  University of Kansas Publications Museum of Natural History Volume 9, No. 23, pp. 579-670, 4 pls., 12 figs. in text June 16, 1960 Speciation and Evolution of the Pygmy Mice, Genus Baiomys BY ROBERT L. PACKARD University of Kansas Lawrence 1960 University of Kansas Publications, Museum of Natural History Editors: E. Raymond Hall, Chairman, Henry S. Fitch, Robert W. Wilson Volume 9, No. 23, pp. 579-670, 4 pls., 12 figs. in text Published June 16, 1960 University of Kansas Lawrence, Kansas PRINTED IN THE STATE PRINTING PLANT TOPEKA, KANSAS 1960 Look for the Union Label 28-3030 BY ROBERT L. PACKARD
Pygmy mice (Genus Baiomys) are the smallest cricetine rodents in North America. They occur from Nicaragua in Central America into the southwestern United States. The principal part of the geographic range of the pygmy mice lies in the Republic of MÉxico. They are notably common in central MÉxico, but are only locally common to the north and to the south, and then only in certain seasons. Pygmy mice were first brought to the attention of biologists in 1887 when Oldfield Thomas described a diminutive species of cricetine rodent, Hesperomys (Vesperimus) taylori. The description was based on a specimen obtained by William Taylor from San Diego, Duval County, Texas. C. Hart Merriam (1892:70) described Sitomys musculus on the basis of specimens from Colima [City of], Colima, MÉxico. Merriam (loc. cit.) mentioned that the two kinds of mice, Hesperomys taylori and Sitomys musculus, "in general appearance look almost precisely like the common house mouse (Mus musculus) but are still smaller and have shorter tails." He placed the two species in the genus Sitomys. Frederick W. True in 1894 regarded them as composing a distinct subgenus of Sitomys, Baiomys. According to True (1894:758), S. taylori and S. musculus possessed a different combination of characters (ascending ramus of mandible short and erect, condyle terminal, coronoid process well-developed, uncinate, and near the condyle, size small, tail short, plantar tubercles six, soles hairy) than either Vesperimus, or Onychomys (which had been considered as a subgenus of Hesperomys until 1889). In 1907, E. A. Mearns accorded Baiomys generic rank. Osgood (1909:252) treated Baiomys us a subgenus of Peromyscus, whereas, Miller, in 1912, regarded Baiomys as a distinct genus. Most recent students of North American mammals have followed Miller, but usually with reservations. Ellerman (1941:402) emphasized that the taxonomic position of the genus was uncertain, and wrote that Baiomys "… seems to be considerably distinct from Peromyscus, and may perhaps be a northern representative of Hesperomys or one of the small South American genera." Only two comprehensive analyses of geographic variation and interspecific taxonomic relationships have been made; the first was by Osgood (1909) who had fewer than a fourth of the specimens of Baiomys available to me; the second was by Hooper (1952a:90-97) who contributed importantly to understanding the relationships of the two living species in central MÉxico. No attempts heretofore have been made to correlate and understand the relationships of the five fossil species to one another and to the living species assigned to the genus. Six objectives of the following report are to: (1) list characters taxonomically useful in recognizing species and subspecies; (2) record amount of variation within and between populations; (3) correlate observed variations with known biological principles; (4) show geographic ranges of the two living species; (5) indicate relationships between fossil and living species of the genus; and (6) clarify the systematic position of the genus. This report is based on the study of approximately 3,520 museum study skins, skulls, complete skeletons, and entire animals preserved in liquid. Most specimens examined were accompanied by an attached label bearing data on locality and date of capture, name of collector, external measurements, and sex. In addition, 49 fossil specimens referable to Baiomys were studied. Nearly two-thirds of the specimens were assembled at the University of Kansas Museum of Natural History; the remainder were examined in other institutions. Specimens studied were grouped by geographic origin, sex, age, and season of capture. Individual variation was then measured in several of the larger samples of each living species and in measurable fossil material. External measurements used were those recorded by the collectors on the labels attached to the skins. Twenty cranial measurements employed in the past in the study of Baiomys and closely related cricetine rodents were statistically analyzed. The coefficient of variation was calculated for each of the 20 measurements in order to determine which varied least. In general, measurements having the least coefficient of variation were used in comparing samples from different geographic areas. Figure 1 shows the points between which measurements were taken. Occipitonasal length.