Speciation and Evolution of the Pygmy Mice, Genus Baiomys Part 2

_Remarks._--The slight development or absence of the anterior median fold in _kolbi_ suggests that it was specialized. The anterior median fold is well-developed in all species of _Baiomys_ save _B.

brachygnathus_ and _B. taylori_, in which the fold is only slightly developed or absent. _B. kolbi_ may have paralleled _B. taylori_ in specialization for a diet of gra.s.ses and for a life in open country.

=Baiomys brachygnathus= (Gidley)

_Peromyscus brachygnathus_ Gidley, U. S. Geol. Surv. Prof. Papers, 131:124, March 15, 1922.

_Baiomys brachygnathus_, Hibbard, Amer. Midland Nat., 26:352, September, 1941.

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

_Type._--No. 10501, U. S. Nat. Mus.; right mandibular ramus bearing m1-m3, and incisor; 2 mi. NE Curtis Ranch house, near a line between sec. 28 and 29, T. 18 S, R. 21 E, Mid-Pleistocene (Hibbard, 1958:25), Cochise County, Arizona.

_Referred material._--None.

_Diagnosis._--Ramus small for the genus; m3 reduced; jaw reduced anteroposteriorly; incisor short, slender, proodont; cingular ridges well-developed, posterior ectolophid continuous from protoconid to hypoconid in m1 and m2; diastema short; length of molar row 2.8 mm.

_Comparisons._--For comparisons with _B. rexroadi_ and _B. kolbi_, see accounts of those species. From _B. minimus_, _B. brachygnathus_ differs in: jaw not so slender anteriorly; ma.s.seteric ridge not so far anterior; cheek-teeth slightly broader, less depressed, therefore, more hypsodont; incisor shorter, more proodont.

From _B. sawrockensis_, _B. brachygnathus_ differs in: molar row slightly longer; teeth slightly less depressed; ma.s.seteric ridge extends farther anteriorly; incisors more proodont.

From _B. musculus_, _B. brachygnathus_ differs in: jaw smaller; molar row slightly shorter; molars less depressed; incisors slender, shorter, narrower, and more proodont.

From _B. taylori_, _B. brachygnathus_ differs in: incisor more slender, shorter, more proodont; 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 proodont 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 proodont; molars slightly more depressed (though worn).

From _B. musculus_, _B. minimus_ differs in: over-all size of jaw and molars smaller; incisors shorter; ma.s.seteric 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; ma.s.seteric 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 proodont 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 proodont 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 gra.s.sland 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 gra.s.sland 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 gra.s.ses." 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 gra.s.sland. This a.s.sumption 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 gra.s.sland.

_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.

[Ill.u.s.tration: FIG. 3. Diagram indicating probable relationships of living and extinct species of pygmy mice.]

_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 s.e.xual 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 dent.i.tion, 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.