Comparative myology of the forelimb of squirrels (Sciuridae)

Comparative Myology of the Forelimb of Squirrels (Sciuridae)RICHARD W. THORINGTON, JR.,* KAROLYN DARROW,AND AIMEE D.K. BETTSDepartment of Vertebrate Zoology, Smithsonian Institution,Washington, D.C. 20004

ABSTRACT The musculature of the shoulder, arm, and forearm was stud-ied in 19 genera of squirrels, representing the Pteromyinae (flying squirrels)and all 7 tribes of the Sciurinae (tree and ground squirrels). The objective wasto locate derived anatomical features of functional or phylogenetic signifi-cance and to determine how much morphological variation underlies thediverse locomotor behavior of squirrels, which includes terrestrial and arbo-real bounding, climbing, digging, and gliding. The fossil evidence suggeststhat arboreality is primitive for squirrels, and in fact tree squirrels appear torepresent the primitive sciurid morphology. Ground squirrels are less uniformand exhibit a few derived features, including a clavobrachialis muscle notseen in other squirrels. Pygmy tree squirrels, which have evolved indepen-dently in three tribes, exhibit convergence of forelimb anatomy, including theloss or reduction of several muscles in the shoulder and forearm. The forelimbanatomy of flying squirrels is the most derived and differs from that of treesquirrels in details of shoulder, arm, and forearm musculature. Some of thesemuscular differences among squirrels have phylogenetic significance, beingshared by closely related genera, but none has significance above the triballevel. Many of the differences suggest a variety of changes in function that areamenable to further study. J. Morphol. 234:155–182, 1997. r 1997 Wiley-Liss, Inc.*

Herein we present a study of the forelimbmusculature of squirrels, which plays a keyrole in gliding, climbing, digging, and qua-drupedal locomotion. There are three pur-poses for this study. First, we are looking forderived features that reflect functionalchanges in the use of the forelimb, especiallyany that have evolved independently in dif-ferent taxa and that may be interpreted asadaptations to particular ways of life. Sec-ond, we are seeking myological charactersthat will be useful in formulating phyloge-netic hypotheses about the genera of squir-rels. Although we do not anticipate findingenough forelimb characters to enable us toconduct a phylogenetic analysis, we hopethat a series of studies of squirrel anatomy(Ball and Roth, ’95; Thorington et al., ’96;Thorington and Darrow, ’96; Thorington etal., in press) will provide adequate data forsuch an analysis in the future. Third, wewish to compile morphological data that canbe used to test phylogenetic hypotheses de-rived from the increasing amount of molecu-lar data on squirrels—immunological, elec-trophoretic, sequencing, etc.—which is being

collected (Hight et al., ’74; Ellis and Maxson,’80; Hafner, ’84; Hafner et al., ’94; Oshida etal., ’96). The muscular anatomy of squirrelsis well known for only 11 genera and 23species (Hoffmann and Weyenbergh, 1870;Parsons, 1894; Alezais, 1900; Brizzee, ’41;Peterka, ’36; Bryant, ’45), with a strong biastoward the two tribes of Holarctic taxa, andno representation among three tribes ofAsian and African squirrels. We have stud-ied the musculature of the forelimb in 19genera and 25 species, including 6 genera offlying squirrels and representatives of all 7recognized tribes of the Sciurinae.

Modern squirrels occur in a diversity ofniches and present a number of cases ofconvergent evolution. Pygmy tree squirrelshave evolved independently in SouthAmerica, Africa, and Southeast Asia, andare members of three different tribes. Groundsquirrels (terrestrial squirrels that often live

Contract grant sponsor: Smithsonian Institution.*Correspondence to: Richard W. Thorington, Jr., NHB 390,

MRC 108, Smithsonian Institution, Washington, D.C. 20004.

JOURNAL OF MORPHOLOGY 234:155–182 (1997)

r 1997 WILEY-LISS, INC. *This article is a US Govern-ment work and, as such, is in the public domain in theUnited States of America.

in subterranean burrows) occur in two tribes,the Holarctic Marmotini, including the chip-munks, marmots, prairie dogs, and groundsquirrels, and the Xerini, which includestwo African genera and one Asian genus.Cursorial terrestrial squirrels (which do notlive in burrows) have evolved independentlyon Borneo (Rheithrosciurus, of uncertaintribal allocation) and in Africa (Epixerus, ofthe tribe Protoxerini). Another terrestrialsquirrel, Rhinosciurus, of the Callosciurini,specializes in eating ants and termites(Askins, ’77). Tree squirrels occur in fivetribes. Because arboreality is primitive forsquirrels (Emry and Thorington, ’82), it isprobable that it evolved only once. It is alsoprobable that gliding has evolved only oncein squirrels (Thorington, ’84), but the phylo-genetic relationships of the genera of flyingsquirrels are unclear (McKenna, ’62). Theyoccur in North America and Eurasia, withthe greatest diversity (12 genera) in South-ern Asia.

A trustworthy phylogenetic hypothesis ofthe family Sciuridae does not exist. Modernsquirrels comprise approximately 50 generaand 273 species (Hoffmann et al., ’93). Theyare usually aligned in 2 subfamilies, theflying squirrels, in the Pteromyinae [14 gen-era, 43 species (Hoffmann et al., ’93), but seeThorington et al., ’96, for justification that15 genera should be recognized], and allother squirrels in the Sciurinae, which is inturn divided into 7 tribes. Moore (’59) di-vided the Sciurinae into eight tribes, but thenumber has since been reduced to seven byBlack (’63), Hight et al. (’74), Hafner et al.(’94). The tribe Sciurini includes the treesquirrels of South America, North America,and northern Eurasia. The Marmotini in-clude the Holarctic chipmunks, marmots,and ground squirrels. Three tribes occurprincipally in Africa: ground squirrels of thetribe Xerini, and tree squirrels of the tribesProtoxerini and Funambulini. Two othertribes occur in southern Asia: Ratufini, thegiant tree squirrels, and Callosciurini, whichincludes a radiation of 13 genera of arborealand terrestrial squirrels.

Some genera are probably inappropriatelyallocated to these tribes. Among these areRheithrosciurus, a terrestrial squirrel of Bor-neo, which is traditionally placed in the Hol-arctic and South American Sciurini, and Sci-urillus, the South American pygmy squirrel,which is also placed in the Sciurini in spite

of its divergent morphological and geneticcharacteristics (Anthony and Tate, ’35;Moore, ’59; Hafner et al., ’94). The place-ment of the Indian Funambulus in the sametribe with the African genera Paraxerus,Funisciurus, and Myosciurus is also ques-tionable (Hight et al., ’74; Thorington andDarrow, ’96). The largest systematic prob-lem, however, is the relationship among thetribes. It is unclear how the flying squirrelsand the seven tribes of tree and groundsquirrels are related one to another. Theflying squirrels have long been recognized asa separate subfamily based on their distinc-tive morphology. Until recently, there wasno evidence relevant to when they divergedfrom other squirrels. Paleontological evi-dence has been used to argue for an earlydate of divergence (Mein, ’70; DeBruijn andUnay, ’89), leading to the suggestion thatflying squirrels should be considered a sepa-rate family from other squirrels (Corbet andHill, ’92). However, Emry and Korth (’96)conclude that the fossil evidence, based juston teeth, is inadequate to determine whetherthey are a distinct family. The immunodiffu-sion evidence of Hight et al. (’74) suggeststhat the flying squirrels are more closelyrelated to the Sciurini than to other tribes oftree squirrels, but the analysis of Ellis andMaxson (’80) suggests that they are diver-gent from other tribes of the Sciurinae. Vari-ous hypotheses have been suggested aboutthe relationships between tribes of the Sci-urinae, such as Moore’s (’59) suggestion thatthe Protoxerini were derived from the Xe-rini, and Hight et al.’s (’74) dendrogram thatsuggests the sequential divergence of theMarmotini, Callosciurini, Ratufini, and Sci-urini, but none of these hypotheses is wellsupported by morphological data, and someseem contradicted by it (Thorington and Dar-row, ’96).

Squirrels are first known from the fossilrecord of the North American Eocene, 35mybp (Emry and Thorington, ’82). Emry andKorth (’96) allocate the genus Douglassia tothe tribe Sciurini. Paleosciurus occurred inEurope in the early Oligocene, approxi-mately 30 mybp (Vianey-Liaud, ’74). By thelate Oligocene, the Marmotini appear in thefossil record of North America (Miosper-mophilus) and the Xerini appear in the fossilrecord of Europe (Heteroxerus) (Black, ’72).Squirrels of undetermined tribal affinity arefirst recorded from Africa in the Miocene(Lavocat, ’73). On the basis of this evidence,

156 R.W. THORINGTON ET AL.

Black (’63) suggested that the tribes of squir-rels diverged in the late Oligocene. However,the paleontological record provides little in-formation useful for showing how the tribesare related one to another.