—From anteriormost projection of nasal bones to posteriormost projection of supraoccipital bone. A to A' Zygomatic breadth.—Greatest distance across zygomatic arches of cranium at right angles to long axis of skull. B to B' Postpalatal length.—From posterior margin of hard palate to anterior margin of foramen magnum. C to C' Least interorbital breadth.—Least distance across top of skull between orbits. D to D' [Pg 585] Length of incisive foramina.—From anteriormost point to posteriormost point of incisive foramina. E to E' Length of rostrum.—The distance in a straight line from the notch that lies lateral to the lacrimal to the tip of the nasal on the same side. F to F' Breadth of braincase.—Greatest distance across braincase, taken at right angles to long axis of skull. G to G' Depth of cranium.—The distance from the dorsalmost part of the braincase to a flat plane touching tips of incisors and ventral border of each auditory bulla. A glass slide one millimeter thick was placed on the ventral side of the skull. One jaw of the caliper was on the lower surface of the slide and the other jaw on the dorsalmost part of the braincase. The depth of the slide was subtracted from the total reading. H to H' Alveolar length of maxillary tooth-row.—From anterior border of alveolus of M1 to posterior alveolus of M3. I to I' Fig. 1. Three views of the skull to show points between which measurements were taken. Based on B. m. pullus, adult, female, No. 71611 KU, 8 mi. S Condega, EstelÍ, Nicaragua. × 11/3. Capitalized color-terms refer to Ridgway (1912). Color terms without initial letters capitalized do not refer to any one standard. The names of the cusps and ridges of the teeth (see Figure 2) are those suggested by Wood and Wilson (1936:389-390). Terminology of the enamel grooves and folds is that of Hershkovitz (1944:17) and Hooper (1952b:20-21). Because secondary sexual variation was not significant (see page 597), both males and females of like age and pelage were used in comparisons of samples designed to reveal geographic variation. The species are arranged from less to more progressive; the subspecies are arranged alphabetically. In the synonymy of each subspecies, the plan has been to cite: (1) the name first proposed; (2) the first usage of the name combination employed by me; (3) all other name combinations in chronological order that have been applied to the subspecies concerned. The localities of specimens examined are listed by country from north to south. Within a country, the listing is by state, beginning with the northwesternmost state and proceeding by tiers (west to east) to the southeasternmost state. Within a state of the United States, the listing is by counties in the same geographic order as described for states. Within any county in the United States, within any state in MÉxico, and within any country in Central America, the listing of localities is from north to south. When more than one locality is on the same line of latitude, the westernmost locality is listed first. Marginal localities for each subspecies are listed in a paragraph at the end of each account. Each marginal locality is mapped by means of a circle. The circles are listed in clockwise order, beginning with the northernmost. When more than one of these localities lies on the same line of latitude, the westernmost is cited first. Localities not represented on the distribution maps, so as to avoid undue crowding of symbols, are italicized in the lists of specimens examined. Fig. 2. Occlusal views of molars. × 13.
The largest single collection of pygmy mice is in the University of Kansas Museum of Natural History, and, unless otherwise indicated, specimens cited in the taxonomic accounts beyond are there. I am indebted to the following named institutions and persons for making specimens available for study: American Museum of Natural History, G. G. Goodwin and R. G. VanGelder. Carnegie Museum, J. K. Doutt. California Academy of Sciences, Robert T. Orr. Chicago Natural History Museum, Phillip H. Hershkovitz. Cleveland Museum of Natural History (Collection now a part of Museum of Zoology, University of Michigan, W. H. Burt, E. T. Hooper). Louisiana State University, Museum of Natural History, George H. Lowery, Jr. Los Angeles County Museum, Charles A. McLaughlin. United States National Museum (Biological Survey Collections), David A. Johnson, and Viola S. Schantz. United States National Museum, Division of Vertebrate Paleontology, C. Lewis Gazin. University of Arizona, E. L. Cockrum, and G. VR. Bradshaw. University of California, Museum of Vertebrate Zoology, Seth B. Benson, and W. Z. Lidicker. University of Illinois, Museum of Natural History, Donald F. Hoffmeister. University of Michigan, Museum of Zoology, W. H. Burt, E. T. Hooper, and Claude W. Hibbard. University of New Mexico, James S. Findley. University of Texas, Frank W. Blair. Texas A & M, Cooperative Wildlife