[Transcriber's Note: Cover compiled from scanned images.]University of Kansas PublicationsMuseum of Natural HistoryVolume 9, No. 23, pp. 579-670, 4 pls., 12 figs. in textJune 16, 1960Speciation and Evolution of thePygmy Mice, Genus BaiomysBYROBERT L. PACKARDUniversity of KansasLawrence1960University of Kansas Publications, Museum of Natural HistoryEditors: E. Raymond Hall, Chairman, Henry S. Fitch,Robert W. WilsonVolume 9, No. 23, pp. 579-670, 4 pls., 12 figs. in textPublished June 16, 1960University of KansasLawrence, KansasPRINTED INTHE STATE PRINTING PLANTTOPEKA, KANSAS1960Look for the Union Label28-3030Speciation and Evolution of the Pygmy Mice, Genus BaiomysBYROBERT L. PACKARDCONTENTSPAGEIntroduction583Materials, Methods and Acknowledgments584Paleontology of the Genus587Baiomys sawrockensis588Baiomys rexroadi589Baiomys kolbi590Baiomys brachygnathus590Baiomys minimus591Phyletic trends592Non-Geographic Variation595Variation with age595Secondary sexual variation597Individual variation597Pelage and molts598Taxonomic Characters and Relationships600External parts600Pelage600Skull600Teeth601Hyoid apparatus601Baculum603Auditory ossicles605Genus Baiomys607Systematic Accounts of Species and Subspecies608Baiomys musculus608Baiomys musculus brunneus612Baiomys musculus grisescens614Baiomys musculus handleyi617Baiomys musculus infernatis618Baiomys musculus musculus620Baiomys musculus nigrescens623Baiomys musculus pallidus625Baiomys musculus pullus628Baiomys taylori630Baiomys taylori allex633Baiomys taylori analogous637Baiomys taylori ater640Baiomys taylori canutus643Baiomys taylori fuliginatus645Baiomys taylori paulus647Baiomys taylori subater650Baiomys taylori taylori651Evolution and Speciation655Formation of the Recent Species658Areas of present differentiation661Zoogeographic position661Conclusions664Literature Cited665INTRODUCTIONPygmy 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) describedSitomys musculuson the basis of specimens from Colima [City of], Colima, México. Merriam (loc. cit.) mentioned that the two kinds of mice,Hesperomys tayloriandSitomys 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 genusSitomys. Frederick W. True in 1894 regarded them as composing a distinct subgenus ofSitomys, Baiomys. According to True (1894:758),S. tayloriandS. musculuspossessed 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 eitherVesperimus, orOnychomys(which had been considered as a subgenus ofHesperomysuntil 1889). In 1907, E. A. Mearns accordedBaiomysgeneric rank. Osgood (1909:252) treatedBaiomysus a subgenus ofPeromyscus, whereas, Miller, in 1912, regardedBaiomysas 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 thatBaiomys"… seems to be considerably distinct fromPeromyscus, and may perhaps be a northern representative ofHesperomysor 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 ofBaiomysavailable 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.MATERIALS, METHODS AND ACKNOWLEDGMENTSThis 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 toBaiomyswere 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 ofBaiomysand 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 1shows the points between which measurements were taken.Occipitonasal length.—From anteriormost projection of nasal bones to posteriormost projection of supraoccipital bone.AtoA'Zygomatic breadth.—Greatest distance across zygomatic arches of cranium at right angles to long axis of skull.BtoB'Postpalatal length.—From posterior margin of hard palate to anterior margin of foramen magnum.CtoC'Least interorbital breadth.—Least distance across top of skull between orbits.DtoD'[Pg 585]Length of incisive foramina.—From anteriormost point to posteriormost point of incisive foramina.EtoE'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.FtoF'Breadth of braincase.—Greatest distance across braincase, taken at right angles to long axis of skull.GtoG'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.HtoH'Alveolar length of maxillary tooth-row.—From anterior border of alveolus of M1 to posterior alveolus of M3.ItoI'Fig. 1.Three views of the skull to show points between which measurements were taken.Based onB. 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 (seeFigure 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 (seepage 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.A.B. taylori analogous, subadult, female, No. 28102 KU, 4 km. ENE Tlalmanalco, 2290 meters, Estado de México. Right, upper molars.B.B. musculus musculus, subadult, male, No. 45456 USNM, Colima, Colima, México. Left, upper molars.A'.B. taylori analogous, subadult, female, No. 28102 KU 4 km. ENE Tlalmanalco, 2290 meters, Estado de México. Left, lower molars.B'.B. musculus musculus, subadult, male, No. 45456 USNM, Colima, Colima, México. Right, lower molars.