MATERIALS AND METHODS

Animals dissected are listed in systematicorder in the Appendix. Dissectable museumspecimens of most of these taxa are rare.During dissection, muscles were bisected be-tween origin and insertion, so that originsand insertions could be examined carefully.We recorded our observations on camera lu-cida drawings made of the appropriate bonesin the collections of the National Museum ofNatural History. Innervation of the muscleswas verified, especially in cases in whichthere was a question about homology. Inner-vations are reported only where they dif-fered with the observations of Hill (’37) andBryant (’45), or where they are of particularinterest.

Anatomical nomenclature remains a prob-lem. The Nomina Anatomica (’66) for hu-mans is inappropriate. The Illustrated Veteri-nary Anatomical Nomenclature (Schaller,’92) does not include rodents. Some termsnormally used in comparative anatomy dif-fer from both. Except where noted, we followthe terminology for muscles used by Hill(’37) to facilitate comparison.

In the remarks sections, we first makecomments on the comparative anatomy ofsquirrels based on Hoffmann and Weyen-bergh (1870) for Sciurus vulgaris; Parsons(1894) for Callosciurus prevostii, Pteromysvolans, Xerus getulus, Spermophilus mexi-cana, and Marmota marmota; Alezais (1900)for Sciurus vulgaris and Marmota marmota;Peterka (’36) for Sciurus niger, Glaucomysvolans, and Cynomys ludovicianus; Hill (’37)for Sciurus griseus and Spermophilus rich-ardsonii; Brizzee (’41) for Spermophilus ar-matus; Bryant (’45) for Spermophilus (sevenspecies), Ammospermophilus (two species),Tamias speciosa, Marmota flaviventer, Cyno-mys gunnisoni, Sciurus griseus, Sciurus ni-ger, Tamiasciurus douglasii, Glaucomysvolans, and Glaucomys sabrinus.

This is followed by comments on the com-parative anatomy of other rodents based onParsons (1894, 1896) for diverse ‘‘hystrico-morph’’ and ‘‘myomorph’’ rodents, Alezais(1900) for Cavia, Mus, and Dipus, Howell(’26) for Neotoma, Hill (’37) for geomyids,Lewis (’49) for Aplodontia, Rinker (’54, ’63)for cricetines, Klingener (’64) for dipodoids,

Woods (’72) for ‘‘hystricomorphs,’’ McEvoy(’82) for Erethizon and Coendou, Stein (’86,’90) for arvicolids and dipodoids, and Ryan(’89) for heteromyids.

RESULTSTrapezius group

M. sternomastoideusOrigin: Arises from the cranial surface of

the manubrium.Insertion (Fig. 1): Inserts on the mastoid

process of the skull. The insertion is re-stricted to a small area of the mastoid in theAfrican genera. It extends from the mastoidslightly onto the nuchal line in other genera,most extensively in Ratufa and Sciurus.

Remarks: In squirrels, variation in theorigins and insertions of this muscle is slight(Parsons, 1894; Brizzee, ’41; Bryant, ’45). Inother rodents, variation is also slight. Thismuscle originates slightly on the clavicle insome cricetines (Rinker, ’54). It inserts onthe posterior edge of the auditory meatus incricetines (Rinker, ’54), on the bulla in Dipod-omys (Ryan, ’89), and on the paraoccipitalprocess in the hystricomorph rodent Myocas-tor (Woods, ’72). Woods (’72) suggests thatthe more extensive insertion on the nuchalline may simply reflect that the muscle isrelatively stronger in some species.

M. cleidomastoideusOrigin (Fig. 2): Arises from the ventral

surface of the clavicle adjacent to the originof the sternomastoid and deep to the originof cleido-occipitalis.

Insertion (Fig. 1): Inserts on the mastoidprocess of the skull just caudal to the inser-tion of sternomastoid. In Spermophilus, theinsertion is dorsal to that of the sternomas-toid, on the superior nuchal line.

Remarks: Like sternomastoid, the originand insertion of this muscle are quite conser-vative in squirrels. Among other rodents,the insertion changes with the insertion ofsternomastoid (Rinker, ’54). This muscle ismissing in Jaculus (Howell, ’32; Klingener,’64).

M. cleido-occipitalis accessoriusOrigin: Arises from the ventral surface of

the clavicle immediately adjacent to the clei-domastoid.

Insertion: Inserts on the nuchal ridge for 2mm adjacent to the mastoid process.

Remarks: Not recorded in other squirrels,this muscle was found in only one specimen

SQUIRREL FORELIMB MUSCLES 157

of Atlantoxerus. It is probably derived fromthe immediately adjacent cleidomastoid, butis completely separable from it.

M. cleido-occipitalisOrigin (Fig. 2): Arises from the caudal

edge of the clavicle. In most genera, theorigin completely covers cleidomastoid. How-ever, in Nannosciurus, Myosciurus, Xerus,Funambulus, Callosciurus, and Spermophi-lus variegatus, the origin was more lateraland only partly overlapped the origin of clei-domastoid.

Insertion (Fig. 1): Inserts on the nuchalline on the cranium, usually between thesternomastoid and the trapezius (in Sper-mophilus, between the cleidomastoid andthe trapezius). Only in Funisciurus are theinsertions of sternomastoid and cleido-occipi-

talis separated by several millimeters. Ingenera with extensive cranial origins of tra-pezius (Xerus and Heliosciurus), the inser-tion is restricted to the lateral portion of thenuchal line. In flying squirrels, which lack acranial origin of trapezius, the insertion ismuch more extensive: in Belomys, Hylope-tes, and Pteromys, it extends from the inser-tion of sternomastoid nearly to the midline;in Eoglaucomys, it extends to the midline; inGlaucomys and Petaurista, it extends to themidline and for a short distance on the nu-chal ligament of the neck. In Atlantoxerus,Sciurus, and Spermophilus, the dorsal edgeof cleido-occipitalis is fused with the ventraledge of trapezius.

Remarks: In squirrels, this muscle is com-pletely separable at its origin from the cleido-mastoid. The degree of fusion of its insertion

Fig. 1. Posterior surface of the skull showing variations in the extent of the origins oftrapezius, occipitoscapularis, and rhomboid, and the insertions of cleido-occipitalis, cleidomas-toid, and sternomastoid in eight squirrel genera. Scale bar 5 5 mm.


with the ventral edge of the trapezius variesin the Marmotini, being completely sepa-rated in Marmota and Cynomys (Bryant,’45). Among other rodents, Woods (’72) notesthat this muscle in hystricomorphs is notseparable from the cleidomastoid at theirorigins.

M. trapeziusOrigin (Fig. 1): In most squirrels, trape-

zius arises from the nuchal line of the cra-nium and the midline of the neck and back.The origin extends for half the length of thenuchal line in Xerus and Heliosciurus andless than half in other tree and ground squir-rels. In the flying squirrels (Belomys, Eoglau-comys, Glaucomys, Hylopetes, Petaurista,and Pteromys), it arises from the neck andback, not from the nuchal line of the skull. Inall squirrels, the origin extends caudally tothe vicinity of T8 to T10. The muscle is not

separable into distinct parts, although theorigin from the midline is aponeurotic oppo-site the scapular spine in some genera (Pro-toxerus, Heliosciurus, Paraxerus, Funisciu-rus, Spermophilus, Atlantoxerus, and Ratufa).

Insertion (Fig. 3): Inserts on the scapularspine from the metacromion almost to thevertebral border. Anterior fibers insert onthe cranial border of the scapular spine; themost posterior fibers insert on the caudalborder of the scapular spine. In Myosciurus,few fibers insert on the caudal border of thespine. In Xerus, the most caudal fibers forman ‘‘auricular slip,’’ passing over the metacro-mion and joining fibers of the M. sphinctercolli superficialis.

Remarks: Among squirrels, Bryant (’45)described the insertion of trapezius extend-ing onto the base of the acromion in theMarmotini. We did not observe this in any ofour dissections and therefore we do not use

Fig. 1. (Continued)


the name acromiotrapezius for the anteriorportion of this muscle. It does not insert onthe acromion and it is not separable from theposterior portion (spinotrapezius) which in-serts on the caudal border of the scapularspine. In some rodents, the insertion ex-tends onto the clavicle: slightly in dipodoidsand heteromyids, extensively in geomyids(Hill, ’37; Klingener, ’64; Ryan, ’89). An au-ricular slip of the trapezius is widely foundamong rodents. Among the sciurids, it wasnoted in Marmota (Bryant, ’45) and in Tami-asciurus (Rinker, ’54). It also occurs inAplodontia (Lewis, ’49; Klingener, ’64), inhystricomorphs (Woods, ’72), and amongbathyergids, microtines, cricetines, and dipo-doids (Klingener, ’64, ’70), but not geomyoids(Hill, ’37; Ryan, ’89).