Research Collection, W. B. Davis. The Museum, Michigan State University, Rollin H. Baker. University of Florida Collections, James N. Layne. I am especially grateful to Professor E. Raymond Hall who guided me in my study and gave critical assistance with the manuscript. Additional appreciated suggestions were made by Professors A. Byron Leonard, Robert W. Wilson, Henry S. Fitch, Ronald L. McGregor, and fellow graduate students. For the illustrations, I am indebted to Mrs. Lorna Cordonnier, Miss Lucy Remple and Mrs. Connie Spitz. Mr. B. J. Wilks of the University of Texas, Department of Zoology, provided a number of living pygmy mice for study in captivity. Mr. J. Raymond Alcorn and his son, Albert, collected a large share of specimens of pygmy mice now in the University of Kansas, Museum of Natural History. My wife, Patricia, aided me in secretarial work and typing of the manuscript. For financial assistance, I am indebted to the National Science Foundation when I was a Research Assistant, to the Sigma Xi-RESA Research Fund for a Grant-in-Aid, and to the Kansas University Endowment Association through its A. Henley Aid Fund, and the Watkins Fund for out-of-state field work by the Museum of Natural History. Five fossil species, all extinct, have been assigned to the genus and range in time from early late Pliocene (Saw Rock Canyon fauna of Hibbard, 1953:408) to Mid-Pleistocene (see Hibbard, 1958:25, who assigns the Curtis Ranch fauna to late Kansan or early Yarmouth). I examined all known fossil material and compared it with Recent material. When the antiquity of the genus is considered, the degree of difference between the oldest fossil species and the two living species is much less than might be expected. Baiomys sawrockensis Hibbard, Papers Mich. Acad. Sci., Arts and Letters, 38:402, April 27, 1953. Type.—No. 27506, Univ. Michigan; left mandibular ramus bearing m1-m3 and incisor; Saw Rock Canyon, early late Pliocene, XI member of the Rexroad formation, sec. 36, T. 34 S, R. 31 W, Seward County, Kansas (University of Kansas, Locality 6). Referred material.—Univ. Michigan, Nos. 25781, 27503-27505, 28159-28165, 29708-29715, 31015. Diagnosis.—Ramus of medium size to small for the genus; lower incisor broad, moderately recurved; diastemal region broad; anterior median fold between anterior labial conulid and anterior lingual conulid of m1 deep; primary first fold between anteroconulid and protoconid of m2 deep; cingular ridge (ectolophid) at entrance to posteroexternal reËntrant valley (major fold, see Figure 2) between protoconid and hypoconid of m1 and m2; average and extreme measurements of lower molar row of eight specimens are, 2.65 (2.5-2.7). Comparisons.—For comparisons with B. brachygnathus, see account of that species. From B. rexroadi, B. sawrockensis differs in: anterior median fold of m1 deeper; incisor narrower; diastemal region broader; coronoid process broader and better developed; cingular ridges (ectolophids and mesolophids) more pronounced in their development; incisors less proÖdont, more retrodont. From B. kolbi, B. sawrockensis differs in: crowns of molars narrower; incisors less proÖdont; cingular ridges (ectolophids and mesolophids) of m1 and m2 more pronounced in their development. From B. minimus, B. sawrockensis differs in: incisor less procumbent; masseteric ridge extending farther anteriorly; anterior cingulum of m2 slightly larger. From B. musculus, B. sawrockensis differs in: over-all size of jaw and molar row less; diastema more acutely curved; incisors shorter; anterior median fold of m1 slightly deeper. From B. taylori, B. sawrockensis differs in: m1 and m2 smaller; cingular ridges in m1 and m2 more pronounced; anterolingual conulid farther forward; incisors shorter, more proÖdont; molar teeth depressed, less hypsodont; diastemal region broader, more acutely curved; masseteric ridge not extending so far anteriorly. Remarks.—B. sawrockensis is the oldest known pygmy mouse. The extreme development of the anterior median fold between the anterolingual conulid and the anterolabial conulid is regarded as a primitive feature in the pygmy mice. In this character, the Recent species can be traced back in time through B. minimus to B. sawrockensis. B. sawrockensis resembles Calomys laucha of South America in general conformation of jaw and tooth structure. The molars of sawrockensis are smaller than those of C. laucha, and the anterolingual conulid of sawrockensis is farther forward. Baiomys rexroadi Hibbard, Amer. Midland Nat., 26:351, September, 1941; Hibbard, Contrib. Mus. Paleo., Univ. Michigan, 8(2):145, June 29, 1950 (part); Hibbard, Papers Mich. Acad. Sci., Arts and Letters, 38:403, April 27, 1953. Type.—No. 4670, Univ. Kansas; left mandibular ramus bearing m1-m3, and incisor; Rexroad fauna, Locality no. 2, Upper Pliocene, Meade County, Kansas. Referred material.—Univ. of Michigan Nos. 24840, 24851, 27493, 27496, 27501, 28862-28867. Diagnosis.