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.PALEONTOLOGY OF THE GENUSFive 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 sawrockensisHibbardBaiomys sawrockensisHibbard, 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, seeFigure 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 withB. brachygnathus, see account of that species. FromB. rexroadi,B. sawrockensisdiffers 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.FromB. kolbi,B. sawrockensisdiffers in: crowns of molars narrower; incisors less proödont; cingular ridges (ectolophids and mesolophids) of m1 and m2 more pronounced in their development.FromB. minimus,B. sawrockensisdiffers in: incisor less procumbent; masseteric ridge extending farther anteriorly; anterior cingulum of m2 slightly larger.FromB. musculus,B. sawrockensisdiffers in: over-all size of jaw and molar row less; diastema more acutely curved; incisors shorter; anterior median fold of m1 slightly deeper.FromB. taylori,B. sawrockensisdiffers 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. sawrockensisis the oldest known pygmy mouse. The extreme development of the anterior median fold between theanterolingual 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 throughB. minimustoB. sawrockensis.B. sawrockensisresemblesCalomys lauchaof South America in general conformation of jaw and tooth structure. The molars ofsawrockensisare smaller than those ofC. laucha, and the anterolingual conulid ofsawrockensisis farther forward.Baiomys rexroadiHibbardBaiomys rexroadiHibbard, 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 withB. sawrockensisandB. minimus, see accounts of those species. FromB. kolbi,B. rexroadidiffers in: over-all size of mandibular ramus, incisors, and molars smaller; anterior median fold of m1 present, though poorly developed.FromB. brachygnathus,B. rexroadidiffers 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.FromB. musculus,B. rexroadidiffers in: over-all size of mandibular ramus less; cingular ridges of m1 and m2 less well-developed; incisors smaller, more proödont; molars less depressed.FromB. taylori,B. rexroadidiffers 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 kolbiHibbardBaiomys kolbiHibbard, 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 withB. sawrockensisandB. rexroadi, see accounts of those species. FromB. brachygnathus,B. kolbidiffers in: molar row longer; m3 and jaw larger; diastema longer; masseteric ridge not so far forward; molars more depressed.FromB. minimus,B. kolbidiffers in: molar row longer; m3 larger; jaw larger; diastema not so acutely curved; incisor shorter, narrower transversely, more proödont.FromB. musculus,B. kolbidiffers 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.FromB. taylori,B. kolbidiffers 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 inkolbisuggests that it was specialized. The anterior median fold is well-developed in all species ofBaiomyssaveB. brachygnathusandB. taylori, in which the fold is only slightly developed or absent.B. kolbimay have paralleledB. tayloriin specialization for a diet of grasses and for a life in open country.Baiomys brachygnathus(Gidley)Peromyscus brachygnathusGidley, 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.[Pg 591]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 withB. rexroadiandB. kolbi, see accounts of those species. FromB. minimus,B. brachygnathusdiffers 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.FromB. sawrockensis,B. brachygnathusdiffers in: molar row slightly longer; teeth slightly less depressed; masseteric ridge extends farther anteriorly; incisors more proödont.FromB. musculus,B. brachygnathusdiffers in: jaw smaller; molar row slightly shorter; molars less depressed; incisors slender, shorter, narrower, and more proödont.FromB. taylori,B. brachygnathusdiffers in: incisor more slender, shorter, more proödont; diastema shorter.Remarks.—The molar teeth ofB. brachygnathus, although worn, resemble those ofB. taylorimore 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) inbrachygnathuswas one of the chief characters separatingbrachygnathusfromtaylori. Intaylori, the anterior median fold characteristically is only slightly developed, and in some specimens is absent.B. brachygnathusdiffers fromtaylorichiefly in proödont incisors, which feature seems to precludebrachygnathusbeing ancestral totaylori.B. brachygnathusmay have been a specialized divergence fromB. minimus.Baiomys minimus(Gidley)Peromyscus minimusGidley, 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 withB. brachygnathus,B. kolbi, andB. sawrockensis, see accounts of those species. FromB. rexroadi,B. minimus[Pg 592]differs in: anterior median fold deeper; incisor longer, more recurved, less proödont; molars slightly more depressed (though worn).FromB. musculus,B. minimusdiffers in: over-all size of jaw and molars smaller; incisors shorter; masseteric ridge more depressed.FromB. taylori,B. minimusdiffers 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 thatB. minimusdiffered considerably fromB. tayloriin 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 ofB. taylorireveals 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 likeB. musculusand is considered to be ancestral to that species.PHYLETIC TRENDSIt seems that the important