Extensor systemCosto-spino-scapular group

M. levator scapulae et M. serratus ventralisOrigin: Arises by slips from the ribs and

cervical vertebrae, converging to a singlesheet. An origin from the atlas is absent inmost genera. In Heliosciurus rufobrachium(three of four specimens), there is a sliporiginating from the ventral surface of theatlas which passes superficial to the scale-nus (Fig. 4).

Insertion: Inserts on the vertebral edge ofthe scapula and the adjacent costal surface.The insertion extends to the superficial sur-face of the scapula between rhomboid andteres major in Sciurus, Callosciurus, Heli-osciurus, Paraxerus, and Funisciurus.

Fig. 2. Ventral surface of the clavicle of ten squirrel genera showing the origin of subclaviusand the insertions of cleidomastoid, cleido-occipitalis, clavodeltoid, and, in Spermophilus,clavobrachialis. Scale bar 5 5 mm.


Remarks: Contrary to our observations,Bryant (’45) reported an origin from atlas asthe common condition for squirrels. Parsons(1894) reported the atlantic origin presentin Callosciurus, Pteromys, and Marmota, butabsent in Xerus. It is present in a widevariety of other rodents as well (Parsons,1894; Ryan, ’89). The occurrence of an atlan-tic slip of levator scapulae, in addition to theatlantoscapularis dorsalis, was cited byWoods (’72) as a reason for doubting that thelatter muscle was derived from the former(see remarks for atlantoscapularis dorsalis).

M. rhomboideusOrigin (Figs. 1, 5): Arises from the supe-

rior nuchal line, the cervical vertebrae, andthe first few thoracic vertebrae. Rhomboidoriginates on the nuchal line in Atlantoxerus,Xerus, Paraxerus, Protoxerus, and Heliosciu-rus. In the other genera, there is no cranialorigin. In Xerus, the cranial origin is sepa-rate from the vertebral origin and forms adistinct rhomboideus capitis. In Funambu-lus and Heliosciurus gambianus, the verte-bral origin is divided into anterior and poste-rior portions.

Insertion (Figs. 3, 5): Inserts on the verte-bral edge of the scapula from the scapularspine nearly to the caudal angle. Rhomboidinserts on the costal side of the scapula in allgenera except for Sciurus. In Funambulusand Heliosciurus gambianus, the posteriorportion inserts superficial to the anteriorportion and the fibers have a medial-lateralorientation. The fibers of the anterior por-tion have a more cranial-caudal orientation.The insertion does not extend to the caudalangle of the scapula in Protoxerus, Ratufa,and Spermophilus variegatus.

Remarks: The part of the rhomboid origi-nating on the nuchal line, as described above,should not be confused with the occipito-scapularis (Fig. 5). The rhomboid and theoccipitoscapularis seem to us always to bedistinguishable in squirrels, contrary to thereport of Bryant (’45). As reported by Par-sons (1894), and noted by us, they are leastseparable in Spermophilus, Bryant’s princi-pal subject. They may not be separable atthe origin, but it is possible to allocate fibersto their respective muscles on the basis oftheir separation more distally. Woods (’72)describes them as forming a single sheet inmost hystricomorphs. The two muscles areobviously closely related. A distinct posteriorportion of the rhomboid, seen by us in Fu-nambulus and Heliosciurus, was not noted

by Parsons (1894) or Bryant (’45). It is foundin some other rodents, e.g., cricetines(Rinker, ’54) and arvicolids (Stein, ’86).

M. occipitoscapularisOrigin (Figs. 1, 5): Arises from the supe-

rior nuchal line of the skull deep to thecleido-occipitalis and medial to the insertionof cleidomastoid. In the pygmy squirrels andin the flying squirrels, the origin is short andis restricted to the lateral half or less of thenuchal line. The origin extends medially tothe cranial origin of the rhomboid in Atlan-toxerus, Xerus, Protoxerus, Paraxerus, andHeliosciurus, and almost to the midline inthe other genera. The origin is most exten-sive in Funisciurus and Spermophilus.

Insertion (Figs. 3, 5): Inserts on the dorsalend of the scapular spine and on the verte-bral border of the scapula anterior to thespine. It also inserts on the costal surface ofthe scapula in the flying squirrels, and in thetree squirrels Callosciurus, Protoxerus, Fu-nisciurus, and Paraxerus ochraceus. In Eo-glaucomys, it also inserts on the surface ofthe rhomboid. In Belomys, Petaurista, andPteromys, the insertion is restricted to thecranial portion of the vertebral border of thescapula. In Protoxerus, occipitoscapularis in-serts on the surface of the supraspinatus. InSpermophilus variegatus, Ratufa, Paraxerus,Funisciurus, and Heliosciurus, the inser-tions of occipitoscapularis and atlantoscapu-laris dorsalis are inseparable.

Remarks: The variation of this muscle insquirrels was not well documented hereto-fore. A similar amount of variation occurs inother rodents (Hill, ’37; Rinker, ’54; Klinge-ner, ’64). The close association of occipito-scapularis and atlantoscapularis dorsalis attheir insertion was noted by Woods (’72). Inaddition, he reported that the origins arecontiguous in Thryonomys and argued thatatlantoscapularis dorsalis was derived fromthe occipitoscapularis.

M. atlantoscapularis ventralis(omocervicalis)

Origin (Fig. 4): Arises from the ventralarch of the atlas, medial to the attachmentof scalenus.

Insertion (Fig. 3): Inserts on the metacro-mion of the scapula adjacent or partiallydeep to the trapezius muscle. The trapeziussignificantly overlaps the atlantoscapularisventralis in Petaurista, Protoxerus, andRatufa. In the other genera, the muscles


may be adjacent or trapezius may slightlyoverlap the insertion of atlantoscapularisventralis. Only in one specimen of Sper-mophilus richardsonii did some fibers of tra-pezius lie deep to fibers of atlantoscapularisventralis. The separability of the two musclesat their insertion is variable.

Remarks: Among squirrels, this muscleexhibits very little variation in origin or

insertion. Hill (’37) reports its origin on thebasioccipital in Spermophilus, but that isnot reported for Spermophilus in other stud-ies (Brizzee, ’41; Bryant, ’45). The primitivemorphology of the origin for rodents is sub-ject to debate. Parsons (1894) suggests thatthe primitive origin is on the basioccipital,as seen in diverse rodents, and that theorigin from the atlas is derived. In many

Fig. 3. Costal and dorsal surfaces of the scapula ofsix squirrel genera showing the origins of teres major,teres minor, and the long head of triceps, and the inser-

tions of occipitoscapularis, rhomboid, atlantoscapularisventralis, atlantoscapularis dorsalis, and trapezius. Scalebar 5 5 mm.


other mammals, e.g., Didelphis (Stein, ’81),and primates (Howell and Straus, ’33), theorigin is on the transverse process of theatlas. If we use these as out-groups, thebasioccipital origin in some rodents wouldappear to be the most derived, not the primi-tive condition as suggested by Parsons(1894).

Hill (’37) suggests that the primitive ori-gin is the transverse process of the atlas,

and that this primitive condition is retainedin Heteromys and Neotoma. However, theorigin is on the ventral arch, not the trans-verse process of the atlas, in Heteromys(Ryan, ’89) and Neotoma (Howell, ’26; Rinker,’54), thus destroying Hill’s (’37) hypothesis.Woods (’72) argues that the origin from theventral arch of the atlas is the primitivemorphology for rodents, as seen in hystrico-morphs and sciurids.

Fig. 3. (Continued.)


This muscle inserts on the metacromion ofall squirrels, most muroids, and most hystri-comorphs. In the dipodoid rodents, it insertsmostly on the acromion (Klingener, ’64). Inthe geomyids and Ctenomys, the burrowinghystricomorph, the insertion is on the clavicle(Hill, ’37). There is a similar disagreementabout which insertion is primitive for ro-dents. Hill (’37) argues that it is the acro-mion and Woods (’72) argues that it is themetacromion.

M. atlantoscapularis dorsalisOrigin (Fig. 4): Arises from the ventral

arch of the atlas, lateral to atlantoscapularisventralis. Atlantoscapularis dorsalis is ab-sent in Xerus, Atlantoxerus, and Myosciu-rus, and is very thin and narrow in Nanno-sciurus.

Insertion (Fig. 3): Inserts on the scapularspine adjacent to the insertion of occipito-scapularis. The insertion is overlapped par-tially by the insertion of occipitoscapularisin Glaucomys, Funambulus, and Spermophi-lus columbianus. In some flying squirrels(Belomys, Eoglaucomys, Petaurista, andPteromys), where the insertion of occipito-scapularis is restricted to the vertebral bor-der of the scapula, the insertion of atlanto-scapularis dorsalis extends from the scapularspine onto the vertebral border. The inser-tions of atlantoscapularis dorsalis and occipi-toscapularis are inseparable in Spermophi-

Fig. 5. Dorsal view of occipitoscapularis and rhom-boid musculature of Funambulus, showing the two partsof rhomboid with rhomboideus posterior overlappingrhomboideus anterior.

