M AMMALIAN SPECIES No. 285, pp. 1- 7, 3 figs. Baiomys taylori. By Bruce D. Eshelman and Guy N. Cameron Published 12 August 1987 by The American Society of Mammalogists Baiomys T rue, 189 4 GENERA L CHA RACTERS. Average adult body mass of B. taylori is 6 to 9.5 g (Hudson, 1965). A weighted average Baiomy s True, 1894:758. Type species Hesp eromy s (Vespe romys) (subspecies combined) of data provided in Packard (1960) yields laylori Thomas. mean s and ranges (in parenth eses) for the following external mea­ Downloaded from https://academic.oup.com/mspecies/article/doi/10.2307/3503776/2600606 by guest on 29 September 2021 sure ments (in mm) for adults: total length, 106.3 (87 to 123); length CONTEXT AND CO NTENT. Order Rodentia, Suborder of tail vertebrae , 42 (34 to 53); body length , 64.1 (53 to 76); length Myornorpha, Family Muridae, Subfamily Sigmodontinae (Carleton of hind foot, 13.5 (12 to 15); length of ear from notch, 10.8 (9 to and Musser, 1984). The genus contains two species. A key to the genus follows (adapted from Packard, 1960 and Hall, 1981 ): 12). Sexual dimorphism has not been reported for measured char­ acters (Packard, 1960 ). Individuals from populations in northern I Longitudinal, dorsal outline of the skull evenly convex; en­ portions of the range or higher elevations have larger externa l toglossal process of the basihyal pointed and directed an­ measurements than counterparts in the south or at lower altitudes teriorly; body of incus flattened, short process knob-shaped; (Packard, 1960). orbicular apoph ysis of malleus round to oblong; baculum Similarly derived means and ranges (in parenth eses) for cranial rounded at tip, 3.0 to 3.9 mm long . ...... B. musculus measurements (in mm) are: occipitonasal length, 18.1 (16.8 to 19.4 ); Longitudinal, dorsal outline of the skull not evenly convex zygomatic breadth, 9.5 (8.7 to 10.2); postpalatal length, 6.6 (5.9 (outline anteriorly deflected ventra lly from frontoparietal to 7.3); least interorbital breadt h, :~ . 5 (3.3 to 3.9); length of incisive suture); entoglossal process of the basihyal rounded or foram ina, 4.0 (3.5 to 4.3); length of rostr um, 6.1 (5.2 to 6.8); absent; orbicular apophysis of malleus rounded to ovoid; breadt h of braincase, 8.7 (8.0 to 9. 1); depth of cranium, 6.6 (6.0 baculum notched at the tip and less than 2.9 mm long . to 7.0); and alveolar length of maxillar y tooth row, 3. 2 (3.1 to 3.4). B. laylori Juveniles are uniformly gray above and lighter below (Packard, 1960). The dorsal pelage of adults varies from reddish brown, through Baiomy s ta ylori (T homas, 1887 ) gray to almost black above and white to crea my buff or gra y below. The tail is covered with short hairs. Color of tail differs among Northe rn Pygmy ~I o u s e subspecies and may be uniformly gra y or bicolored (lighter ventrally). Hesperomy s ( Fespe rim us) laylori Thomas. 188 7:66. Type locality DI STRIBUTIO N. The southern limit of the geographic range San Diego, Duval Co., Texas. of B. tavlori (Fig. 3) is in central Mexico (approximately 19°N) and Peromvs cus paulu s J. A. Allen, 1903:598. Type locality Rio Sestin, extends north in three projections (Hall. 198 1; Packard, 1960). The northwestern Durango. western projection extends along the west coast of Mexico to southern Peromvscus allex Osgood, 1904:76. Type locality Colima, Colima. Sonora. A middle projection extends northwest through Durango Baiomvs laylori Mearns. 190 7:381 ; first use of name combination. and Chihuahua and reac hes its northe rn limit in southeastern Ari­ zona. The easte rn projection extends from central Mexico, north to CONTEXT AN D CONTENT. Conte xt noted in generic centra l Texas, and east along the Texas coast. Recent range ex­ summary above. The following eight subspecies were recognized by pansions (Fig. 3) have occur red toward the Oklahoma border (Cok­ Hall (1981 ): endolpher et al., 1979), and northwest and west onto the Texas B. I. Ill/ex Osgood, 1904:76, see above. High Plains (Diersing and Diersing, 1979; Stangl et aI., 1983). B. I. analogus Osgood, 1909:256. Type locality Zamora. Michoa­ FOSSIL RECORD. Seven fossil species have been described can. for the genus (Gidley, 1922; Hibbard, 1953; Packard, 1960; Pack· B. I. Iller Blossom and Burt, 1942:2. Type locality 7 mi W Hereford, ard and Alvarez , 1965; Zakrzewski. 1969) . According to Hibbard Cochise Co.• Arizona. R. I. cn nu tus Packard. 1 96 0 : 6 4 :~ . Type locality I mi S Pericos, Sinaloa. B. I. .