trends in phyletic development in the pygmy mice have been from an ancestral stock (seeFigure 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. kolbiseems 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. kolbimay have been an early, specialized derivation from the ancestral stock. From his knowledge of the habitats ofB. musculus, the larger species, andB. taylori, the smaller species, Hibbard (1952:203) suggests thatB. kolbi, a large species, might have inhabited lowlands, andB. rexroadi, a small species, highlands. I have no evidence to dispute this suggestion except thatB. musculushas more prominent cingular ridges (or at least vestiges of this lophid condition) than eitherB. kolbiorB. rexroadi.B. musculus(seepage 610) is less of an open grassland inhabitant than isB. taylori. Therefore, bothB. kolbiandB. rexroadi, because of their poorly developed cingular ridges, might be expected to have lived in a relatively open grassland habitat.The relationship ofB. rexroadito fossil species other thanB. kolbiis not clear. Superficially, the former resemblesB. taylori, but, owing to the specialized development of the molars ofrexroadi, it could hardly have been ancestral to either of the living species. The resemblance ofB. rexroaditoB. taylorimay result from each having occupied the same ecological niche in different periods. The incisors ofB. rexroadi, however, are much shorter than those ofB. tayloriand suggest somewhat different food habits.B. minimusseemingly is more closely related toB. sawrockensisandB. musculusthan to the other described species. The development of the cingular ridges leads one to suspect thatB. minimuswas the ancestor ofB. musculus.B. minimusmay have been derived from asawrockensis-like stock and probably gave rise toB. 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 genusBaiomyscan be divided into two morphological groups. One group, composed ofB. sawrockensis,B. minimus, andB. 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. sawrockensisprobably 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. sawrockensisprobably inhabited denser vegetation than didB. minimusor than doesB. musculus. The teeth of the second group (B. kolbi,B. rexroadi,B. brachygnathus, andB. 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 ofB. taylori(seepage 632).The suggested grouping, based on supposed similarities in niches inhabited by the extinct species, does not necessarily indicate degree of relationship.B. tayloriprobably was not derived from anancestor likeB. rexroadiorB. kolbi, although, in certain characters, the three species resemble one another.B. kolbiandB. rexroadiwere already specialized in Blancan times, probably for living on grassland.B. taylorishows 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 amongB. taylori,B. rexroadi, andB. kolbi. The available evidence (seepage 658) suggests, however, thatB. tayloriwas derived from theB. sawrockensis-B. minimus-B. musculusline.Fig. 3.Diagram indicating probable relationships of living and extinct species of pygmy mice.Baiomysseems 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 genusCalomys.
[Transcriber's Note: Cover compiled from scanned images.]
[Transcriber's Note: Cover compiled from scanned images.]
University of Kansas PublicationsMuseum of Natural HistoryVolume 9, No. 23, pp. 579-670, 4 pls., 12 figs. in textJune 16, 1960Speciation and Evolution of thePygmy Mice, Genus BaiomysBYROBERT L. PACKARDUniversity of KansasLawrence1960
Speciation and Evolution of thePygmy Mice, Genus Baiomys
BY
ROBERT L. PACKARD
University of Kansas Publications, Museum of Natural HistoryEditors: E. Raymond Hall, Chairman, Henry S. Fitch,Robert W. WilsonVolume 9, No. 23, pp. 579-670, 4 pls., 12 figs. in textPublished June 16, 1960University of KansasLawrence, KansasPRINTED INTHE STATE PRINTING PLANTTOPEKA, KANSAS1960Look for the Union Label28-3030
Speciation and Evolution of the Pygmy Mice, Genus Baiomys
BY
ROBERT L. PACKARD
CONTENTS
INTRODUCTION
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) describedSitomys musculuson the basis of specimens from Colima [City of], Colima, México. Merriam (loc. cit.) mentioned that the two kinds of mice,Hesperomys tayloriandSitomys 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 genusSitomys. Frederick W. True in 1894 regarded them as composing a distinct subgenus ofSitomys, Baiomys. According to True (1894:758),S. tayloriandS. musculuspossessed 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 eitherVesperimus, orOnychomys(which had been considered as a subgenus ofHesperomysuntil 1889). In 1907, E. A. Mearns accordedBaiomysgeneric rank. Osgood (1909:252) treatedBaiomysus a subgenus ofPeromyscus, whereas, Miller, in 1912, regardedBaiomysas 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 thatBaiomys"… seems to be considerably distinct fromPeromyscus, and may perhaps be a northern representative ofHesperomysor 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 ofBaiomysavailable 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.