Fig. 4. Ventral surface of the atlas of Sciurus carolin-ensis and Heliosciurus rufobrachium illustrating theorigin of an atlantic slip of serratus ventralis in Heliosciu-

rus. In S. carolinensis, as in most squirrel genera, serra-tus ventralis does not have an origin on the atlas. Scalebar 5 5 mm.


lus variegatus, Ratufa, Paraxerus, Funisciurus,and Heliosciurus.

Remarks: This muscle is reported absentin Aplodontia (Hill, ’37; Lewis, ’49), foursubgenera of Spermophilus (Bryant, ’45), thedipodoids (Klingener, ’64), the muroids(Rinker, ’54), and four genera of hystrico-morphs (Woods, ’72). Bryant reported themuscle absent in the subgenus Spermophi-lus based on his examination of S. beldingi.We found it present in two other species ofthis subgenus, S. columbianus and S. rich-ardsonii. There appears to be a tendency forit to be lost in burrowing rodents as well asin pygmy squirrels.

There are two hypotheses about the deri-vation of this muscle. Howell (’37) suggestedthat it was derived from the serratus sheet,because of its origin on the atlas. Woods (’72)argued that it was derived from the rhom-boid sheet via the occipitoscapularis becausethe two are closely associated at their inser-tions and because in Thryonomys their ori-gins are also adjacent. He submitted thatHowell’s hypothesis was falsified by the co-occurrence of atlantic slips of both the serra-tus sheet and the atlantoscapularis dorsalisin the same animal, such as we observed inHeliosciurus. However, this is not fatal toHowell’s argument if one accepts the possibil-ity of a doubling of the atlantic slip of theserratus sheet.Atlantoscapularis dorsalis oc-curs in marsupials (Stein, ’81), primates(Howell and Straus, ’33), insectivores (Camp-bell, ’39; Reed, ’51), and other orders of mam-mals. Therefore, the condition in Thryono-mys is probably derived and irrelevant tothe origin of this muscle in the Mammalia.

Latissimus-subscapular groupM. latissimus dorsi

Origin: This somewhat variable musclecommonly has a fleshy origin from the spi-nous process of the posterior six thoracicvertebrae, a fascial origin from the first oneor two lumbar vertebrae, and again a fleshyorigin from the last four ribs.

Insertion (Fig. 6): Inserts with teres majoron the medial surface of the humerus, me-dial to the deltoid ridge. In Eoglaucomys andFunambulus, latissimus dorsi has an achsel-bogen that inserts deep to pectoralis majoron the deltoid ridge.

Remarks: Bryant (’45) stated that the in-sertions of latissimus dorsi and teres majorwere separate in Glaucomys. We did not findthis in Glaucomys or any other squirrels. Inother rodents, the two muscles insert sepa-

rately in Neotoma and Peromyscus (Rinker,’54), and Phodopus (Stein, ’90). An achselbo-gen is less common in squirrels than in hys-tricomorphs, in which it has been carefullydescribed and discussed (Woods, ’72).

M. teres majorOrigin (Fig. 3): Arises from the axillary

edge of the scapula, ventral to the caudalangle, and sometimes from the caudal sur-face of the axillary border and the adjoiningsurface of subscapularis. Its origin extendsone- to two-thirds of the length of the axil-lary edge. In tree squirrels, there is a dis-tinct teres fossa on the axillary border ofscapula for the origin of this muscle. InCallosciurus and Funambulus, teres majororiginates partially from infraspinatus aswell.

Insertion (Fig. 6): Inserts with latissimusdorsi on the medial surface of humerus, me-dial to the deltoid ridge.

Remarks: In the Sciuridae, a prominentteres major fossa is characteristic of treesquirrels and is absent from the marmotineand xerine ground squirrels. In contrast,Lehmann (’63) noted a prominent teres ma-jor fossa in the burrowing rodents Geomysand Ctenomys which was lacking in the non-fossorial rodents she studied.

M. subscapularisOrigin: Arises from the subscapular fossa,

except for its vertebral edge.Insertion: Inserts on the lesser tuberosity

of the humerus.Remarks: The internal structure and sub-

division of this muscle (Lehmann, ’63) werenot studied.

Deltoid groupM. deltoideus

Divisible into three parts: the clavodeltoid,the acromiodeltoid, and the spinodeltoid.

Origin (Fig. 2.): Clavodeltoid takes originfrom the caudoventral surface of the clavicleover the lateral two-thirds of its length. Itextends further medially in Heliosciurus,Eoglaucomys, and Glaucomys. It is alwaysseparable from acromiodeltoid, althoughtheir origins closely approximate one an-other in most genera. The origins are mostseparated in Atlantoxerus and Glaucomys.In Spermophilus, the most superficial fibersform a separate M. clavobrachialis, whicharises from the clavicle, superficial to theclavodeltoid.


The acromiodeltoid arises from the acro-mion, lateral to the insertion of atlantoscapu-laris ventralis.

Spinodeltoid takes origin from the ventralhalf of the scapular spine to the vicinity ofthe metacromion. In Atlantoxerus, Xerus,Callosciurus, Funambulus, Myosciurus, Eo-glaucomys, and Glaucomys, it also origi-nates slightly from the surface of the infra-spinatus. In Myosciurus and Nannosciurus,

the origin extends nearly to the vertebralborder of the scapula.

Insertion (Figs. 6, 7): Clavodeltoid insertsalong the distal three-fourths of the medialside of the deltoid ridge of the humerus. InPetaurista, Atlantoxerus, Myosciurus,Ratufa, and Funambulus, the insertion ofclavodeltoid extends to the proximal end ofthe humerus. In all genera except Atlan-toxerus, spinodeltoid fuses with acromiodel-

Fig. 6. Medial surface of the humerus for eight squirrel genera, showing insertions of sevenmuscles. Scale bar 5 5 mm.


toid, which then inserts on the lateral side ofthe deltoid ridge. In Atlantoxerus, the spi-nodeltoid inserts tendinously, separate fromthe acromiodeltoid. In Myosciurus, Pe-taurista, and Funambulus, this insertion ex-tends to the top of the humerus. In mostgenera, the insertion of acromiodeltoid isrelatively thin, but in Atlantoxerus, Pe-taurista, Eoglaucomys, Glaucomys, andRatufa, the insertion is thick. In Spermophi-lus, the clavobrachialis inserts with brachia-lis on the proximal end of the ulna.

Remarks: In squirrels, a clavobrachialis,innervated by the axillary nerve, occurs inSpermophilus, Cynomys, and Marmota, butis absent in other members of this tribe:Ammospermophilus, Tamias, and Sciurot-amias (Parsons, 1894; Brizzee, ’41; Bryant,’45). In the hystricomorphs Cavia and Dasy-procta, the clavodeltoid inserts distally onthe humerus, but this is very different fromthe clavobrachialis of Spermophilus, whichcrosses the elbow joint and inserts on the ulna.

M. teres minorOrigin (Fig. 3): Arises from the axillary

surface of the scapula adjacent to the gle-noid process, superficial to the origin of thelong head of triceps. In most genera, the

origin of the long head of triceps extendsfurther from the glenoid than does the originof the teres minor. The opposite is true inSpermophilus columbianus, in the flyingsquirrels Glaucomys and Belomys, and espe-cially in Petaurista.

Insertion: Inserts on the distal surface ofgreater tuberosity of the humerus, distal tothe insertion of the infraspinatus muscle. InRatufa, and to a greater degree in Petaurista,the insertion is elongated, rather than circu-lar.

Remarks: In many squirrels this muscle isminuscule and therefore is not easily differ-entiated from the infraspinatus.

Suprascapular groupM. infraspinatus

Origin: Arises from the infraspinous fossaof the scapula.

Insertion: Inserts on the greater tuberos-ity of the humerus, between the insertions ofsupraspinatus and teres minor.

Remarks: Differences in the relative sizesof the infraspinatus and supraspinatusmuscles and their fossae vary among gen-era, but this variation is better studied in aseries of scapulae than in dissections (e.g.,Swidersky, ’93).

Fig. 7. Deltoid musculature of Atlantoxerus and Spermophilus, showing the special clavobra-chialis, found in the marmotine ground squirrels Spermophilus, Cynomys, and Marmota, butnot in the African xerine ground squirrels Atlantoxerus and Xerus. Scale bar 5 5 mm.


M. supraspinatusOrigin: Arises from the supraspinous fossa

of the scapula.Insertion: Inserts on the greater tuberos-

ity of the humerus, proximal to the insertionof infraspinatus.

Triceps groupM. triceps brachii

Origin (Fig. 3): The long head arises fromthe axillary border of the scapula adjacent tothe glenoid fossa, deep to the origin of teresminor. In Funambulus, the origin of the longhead has shifted away from the glenoid.