—Ramus medium in size for the genus; incisors small, proÖdont; anterior median fold of m1 slight; cingulum of all molars poorly developed; average and external measurements of lower molar row of seven specimens are, 2.7 (2.6-3.0). Comparisons.—For comparisons with B. sawrockensis and B. minimus, see accounts of those species. From B. kolbi, B. rexroadi differs in: over-all size of mandibular ramus, incisors, and molars smaller; anterior median fold of m1 present, though poorly developed. From B. brachygnathus, B. rexroadi differs in: over-all size of mandibular ramus smaller; m3 larger; posterior cusps (hypoconid and entoconid) elongated; diastema shorter, less acutely recurved; incisors less proÖdont; cingular ridges of m1 and m2 less well-developed. From B. musculus, B. rexroadi differs in: over-all size of mandibular ramus less; cingular ridges of m1 and m2 less well-developed; incisors smaller, more proÖdont; molars less depressed. From B. taylori, B. rexroadi differs in: m3 more triangular, posterior part narrower; mental foramen closer to anterior root of m1; masseteric ridge closer to alveolus of m1; incisor shorter, more proÖdont; molars more depressed. Remarks.—Two maxillary tooth-rows and associated parts were studied. On one of these specimens, the M2 has a well-developed mesostyle; the anterior median fold of M1 is also well-developed. The other specimen possesses a low cingular ridge (enteroloph) between the protocone and the hypocone, a reduced cingular ridge (mesoloph) between the paracone and metacone of M1. On the second molar, M2, a mesostyle joins with the mesoloph somewhat in the fashion indicated by Hooper (1957:9, encircled number 2). Baiomys kolbi Hibbard, Trans. Kansas Acad. Sci., 55:201, June 18, 1952; Hibbard, Papers Mich. Acad. Sci., Arts and Letters, 38:403, April 27, 1953. Type.—No. 24846, Univ. Michigan; right mandibular ramus bearing m1-m3 and incisor; Fox Canyon, upper Pliocene, Rexroad formation, Rexroad fauna, Univ. Michigan Locality K1-47, sec. 35, T. 34 S, R. 30 W, XI Ranch, Meade County, Kansas. Referred material.—Univ. Michigan Nos. 24845-24848, 27494, 27497, 27499, 28566, 28861, 28878, 28880-28882, 28884, 28886. Diagnosis.—Ramus of medium size to large for the genus; lower incisor short, narrow transversely, proÖdont; anterior median fold of m1 reduced or absent; cingular ridges of m1 and m2 moderately well-developed; m3 large relative to m1 and m2; average and extreme measurements of lower molars of seven specimens are, 3.0 (3.0-3.1). Comparisons.—For comparisons with B. sawrockensis and B. rexroadi, see accounts of those species. From B. brachygnathus, B. kolbi differs in: molar row longer; m3 and jaw larger; diastema longer; masseteric ridge not so far forward; molars more depressed. From B. minimus, B. kolbi differs in: molar row longer; m3 larger; jaw larger; diastema not so acutely curved; incisor shorter, narrower transversely, more proÖdont. From B. musculus, B. kolbi differs in: anterior median fold of m1 slightly developed or absent, instead of well-developed; m3 larger (not reduced), external reËntrant valley broad and extending farther across crown of tooth; incisor smaller, and more proÖdont; cingular ridges of m1 and m2 less well-developed. From B. taylori, B. kolbi differs in: molars larger, more depressed; incisor shorter, more proÖdont; m3 smaller relative to m1 and m2; external reËntrant valley of m3 broad, extending farther across crown of tooth. 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 grasses and for a life in open country. 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, proÖdont; 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; masseteric ridge not so far anterior; cheek-teeth slightly broader, less depressed, therefore, more hypsodont; incisor shorter, more proÖdont. From B. sawrockensis, B. brachygnathus differs in: molar row slightly longer; teeth slightly less depressed; masseteric ridge extends farther anteriorly; incisors more proÖdont. From B. musculus, B. brachygnathus differs in: jaw smaller; molar row slightly shorter; molars less depressed; incisors slender, shorter, narrower, and more proÖdont. From B. taylori, B. brachygnathus differs in: incisor more slender, shorter, more proÖdont; 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 proÖdont incisors, which feature seems to preclude brachygnathus being ancestral to taylori. B. brachygnathus may have been a specialized divergence from B. minimus. 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 reËntrant 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 [Pg 592] differs in: anterior median fold deeper; incisor longer, more recurved, less proÖdont; 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. 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 proÖdont 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 proÖdont 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. |
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