MATERIALS, METHODS AND ACKNOWLEDGMENTS
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 toBaiomyswere 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 ofBaiomysand 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 1shows the points between which measurements were taken.
Occipitonasal length.—From anteriormost projection of nasal bones to posteriormost projection of supraoccipital bone.AtoA'Zygomatic breadth.—Greatest distance across zygomatic arches of cranium at right angles to long axis of skull.BtoB'Postpalatal length.—From posterior margin of hard palate to anterior margin of foramen magnum.CtoC'Least interorbital breadth.—Least distance across top of skull between orbits.DtoD'[Pg 585]Length of incisive foramina.—From anteriormost point to posteriormost point of incisive foramina.EtoE'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.FtoF'Breadth of braincase.—Greatest distance across braincase, taken at right angles to long axis of skull.GtoG'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.HtoH'Alveolar length of maxillary tooth-row.—From anterior border of alveolus of M1 to posterior alveolus of M3.ItoI'
Fig. 1.Three views of the skull to show points between which measurements were taken.Based onB. 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 (seeFigure 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 (seepage 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.A.B. taylori analogous, subadult, female, No. 28102 KU, 4 km. ENE Tlalmanalco, 2290 meters, Estado de México. Right, upper molars.B.B. musculus musculus, subadult, male, No. 45456 USNM, Colima, Colima, México. Left, upper molars.A'.B. taylori analogous, subadult, female, No. 28102 KU 4 km. ENE Tlalmanalco, 2290 meters, Estado de México. Left, lower molars.B'.B. musculus musculus, subadult, male, No. 45456 USNM, Colima, Colima, México. Right, lower molars.
Fig. 2.Occlusal views of molars. × 13.A.B. taylori analogous, subadult, female, No. 28102 KU, 4 km. ENE Tlalmanalco, 2290 meters, Estado de México. Right, upper molars.B.B. musculus musculus, subadult, male, No. 45456 USNM, Colima, Colima, México. Left, upper molars.A'.B. taylori analogous, subadult, female, No. 28102 KU 4 km. ENE Tlalmanalco, 2290 meters, Estado de México. Left, lower molars.B'.B. musculus musculus, subadult, male, No. 45456 USNM, Colima, Colima, México. Right, lower molars.
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.
PALEONTOLOGY OF THE GENUS
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 sawrockensisHibbard
Baiomys sawrockensisHibbard, 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, seeFigure 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 withB. brachygnathus, see account of that species. FromB. rexroadi,B. sawrockensisdiffers 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.FromB. kolbi,B. sawrockensisdiffers in: crowns of molars narrower; incisors less proödont; cingular ridges (ectolophids and mesolophids) of m1 and m2 more pronounced in their development.FromB. minimus,B. sawrockensisdiffers in: incisor less procumbent; masseteric ridge extending farther anteriorly; anterior cingulum of m2 slightly larger.FromB. musculus,B. sawrockensisdiffers in: over-all size of jaw and molar row less; diastema more acutely curved; incisors shorter; anterior median fold of m1 slightly deeper.FromB. taylori,B. sawrockensisdiffers 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.
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, seeFigure 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 withB. brachygnathus, see account of that species. FromB. rexroadi,B. sawrockensisdiffers 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.
FromB. kolbi,B. sawrockensisdiffers in: crowns of molars narrower; incisors less proödont; cingular ridges (ectolophids and mesolophids) of m1 and m2 more pronounced in their development.
FromB. minimus,B. sawrockensisdiffers in: incisor less procumbent; masseteric ridge extending farther anteriorly; anterior cingulum of m2 slightly larger.