In most squirrels, the lateral head arisesaponeurotically distal to the insertion of teresminor, along the lateral surface of the del-toid ridge, adjacent to the insertion of del-toid. It extends distally to the crest of thedeltoid ridge in Myosciurus, Protoxerus, He-liosciurus, and Funambulus, while in Funis-ciurus and Ratufa, the origin barely extendsto the deltoid ridge. The condition in theother genera is intermediate. In some flyingsquirrels (Belomys, Eoglaucomys, and Ptero-mys), the lateral head has a fleshy origindistal to the insertion of teres minor. Thiscontrasts with the strictly tendinous originin Glaucomys and Hylopetes. In Petaurista,the fleshy origin covers a larger area on thelateral surface of the humerus and extendsonto the proximal third of the posterior sur-face of the humerus.

In most squirrels, the medial head of tri-ceps arises extensively from the posteriorsurface of the humerus, from a point be-tween the head and the lesser tuberosity to apoint only slightly proximal to the medialepicondyle. In Petaurista, the origin of themedial head is restricted to the distal two-thirds of the humerus. In Funambulus, themedial head is divided into superficial anddeep parts by the radial nerve.

Insertion (Figs. 8, 9): The long head andthe medial head insert on the distal tip of theolecranon. The lateral head inserts on theolecranon with the long head, and along thelateral surface of the ulna to a point slightlydistal to the semilunar notch. In Petaurista,the insertion of the lateral head is restrictedto the distal end of the olecranon process.

Remarks: The separation of the medialhead into superficial and deep parts is rarein squirrels, but is seen in nine genera ofhystricomorph rodents (Woods, ’72). A tendi-

nous or aponeurotic origin of the lateral head,common in squirrels, is found in most otherrodents, also.

M. anconeusOrigin: Arises from the posterior surface

of the lateral epicondylar ridge of the hu-merus.

Insertion (Fig. 9): Inserts on the lateralsurface of the olecranon process of the ulnaand extends distal to the semilunar notch.

Remarks: The most distal fibers pass onthe flexor side of the elbow joint when thearm is flexed. Only when the elbow is partlyextended could they act to further extendthe arm.

M. dorsoepitrochlearisOrigin: Arises from both teres major and

latissimus dorsi near their common inser-tion on the humerus.

Insertion (Fig. 10): Inserts on the medialside of the proximal end of the ulna. InAtlantoxerus and Xerus, the dorsoepitrochle-aris inserts on the dense connective tissueon the medial side of the elbow, overlyingflexor carpi ulnaris. In Protoxerus, there isan additional insertion on the superficialsurface of anconeus.

Remarks: The usual origin of this musclein squirrels is from both teres major andlatissimus dorsi. Bryant (’45) reports an ori-gin, in some species of Spermophilus, fromthe teres major and the scapula, but notfrom latissimus dorsi. In contrast, the muscletakes origin almost exclusively from the latis-simus dorsi in hystricomorphs (Woods, ’72).Howell (’37) and Cheng (’55) consider thismuscle to be derived from the latissimusdorsi, in spite of its innervation by the radialnerve. Therefore, Woods (’72) suggests thatthe origin from the latissimus dorsi is primi-tive. Origin from the teres major is com-monly found in other rodents as well assquirrels.

Extensor group of the forearmM. brachioradialis

Origin: Arises from the anterior surface ofthe lateral epicondylar ridge of the hu-merus, proximal to extensor carpi radialislongus.

Insertion (Fig. 9): Inserts tendinously onthe distal end of the radius. In Petaurista,the tendinous insertion is elongated over thedistal one-third of the radius. In Ratufa in-


dica, but not in other species of Ratufa, theinsertion is fleshy, rather than tendinous,and extends over the distal one-fourth of theradius.

Remarks: Although universally present insquirrels, this muscle is commonly lost inother rodents. It is absent in Aplodontia(Hill, ’37; Lewis, ’49), all hystricomorphs ex-cept Erethizon and Coendou (Woods, ’72;

McEvoy, ’82), Castor (Parsons, 1894), geomy-ids (Hill, ’37), and some muroids (Rinker, ’54).

M. extensor carpi radialis longus

Origin (Fig. 9): Arises from the anteriorsurface of the lateral epicondylar ridge ofthe humerus, between brachioradialis andextensor carpi radialis brevis.

Fig. 8. Lateral view of the humerus for six genera of squirrels, demonstrating the variationin the origin of the lateral head of triceps. Scale bar 5 5 mm.


Insertion: Inserts on the proximal end ofthe second metacarpal.

Remarks: In sciurids, this is always largerthan extensor carpi radialis brevis. In thehystricomorphs Myocastor and Thryonomys,geomyids, and others, the reverse is true(Hill, ’37; Woods, ’72).

M. extensor carpi radialis brevisOrigin (Fig. 9): Arises from the anterior

surface of the lateral epicondylar ridge ofthe humerus, distal to extensor carpi radia-lis longus.

Insertion: Inserts on the proximal end ofthe third metacarpal.

M. supinatorOrigin (Fig. 9): Arises from the anterior

portion of the lateral epicondyle of the hu-merus.

Insertion (Fig. 9): Inserts on the lateralsurface of the proximal half of the radius. Atits proximal end, the insertion also wrapsaround to the medial surface of the radius.In Callosciurus and Spermophilus, supina-tor inserts on more than half the length ofthe radial shaft.

Remarks: In squirrels, a sesamoid bonedoes not occur in the tendon of origin of thesupinator. Such a sesamoid is found in otherrodents: geomyids (Hill, ’37), cricetines

Fig. 9. Extensor surface of the forearm (radius, ulna, and distal end of humerus) of tengenera of squirrels, showing the origins of seven muscles and the insertions of brachioradialis,supinator, anconeus, and triceps. Scale bar 5 5 mm.


(Rinker, ’54), and microtines (Stein, ’86), butnot in dipodoids (Stein, ’90).

M. abductor pollicis longusOrigin (Fig. 9): Arises linearly from the

lateral surface of the ulna, from the lateralsurface of the radius, and from the interosse-ous membrane. The ulnar origin begins nearthe elbow joint and continues distally two-thirds of the way down the shaft. The radialorigin begins at the radial head and extendsdistally halfway down the shaft. In all gen-era except for Myosciurus, Petaurista, Glau-comys, and Atlantoxerus, the ulnar origin ismuch longer than the radial origin.

Insertion: Inserts via a bifurcated tendonon the radial side of the first metacarpal andon the falciform. In Ratufa and Funambu-lus, there is only one broad tendon, whichinserts across the two bones.

Remarks: In sciurids, the tendon variesfrom a single broad insertion on both thefirst metacarpal and the falciform to a dis-tinctly bifurcated tendon. Bryant (’45) re-ported the absence of the insertion on thefalciform in Sciurus, Tamiasciurus, andGlaucomys. We observed a falciform inser-tion in all specimens we dissected. In flyingsquirrels, the falciform is connected by aligament to the styliform cartilage by meansof which the abductor has the importantfunction of extending the wingtip (Thoring-ton et al., in press). Therefore, reports of theabsence of a falciform insertion in flyingsquirrels should be viewed as dubious.

In seven genera of hystricomorphs, themuscle is described as having two distincttendons. In dipodoids and some muroids,Rinker (’54) and Klingener (’64) consideredit to be two separate muscles, using the

Fig. 9. (Continued.)


name M. extensor pollicis brevis for the partthat inserted on the falciform. Woods (’72)argued that it was not the extensor, butrather a subdivided abductor. The differencebetween the two hypotheses is moot, be-cause the extensor pollicis brevis is a de-rived feature in primates, found in Homoand Gorilla (Howell and Straus, ’33). There-fore, the subdivision of the abductor pollicisin rodents, no matter what it is called, isindependent of the evolution of the extensorpollicis brevis in primates.

M. extensor digitorum communisOrigin: Arises from the proximal surface

of the lateral epicondyle of the humerus.Insertion: Inserts on digits 2–5. In Xerus,

Atlantoxerus, Ratufa, Sciurus, Callosciurus,Funisciurus, and Spermophilus, there are

two tendons to digit 3. In Eoglaucomys, thetendon to digit 2 arises from a separate headof the muscle.

M. extensor indicisOrigin (Fig. 9): Arises from the lateral

surface of the ulna toward the distal end ofthe bone. The origin is distal to the origin ofabductor pollicis longus in all genera exceptAtlantoxerus, Sciurus, and Spermophilus co-lumbianus.

Insertion: Inserts on digit 2. In Ratufa,Protoxerus, Heliosciurus, Callosciurus,Paraxerus, Funisciurus, and Spermophiluscolumbianus, there was an additional inser-tion on digit 1.

