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.
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 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).