[uligi natus Packard, 1960:645. Type locality 2 mi N. 10 mi E Ciudad del Maiz, 4,000 ft, San Luis Potosi. B. I. pa ulus J. A. Allen, 1903:598, see above. B. I. subater V. Bailey, 1905: I02, Type locality Bernard Creek. near Columbia, Brazoria Co., Texas. B. I. taylori Thomas, 1887:66, see above. DIAGNO SIS. Baiomys taylor! (Fig. I) is the smaller of the two living species of pygmy mice and is the smallest North American rodent (Packard, 1960). Total length ranges from 87 to 123 mm. B. tavlori is distinguished from B. musculus by a hind foot length of le;s than 16 mm. occipitonasal length less than 19 mrn, and zygomatic breadth less tha n 10 mm. The rostrum of B. taylori (Fig. 2) is deflected ventrally at the frontoparietal suture rather than gradually curving toward the anteriormost point of the nasals (Pac k. ard, 1960). The dental formu la is i Ill, c 0/0, I' 0/0, m 3/3, total 16. Molars of B. taylori are more hypsodont than those of B. musculu s and the cingular ridges and secondary cusps of the teeth are reduced or absent (Pa ckard, 1960; Packard and Montgomery, 1978). The entoglossal process of the basihyal is much reduced or absent (Pa ckard, 1960; Hall, 1981 ). The baculum is narrow, has a notched tip. and usually is less than 2.9 mm in length (Blair. 1942; Hall, 198 1). The short processes of the incus are attenuated (Pack. ard, 1960; Hall, 1981). FIG. I. The northern pygmy mouse (Baiomy s tay lori laylori). 2 MAMMALIAN SPEC IES 285 Downloaded from https://academic.oup.com/mspecies/article/doi/10.2307/3503776/2600606 by guest on 29 September 2021 2 Frc, 3. Geographi c distribution of B. taylo ri (after Hall, 1981). Closed circles indicate ran ge extensions described in text. l , B. I. Ill/ex; 2, B. I. an alogue: 3, B. I. aler; 4, B. I. call/llus ; 5 , B. I. .[ulignatus: 6, B. I. paulus: 7, B. I. subater; 8, B. I. taylori. that of B. rexroadi. Packard (1960) and Packa rd and Alvarez (1965) conclud ed that B. ta ylori most likely was derived from a B. sa w­ rock ensis -l], minimus-B. int erm edius-B. mu sculus line and split from B . mu sculus in the midPleistocene. These authors also proposed central Mexico as the center of speciation for the genu s and con­ sidered B. tavlori the more recent of the living species. Fossil remain s of B. ta vlori were identified from the Pleistocene fauna s of Schulze Cave, E~ l wa rd s Co., Texas (Dalquest et aI., 1969) and Cueva Que­ brad a. Val Verde Co., Texas (Lundeliu s, 1984). FO RM . Pelage color of B. ta.ylori varies widely amon g the subspecies (Packard, 1960) . The first eviden ce of postjuvenal molt is presence of bright brownish hair s on the head at 38 to 46 days of age (Blair, 1941 ; Packard , 1960). Sequence of the molt (Pa cka rd, 1960) is similar to that reported for Peromyscus (Collins, 1918, 1924; Hoffmeister, 1951). Postjuvenal molt is complet e at 60 to 74 days (Blair, 1941 ; Packard, 1960). Duration of the molt is similar to that reported for R eithrodontomys (Layne, 1959) . Adult pygmy mice molt during all month s of the year; however , the annual molt usually starts at the beginning of the ra iny season. This molt Fu.. 2. Dorsal, ventral, and lateral views of the cranium and begins ant eriorly and proceeds posteriorl y over the back (Packar d, lateral view of mandible of B . ta.ylor i. (Skull from the Texa s Co­ 1960). Th e pelage of a melanistic individual contained longer and operativ e Wildlife Collection, Texas A&M University.) Greate st length finer hair s than "normal" animals of the same sex and age (Packard, of skull is 18.8 mm. 1960). The rostrum ante rior to the frontonasal suture typically is deflected ventrally 3 to 5° (Packard, 1960). Th e shape of the basihyal (1953), the genus arose in the Blancan Land-Mammal Age of the is parti cularl y useful in discriminating between B. taylori and B. late Pliocene. The oldest of the species seems to have been B. muscu lus. The entog lossal proce ss points anteriorly in B. musculus sa uirocke nsis from Saw Rock Canyon, Seward Co., Kansas (earl y and is round ed or absent in B. ta ylori. Mean length of the basihyal late Pliocene) . Type specimens of B. minimus and B . brachygnathus is 2.18 mm and its shoulders are flattened. The hypohyals remain are from the late Pliocene and midPleistocene , respectiv ely, of Co­ distinct throughout life. The body of the incus is roun d and the short chise Co., Arizona (Cazin, 194 2; Hibbard , 1958).