Remarks: The insertion on the pollex wasnot observed by Hill (’37) or Bryant (’45) innearctic sciurids, but it was present in speci-

Fig. 10. Flexor surface of the forearm (radius, ulna,and distal end of humerus) of ten genera of squirrels,showing the origins of epitrochleo-anconeus, flexor carpiulnaris, flexor digitorum profundus, and pronator quadra-

tus, and the insertions of epitrocleo-anconeus, dorsoepitro-clearis, brachialis, biceps, pronator teres, and pronatorquadratus. Scale bar 5 5 mm.


mens of Sciurus carolinensis dissected byWoods (’72). Our dissections demonstratethat it is common in sciurids as a bifurcationof the tendon to the index finger. Woods (’72)found it in ten genera of hystricomorphs,and in one of these, Mesomys, there was acompletely separate tendon to the pollex.Hill (’37) suggested that the extensor pollicisbrevis has been incorporated into the exten-sor indicis, but this interpretation assumesthat the extensor pollicis brevis of humans isprimitive for rodents, which it surely is not.

M. extensor digiti quinti propriusOrigin: Arises from the lateral epicondyle

of the humerus and is inseparable from theorigin of the extensor digitorum communis,except in Spermophilus columbianus.

Insertion: Inserts on digits 3–5. In Xerus,Atlantoxerus, Ratufa, Sciurus, Callosciurus,Heliosciurus, and Spermophilus, the inser-tion is restricted to digits 4 and 5. In Ratufaand Eoglaucomys, the muscle belly that in-

serts on digits 3 and 4 is completely separatefrom the belly that inserts on digit 5.

Remarks: The frequent insertion of thismuscle into digit 3 suggests that the exten-sor digiti tertii has been incorporated intothe extensor digiti quinti, as suggested byKlingener (’64).

M. extensor carpi ulnarisOrigin: Arises from the distal end of the

lateral epicondyle of the humerus.Insertion: Inserts on the proximal end of

the fifth metacarpal.

Flexor systemPectoral group

M. pectoralis major (ectopectoralis of Hill, ’37)Origin: Arises from the sternebrae, from

the manubrium to the xiphisternum in allgenera.

Insertion (Fig. 6): Inserts on the deltoidridge of the humerus, deep to the insertionof clavodeltoid, usually along the distal three-

Fig. 10. (Continued)


fourths of the ridge. In Petaurista and Fu-nambulus, the insertion of pectoralis majorextends to the proximal end of the humerus.

Remarks: The origin of a few fibers fromthe clavicle was reported in Tamias andGlaucomys (Bryant, ’45) and in Aplodontia(Lewis, ’49).

M. pectoralis minorOrigin: Takes origin deep to the pectoralis

major, commonly from sternebrae 2–5 andthe sternal ends of costal cartilages 2–4, andsometimes 5. There is some variation in theextent of the origin. It has shifted slightlycaudally in Heliosciurus, Protoxerus, andSpermophilus, in which it did not take originfrom sternebra 2 or costal cartilage 2. InAtlantoxerus, the origin extended from ster-nebra 2 to the xiphisternum.

Insertion (Fig. 6): Inserts on the deltoidridge of the humerus, deep to the insertionof pectoralis major. It extends to the distalend of the pectoralis major insertion in oneflying squirrel (Petaurista), but not the oth-ers; in three tree squirrels (Ratufa,Paraxerus, and Callosciurus), but not theother tree squirrels; and in one ground squir-rel (Atlantoxerus), but not the others (Xerusand Spermophilus). The proximal end of theinsertion extends to the proximal end of thehumerus, distal to the insertion of supraspi-natus, in all genera except Protoxerus. Theinsertion continues onto the coracoid pro-cess of the scapula in the large flying squir-rels, Petaurista and Eoglaucomys. In Myo-sciurus and Glaucomys, the insertion isrestricted to the proximal end of the hu-merus, near the insertion of supraspinatus.The insertion is short and is proximal to andaligned with the insertion of pectoralis ma-jor in Xerus. In Callosciurus, pectoralis mi-nor splits into two slips, the dorsal slip insert-ing just distal to supraspinatus insertionand the ventral slip inserting further dis-tally on the deltoid ridge.

Remarks: In other rodents, insertion onthe coracoid process is common (Parsons,1896; Rinker, ’54). An insertion onto theclavicle, as seen in many hystricomorphs(Woods, ’72), was not found in any sciurids.

M. pectoralis abdominalisOrigin: Arises from abdominal facia, cau-

dal to the xiphisternum. It is most extensivein Funambulus. It has a very narrow originin Petaurista.

Insertion (Fig. 6): Inserts on the proximalend of the medial side of the deltoid ridge.

The proximal end of the insertion is mostextensive in Funambulus, Petaurista, andEoglaucomys, where both insert on the proxi-mal end of the humerus; in Petaurista andEoglaucomys, the insertion continues ontothe coracoid process of the scapula. The inser-tion does not extend onto the deltoid ridge inCallosciurus, Paraxerus, Myosciurus, andFunisciurus. The insertion of pectoralis ab-dominalis extends onto the deltoid ridge inSciurus, Protoxerus, and Heliosciurus, andis most extensive in Atlantoxerus and Fu-nambulus. In Glaucomys, the insertion isrestricted to the proximal end of the hu-merus, just distal to the insertion of supra-spinatus.

Remarks: We found the insertion to ex-tend further distally on the humerus in somegenera than was reported by Bryant (’45) fornearctic sciurids. If the insertion on theshoulder capsule is the primitive conditionfor rodents (Woods, ’72), then the more distalinsertion in Atlantoxerus and Funambulusis derived.

M. entopectoralis profundusOrigin: Arises as a thin membranous sheet

from 2 to 4 costal cartilages (between carti-lages 2 and 5) and adjoining sternebrae.

Insertion: Inserts on the first rib, lateralto the origin of the subclavius.

M. cutaneus maximusOrigin: Arises from the skin over the tho-

racic and lumbar regions.Insertion: Inserts on the shoulder joint

(anterior fibers) and further down on thehumerus deep to pectoralis major, usuallywith pectoralis minor, pectoralis abdomina-lis, and latissimus dorsi. In Myosciurus, itinserts independently on the humeral head.In Petaurista, cutaneus maximus inserts in-dependently as well, but its insertion ex-tends onto the coracoid of the scapula. InEoglaucomys, cutaneus maximus insertsonly on the coracoid process.

Remarks: The derived condition of thismuscle in the gliding membrane of flyingsquirrels was not examined. See Johnson-Murray (’77).

M. subclaviusOrigin: Arises from the proximal end of

the first costal cartilage.Insertion (Figs. 2, 11): Inserts on the dor-

sal surface of the distal half of the clavicle.The insertion of subclavius extends to the


distal end of clavicle in most genera. It doesnot extend quite to the distal end in Sciurus,Heliosciurus, Funisciurus, Myosciurus, Sper-mophilus, and Atlantoxerus. In Ratufa, theinsertion extends onto the surface of thesupraspinatus muscle of the scapula.

Remarks: In hystricomorphs (Woods, ’72),bathyergids (Parsons, 1896), and other mam-mals, there is a scapuloclavicularis musclewhich originates on the clavicle and insertson the scapular spine or vertebral border.This is superficially similar to the scapularinsertion of subclavius in Ratufa, but theorigins and insertions are different. Thus,the morphology in Ratufa is probably inde-pendently derived from the scapuloclavicula-ris of other rodents.

Flexor group of the armM. coracobrachialis

Origin: Arises from the coracoid process.Insertion (Fig. 6): Short head inserts on

the medial surface of the humerus slightlydistal to the greater tuberosity. The linearinsertion is perpendicular to the insertion ofteres major. Atlantoxerus, Glaucomys, andRatufa have a more dot-like insertion. InPetaurista, the short head has two points ofinsertion, adjacent to each other. The longhead inserts on the distal half of the medialsurface of the humerus and on the medialepicondylar ridge. The insertion of coracobra-

chialis begins near the peak of deltoid ridgein Sciurus, Protoxerus, Heliosciurus, Atlan-toxerus, Ratufa, Petaurista, Eoglaucomys,Glaucomys, Spermophilus columbianus, andParaxerus. In Myosciurus, Callosciurus, Xe-rus, Spermophilus variegatus, Funambulus,and Funisciurus, the insertion begins distalto the deltoid ridge.

Remarks: The short head has been foundin all genera of squirrels studied, in Aplodon-tia and in the geomyids (Hill, ’37), heteromy-ids (Howell, ’32), dipodoids (Klingener, ’64),and some muroids (Rinker, ’54). The distalinsertion in sciurids, Aplodontia (Hill, ’37;Lewis, ’49), and most myomorphs (Parsons,1896) reputedly includes the middle and longheads described by Wood (1867), although itis not clear how to distinguish this conditionfrom an extended middle head.

M. biceps brachiiOrigin: The long head originates from the

supraglenoid tuberosity. The short headoriginates from the coracoid process.

