weaned when 17 to 24 days old. At that time, the mice possess a fine, but not dense, dusky-gray fur.