FromB. musculus,B. sawrockensisdiffers in: over-all size of jaw and molar row less; diastema more acutely curved; incisors shorter; anterior median fold of m1 slightly deeper.
FromB. taylori,B. sawrockensisdiffers 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. sawrockensisis the oldest known pygmy mouse. The extreme development of the anterior median fold between theanterolingual 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 throughB. minimustoB. sawrockensis.B. sawrockensisresemblesCalomys lauchaof South America in general conformation of jaw and tooth structure. The molars ofsawrockensisare smaller than those ofC. laucha, and the anterolingual conulid ofsawrockensisis farther forward.
Baiomys rexroadiHibbard
Baiomys rexroadiHibbard, 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 withB. sawrockensisandB. minimus, see accounts of those species. FromB. kolbi,B. rexroadidiffers in: over-all size of mandibular ramus, incisors, and molars smaller; anterior median fold of m1 present, though poorly developed.FromB. brachygnathus,B. rexroadidiffers 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.FromB. musculus,B. rexroadidiffers in: over-all size of mandibular ramus less; cingular ridges of m1 and m2 less well-developed; incisors smaller, more proödont; molars less depressed.FromB. taylori,B. rexroadidiffers 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.
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 withB. sawrockensisandB. minimus, see accounts of those species. FromB. kolbi,B. rexroadidiffers in: over-all size of mandibular ramus, incisors, and molars smaller; anterior median fold of m1 present, though poorly developed.
FromB. brachygnathus,B. rexroadidiffers 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.
FromB. musculus,B. rexroadidiffers in: over-all size of mandibular ramus less; cingular ridges of m1 and m2 less well-developed; incisors smaller, more proödont; molars less depressed.
FromB. taylori,B. rexroadidiffers 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 kolbiHibbard
Baiomys kolbiHibbard, 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 withB. sawrockensisandB. rexroadi, see accounts of those species. FromB. brachygnathus,B. kolbidiffers in: molar row longer; m3 and jaw larger; diastema longer; masseteric ridge not so far forward; molars more depressed.FromB. minimus,B. kolbidiffers in: molar row longer; m3 larger; jaw larger; diastema not so acutely curved; incisor shorter, narrower transversely, more proödont.FromB. musculus,B. kolbidiffers 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.FromB. taylori,B. kolbidiffers 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.
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 withB. sawrockensisandB. rexroadi, see accounts of those species. FromB. brachygnathus,B. kolbidiffers in: molar row longer; m3 and jaw larger; diastema longer; masseteric ridge not so far forward; molars more depressed.
FromB. minimus,B. kolbidiffers in: molar row longer; m3 larger; jaw larger; diastema not so acutely curved; incisor shorter, narrower transversely, more proödont.
FromB. musculus,B. kolbidiffers 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.
FromB. taylori,B. kolbidiffers 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 inkolbisuggests that it was specialized. The anterior median fold is well-developed in all species ofBaiomyssaveB. brachygnathusandB. taylori, in which the fold is only slightly developed or absent.B. kolbimay have paralleledB. tayloriin specialization for a diet of grasses and for a life in open country.
Baiomys brachygnathus(Gidley)
Peromyscus brachygnathusGidley, 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.[Pg 591]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 withB. rexroadiandB. kolbi, see accounts of those species. FromB. minimus,B. brachygnathusdiffers 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.FromB. sawrockensis,B. brachygnathusdiffers in: molar row slightly longer; teeth slightly less depressed; masseteric ridge extends farther anteriorly; incisors more proödont.FromB. musculus,B. brachygnathusdiffers in: jaw smaller; molar row slightly shorter; molars less depressed; incisors slender, shorter, narrower, and more proödont.FromB. taylori,B. brachygnathusdiffers in: incisor more slender, shorter, more proödont; diastema shorter.
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.
[Pg 591]
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 withB. rexroadiandB. kolbi, see accounts of those species. FromB. minimus,B. brachygnathusdiffers 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.
FromB. sawrockensis,B. brachygnathusdiffers in: molar row slightly longer; teeth slightly less depressed; masseteric ridge extends farther anteriorly; incisors more proödont.
FromB. musculus,B. brachygnathusdiffers in: jaw smaller; molar row slightly shorter; molars less depressed; incisors slender, shorter, narrower, and more proödont.
FromB. taylori,B. brachygnathusdiffers in: incisor more slender, shorter, more proödont; diastema shorter.