Insertion (Fig. 10): Inserts on the bicipitaltuberosity on the radius, distal to or slightlyoverlapping the insertion of brachialis onthe ulna. The short head merges into andinserts with the long head.

Remarks: In sciurids, the short head ofbiceps is usually present, but has been re-ported absent in Sciurus vulgaris (Hoff-

Fig. 11. Cranial view of clavicle and scapula, showing the insertion of subclavius inCallosciurus, as it is in most squirrels, and the insertion extending onto the supraspinatusmuscle of the scapula in Ratufa. Scale bar 5 5 mm.


mann and Weyenbergh, 1870) and in Sper-mophilus variegatus and S. beecheyi (Bryant,’45). The biceps always inserts on the radius(Parsons, 1894; Alezais, 1900; Hill, ’37;Brizzee, ’41; Bryant, ’45). In other rodents,the short head of biceps is absent in somehystricognaths and Castor (Parsons, 1894),Zapus and Jaculus (Klingener, ’64), and Alac-taga (Howell, ’32). Insertion on both radiusand ulna, or just on ulna, is common (Par-sons, 1894; Alezais, 1900; Hill, ’37; Rinker,’54; Klingener, ’64; Ryan, ’89).

M. brachialisOrigin: Arises extensively as a single head

from the lateral surface of the humerus dis-tal to the origin of the lateral head of triceps.Distal to the deltoid ridge, the origin extendsmore medially, adjoining the insertion of thecoracobrachialis.

Insertion (Fig. 8): Inserts on the ulna dis-tal to the coronoid process.

Remarks: The recognition of a small me-dial head of brachialis, seen in other rodents(Parsons, 1894; Hill, ’37; Rinker, ’54; Klinge-ner, ’64; Woods, ’72), does not seem war-ranted for squirrels. Parsons (1894) de-scribed the two heads in sciurids as ‘‘soclosely blended as to be indistinguishable,’’and Bryant (’45) remarked that brachialis‘‘has two incompletely separated heads.’’

Flexor group of the forearmM. epitrochleo-anconeus

Origin (Fig. 10): Arises from the extensorside of the medial epicondyle of the hu-merus. Petaurista has a more extensive ori-gin that includes the epicondylar ridge.

Insertion: Inserts on the medial surface ofthe olecranon process deep to the dorsoepi-trochlearis.

M. flexor carpi ulnarisOrigin (Fig. 10): Arises from the base of

the medial epicondyle of the humerus, ei-ther on its own or in common with the otherforearm flexors, and from the medial surfaceof the olecranon, deep to the insertion ofdorsoepitrochlearis.

Insertion: Inserts on the distal tip of thepisiform bone. In the flying squirrels, flexorcarpi ulnaris inserts both on the pisiformand on the styliform cartilage.

Remarks: In squirrels, the epicondylarhead is almost always present, but has beenreported absent in Spermophilus (Parsons,1894). Gupta (’66) and Johnson-Murray (’77)described two tendons of insertion in flyingsquirrels, one inserting on the styliform car-

tilage. This latter is the tendon of the pal-maris longus (Thorington et al., ’97a). Inflying squirrels, the flexor carpi ulnarisserves the additional function of retractingthe wing tip when the animals are not glid-ing (Thorington et al., ’97a). In other ro-dents, the epicondylar head is absent in geo-myids (Hill, ’37), Oryzomys (Rinker, ’54),dipodoids (Klingener, ’64), microtines (Stein,’86), and heteromyids (Ryan, ’89).

M. palmaris longusOrigin: Arises from the medial epicondyle

of the humerus with flexor digitorum subli-mis. It is completely separable from the flexorin Spermophilus columbianus, Callosciurus,Funisciurus, Paraxerus, and Sciurus.

Insertion: Inserts on the palmar facia ofthe hand. In flying squirrels, it also insertson the styliform cartilage. In Petaurista, abroad tendon separates into two parts beforeinserting on the palmar fascia.

Remarks: In other mammals, this muscleis usually innervated by the median nerve.In sciurids and hystricomorphs, it is inner-vated by the ulnar nerve (Woods, ’72).

M. flexor carpi radialisOrigin: Arises from the medial epicondyle

of the humerus, distal to the origin of prona-tor teres, either separately or with the otherflexor muscles. It is completely separate inSpermophilus columbianus, Heliosciurus,Paraxerus, Callosciurus, and Funisciurus.

Insertion: Inserts on the base of the sec-ond metacarpal.

M. pronator teresOrigin: Arises from the proximal portion

of medial epicondyle of the humerus, sepa-rate from the other forearm flexors in allgenera except Eoglaucomys.

Insertion (Fig. 10): Inserts on the medialsurface of the radius, over approximatelythe middle third of the bone. Pronator teresinserts more distally in Eoglaucomys, Cal-losciurus, and Xerus. The length of the inser-tion varies. Spermophilus, Ratufa, Funam-bulus, Sciurus, Protoxerus, Eoglaucomys,and Glaucomys have relatively long inser-tions, but the insertion is relatively short inthe other genera. In Glaucomys and Ratufa,the proximal portion of the insertion curvesmedially toward the insertion of biceps.

M. flexor digitorum sublimisOrigin: Arises from the medial epicondyle

of the humerus, between the condyloradialand central heads of flexor digitorum profun-


dus, either separately or in common with theother forearm flexors. It arises separately inSpermophilus columbianus, Callosciurus,Paraxerus, and Funisciurus.

Insertion: Inserts on digits 2–5. The ten-don to each digit is perforated by the corre-sponding tendon of the flexor digitorum pro-fundus. It divides over the distal end of themetacarpal and reunites deep to the profun-dus tendon over the first phalanx. Each in-serts as a single broad tendon on the palmarsurface of the base of the second phalanx.

Remarks: Contrary to the normal innerva-tion by the median nerve in other mammals,this muscle is commonly innervated by theulnar nerve in squirrels. We found only ul-nar innervation in Xerus, Protoxerus, Ratufa,and Petaurista, and Woods (’72) found it inSciurus and Eutamias. We found only me-dian innervation in Funisciurus and Glauco-mys and in two specimens of Callosciurus.We found both median and ulnar innerva-tion in one specimen of Callosciurus. Thusulnar innervation of the flexor digitorumsublimis has been observed in both subfami-lies and five of the seven tribes of the Sciuri-nae, and median innervation in flying squir-rels and two tribes of the Sciurinae. Theinsertion of this muscle is commonly re-stricted to digits 2–4 in other rodents (Par-sons, 1894; Rinker, ’54; Klingener, ’64; Stein,’86; Ryan, ’89).

M. flexor digitorum profundusOrigin: Arises by four heads. The condylo-

radial head arises from the posterior portionof the medial epicondyle of the humerus,anterior to the origin of epitrochleo-anco-neus. The central head arises from the ante-rior portion of the medial epicondyle, deep tothe other forearm flexors. These two headsarise separately in Spermophilus columbi-anus, Paraxerus, Funisciurus, and Callosciu-rus. Only the central head is separate inSciurus. In other genera, these arise in com-mon with the other forearm flexors. Theradial head arises from the medial surface ofthe radius, deep to the insertion of pronatorteres. The ulnar head arises from the medialsurface of the ulna, deep to the origin offlexor carpi ulnaris.

Insertion: All four heads converge to asingle tendon in the carpal tunnel and di-verge into four tendons at the level of themetacarpals. These tendons insert on theterminal phalanges of digits 2 through 5.The tendon to digit 2 commonly bifurcatesand sends a branch to digit 1. This tendon to

digit 1 was absent in Myosciurus, Sciurus,Callosciurus, and Petaurista.

Remarks: The innervation of this muscleis also variable. Innervation of the ulnarhead by the ulnar nerve, and the other headsby the median nerve, is probably most com-mon, as in other rodents (Hill, ’37), but weobserved innervation of the two condylarheads by both median and ulnar nerves intwo specimens of Glaucomys, and of the con-dyloradial head in Funisciurus.

The tendon to digit 1 was not found byParsons (1894) in Callosciurus or Pteromys,and by Alezais (1900) in Marmota. It wasfound in Sciurus by Alezais (1900). Bryant(’45) found it in all of his specimens, includ-ing Sciurus and Marmota.

M. pronator quadratusOrigin (Fig. 10): Arises from the medial

surface of the ulna near its distal end.Insertion (Fig. 10): Inserts on the medial

surface of the radius near its distal end. InCallosciurus, Spermophilus, Paraxerus,Sciurus, Atlantoxerus, and Xerus, the originis more extensive than the insertion.

Remarks: This muscle is absent in thepygmy squirrels Nannosciurus and Myosciu-rus and in the smaller flying squirrels Glau-comys, Pteromys, and Hylopetes. In the largerflying squirrels Petaurista, Eupetaurus, Eo-glaucomys, and Belomys, it is a narrow, thin,interosseus muscle, in contrast to the robustpronator quadratus of tree and ground squir-rels.