Remarks.—The molar teeth ofB. brachygnathus, although worn, resemble those ofB. taylorimore 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) inbrachygnathuswas one of the chief characters separatingbrachygnathusfromtaylori. Intaylori, the anterior median fold characteristically is only slightly developed, and in some specimens is absent.B. brachygnathusdiffers fromtaylorichiefly in proödont incisors, which feature seems to precludebrachygnathusbeing ancestral totaylori.B. brachygnathusmay have been a specialized divergence fromB. minimus.
Baiomys minimus(Gidley)
Peromyscus minimusGidley, 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 withB. brachygnathus,B. kolbi, andB. sawrockensis, see accounts of those species. FromB. rexroadi,B. minimus[Pg 592]differs in: anterior median fold deeper; incisor longer, more recurved, less proödont; molars slightly more depressed (though worn).FromB. musculus,B. minimusdiffers in: over-all size of jaw and molars smaller; incisors shorter; masseteric ridge more depressed.FromB. taylori,B. minimusdiffers in: anterior median fold slightly deeper; molar teeth more depressed; cingular ridges on m1 and m2 better developed; masseteric ridge more depressed.
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 withB. brachygnathus,B. kolbi, andB. sawrockensis, see accounts of those species. FromB. rexroadi,B. minimus[Pg 592]differs in: anterior median fold deeper; incisor longer, more recurved, less proödont; molars slightly more depressed (though worn).
FromB. musculus,B. minimusdiffers in: over-all size of jaw and molars smaller; incisors shorter; masseteric ridge more depressed.
FromB. taylori,B. minimusdiffers 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 thatB. minimusdiffered considerably fromB. tayloriin 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 ofB. taylorireveals 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 likeB. musculusand 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 (seeFigure 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. kolbiseems 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. kolbimay have been an early, specialized derivation from the ancestral stock. From his knowledge of the habitats ofB. musculus, the larger species, andB. taylori, the smaller species, Hibbard (1952:203) suggests thatB. kolbi, a large species, might have inhabited lowlands, andB. rexroadi, a small species, highlands. I have no evidence to dispute this suggestion except thatB. musculushas more prominent cingular ridges (or at least vestiges of this lophid condition) than eitherB. kolbiorB. rexroadi.B. musculus(seepage 610) is less of an open grassland inhabitant than isB. taylori. Therefore, bothB. kolbiandB. rexroadi, because of their poorly developed cingular ridges, might be expected to have lived in a relatively open grassland habitat.
The relationship ofB. rexroadito fossil species other thanB. kolbiis not clear. Superficially, the former resemblesB. taylori, but, owing to the specialized development of the molars ofrexroadi, it could hardly have been ancestral to either of the living species. The resemblance ofB. rexroaditoB. taylorimay result from each having occupied the same ecological niche in different periods. The incisors ofB. rexroadi, however, are much shorter than those ofB. tayloriand suggest somewhat different food habits.
B. minimusseemingly is more closely related toB. sawrockensisandB. musculusthan to the other described species. The development of the cingular ridges leads one to suspect thatB. minimuswas the ancestor ofB. musculus.B. minimusmay have been derived from asawrockensis-like stock and probably gave rise toB. 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 genusBaiomyscan be divided into two morphological groups. One group, composed ofB. sawrockensis,B. minimus, andB. 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. sawrockensisprobably 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. sawrockensisprobably inhabited denser vegetation than didB. minimusor than doesB. musculus. The teeth of the second group (B. kolbi,B. rexroadi,B. brachygnathus, andB. 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 ofB. taylori(seepage 632).
The suggested grouping, based on supposed similarities in niches inhabited by the extinct species, does not necessarily indicate degree of relationship.B. tayloriprobably was not derived from anancestor likeB. rexroadiorB. kolbi, although, in certain characters, the three species resemble one another.B. kolbiandB. rexroadiwere already specialized in Blancan times, probably for living on grassland.B. taylorishows 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 amongB. taylori,B. rexroadi, andB. kolbi. The available evidence (seepage 658) suggests, however, thatB. tayloriwas derived from theB. sawrockensis-B. minimus-B. musculusline.
Fig. 3.Diagram indicating probable relationships of living and extinct species of pygmy mice.
Fig. 3.Diagram indicating probable relationships of living and extinct species of pygmy mice.
Baiomysseems 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 genusCalomys.