Miscellaneous musclesM. omohyoid

Origin: Arises from the ventral portion ofthe cranial border of the scapula, in thecranial notch.

Insertion: Inserts on the hyoid.

Lateral retractor of pouchWe found this muscle present in Spermophi-

lus and Sciurotamias.Origin: Arises from the metacromion of

the scapula, between the insertion of trape-zius and atlantoscapularis ventralis and theorigin of spinodeltoid.

Insertion: Inserts on the cheek pouch inSpermophilus and Sciurotamias. There is asimilar muscle taking origin on the scapulaand inserting on the cheek in Heliosciurus,Callosciurus, and Funambulus, althoughthese squirrels lack cheek pouches.

Remarks: Bryant (’45) found this musclein Spermophilus, Cynomys, Ammosper-mophilus, and Tamias, but not in Marmota.


The lateral retractor of the cheek pouch insquirrels is formed of facial musculature. Inhamsters, it is derived from the auricularslip of the trapezius muscle. In geomyidsand heteromyids, both facial musculatureand trapezius muscle contribute to the lat-eral retractor (Hill, ’35; Priddy and Brodie,’48; Klingener, ’64, ’70). The presence of anauricular portion of trapezius in Marmotaand Xerus suggests that another form ofcheek retractor could readily have evolvedin sciurids.

DISCUSSION

Our goal in this study was to locate de-rived myological features of phylogenetic orfunctional significance. Forelimb muscula-ture alone is an inadequate base for a phylo-genetic analysis, but it is clear that some ofthe myological differences we describe haveimportant phylogenetic implications. An ad-equate understanding of how muscles func-tion during locomotion requires a great dealof information not currently available forsquirrels. These include differences in musclemasses, distributions of slow and fast twitchmuscle fibers, differences in recruitment pat-terns, etc. (Jenkins and Weijs, ’79; Arm-strong, ’81; Fleagle, ’82; Stalheim-Smith, ’84;Larson, ’93). However, even if we had all ofthis information for squirrels, it would bedifficult to interpret it in view of the diverselocomotor behaviors of these animals, whichinclude walking, bounding on horizontal tovertical surfaces, digging, leaping, gliding,landing, and head-first descent of verticalsurfaces. Nonetheless, comparative anatomyprovides data for initial hypotheses aboutthe functional differences between animals,and it seems appropriate to speculate on thebiological significance of the data presented.

The 19 genera of squirrels we selected formorphological comparison are geographi-cally and ecologically diverse. The two gen-era of pygmy tree squirrels, Myosciurus ofAfrica and Nannosciurus of Southeast Asia,occupy the bark gleaner niche (Emmons,’80). Atlantoxerus and Xerus are genera ofAfrican ground squirrels. Like many of theNorth American ground squirrels, Sper-mophilus, they live in burrows, but also climbreadily (Herzig-Straschil, ’78; Herzig-Stra-schil and Herzig, ’89; O’Shea, ’91). The twotribes of African tree squirrels are repre-sented by the highly arboreal Protoxerusand Heliosciurus, and by the less arborealmembers of the genera Funisciurus andParaxerus (Kingdon, ’74; Emmons, ’80). Fu-nambulus, the Indian striped squirrels, are

traditionally, but probably incorrectly, allo-cated to the same tribe as the African squir-rels Funisciurus, Paraxerus, and Myosciu-rus. Like chipmunks, they are active both onthe ground and in trees. Ratufa, the gianttree squirrels of Southeast Asia, are highlyarboreal and seldom come to the ground(MacKinnon, ’78; Payne, ’80; Thorington andCifelli, ’90). Callosciurus, also of SoutheastAsia, are usually highly arboreal, but thespecies we studied, C. notatus, frequentlycomes to the ground like North Americantree squirrels of the genus Sciurus (MacKin-non, ’78; Payne, ’80). The six genera of flyingsquirrels we dissected include Petaurista,the giant flying squirrels of Southern Asia;Eoglaucomys, the large Himalayan flyingsquirrels; Belomys and Hylopetes, smallerflying squirrels of Southern Asia; Pteromys,the northern flying squirrels of Eurasia; andGlaucomys, the NorthAmerican flying squir-rels. In addition, we examined a few musclesin a forearm of the large wooly flying squir-rel, Eupetaurus cinereus, and one specimenof the Chinese rock squirrel, Sciurotamiasdavidianus.

Arboreality is presumed to be primitivefor the Sciuridae because the earliest fossilsquirrel, Douglassia jeffersoni, has so manyfeatures of an arboreal squirrel (Emry andThorington, ’82). Accordingly, it is likely thatthe forelimb anatomy of tree squirrels isprimitive for the family. The similarity ofmusculature among tree squirrels in fivetribes of the Sciurinae is in keeping withthis assumption. Only in the Ratufini is therea noteworthy derived myological feature, theinsertion of the subclavius on the supraspi-natus muscle in addition to the insertion onthe clavicle (Fig. 11). This muscle stabilizesthe distal end of the clavicle, which serves asa strut for positioning the shoulder joint.The extended insertion of the subclaviuspresumably strengthens this role. The divi-sion of the rhomboid (Figs. 1, 3, 5) intoanterior and posterior sections occurs in Fu-nambulus and Heliosciurus gambianus,which are not closely related to one another.Functional reasons for this morphology areunclear. There are other differences amongthe tree squirrels in the relative positions oforigins and insertions of muscles, but theseare subtle and do not distinguish any of thetribes from each other. The scansorial andarboreal locomotion of tree squirrels in allfive tribes is accomplished with the samebasic arrangement of forelimb muscles. Onefeature shared by all tree squirrels, but notby other sciurids, is a well-developed teres


major fossa at the posterior angle of thescapula (Fig. 3). The teres major functions inretroflexion of the shoulder—decreasing theangle between the humerus and the axillaryborder of the scapula. In terrestrial locomo-tion, retroflexion occurs during the supportphase as the forelimb is retracted, but theweight of the animal probably contributes tothis movement and little force is required bythe teres major. In a climbing animal, how-ever, gravity works against retroflexion andthe teres major must overcome it (Jenkinsand Weijs, ’79). This may explain why treesquirrels have the enlarged fossa and whyStalheim-Smith (’84) found that the treesquirrel Sciurus niger has a much strongerteres major than does the prairie dog Cyno-mys gunnisoni.

Pygmy tree squirrels are bark gleanersand forage on large tree trunks with theirlimbs placed far more laterally than is seenin larger tree squirrels. The limbs are elon-gated, particularly the forearm, and thethumb is reduced or absent (Anthony andTate, ’35; Emmons, ’79; Thorington and Thor-ington, ’89). The atlantoscapularis dorsalis(Fig. 3) is lost in Myosciurus and is greatlyreduced in Nannosciurus. This muscle func-tions to rotate the scapula during retractionof the limb, but it may be in a poor positionto effect this when the limbs are held later-ally. The spinodeltoid is extensive in pygmysquirrels and probably plays a major role inretroflexion of the shoulder joint. Pronatorquadratus (Fig. 10) is lost in these two ani-mals, correlating with the elongation of theforearm and presumably with a reduction inthe amount of pronation and supination atthe wrist.

Terrestrial locomotion takes several formswithin the Sciurinae. Cursorial terrestrialsquirrels (Rheithrosciurus and Epixerus) andthe terrestrial Asiatic squirrel Rhinosciuruswere not included in this study because ofthe unavailability of specimens. The twotribes of burrowing ground squirrels, Xeriniand Marmotini, were included. These ani-mals both climb less and dig more than dotree squirrels. A distinctive derived musclein the Marmotini is the superficial layer ofthe clavodeltoid, which inserts on the ulnaand forms a clavobrachialis (Fig. 7). This isfound only in some genera (Marmota, Sper-mophilus, and Cynomys) forming a singleclade of the Marmotini (Bryant, ’45), and isabsent in others (Tamias, Sciurotamias, andAmmospermophilus). This muscle has a com-pletely different function from the rest of the

deltoid, to flex the elbow in support of thenormal function of the brachialis. Thesemuscles flex the elbow without causing supi-nation of the forearm, in contrast with thebiceps, which causes both flexion and supina-tion. In comparing Sciurus and Cynomys,Stalheim-Smith (’84) noted that the bicepsof Sciurus generated more than twice thetorque as that of Cynomys. Tree climbingprobably requires both strong elbow flexionand forearm supination, while digging mayrequire flexion without supination. The at-lantoscapularis dorsalis is lost repeatedly inthe xerine ground squirrels, and in four sub-genera of the marmotine genus Spermophi-lus. It is not clear to us why this musclewould be lost in burrowing squirrels. It wouldappear to function as a rotator of the scapula,a movement that may be more effectivelyaccomplished by the occipitoscapularis inthese animals.

Flying squirrels exhibit a number of de-rived myological features. The trapeziuslacks a cranial origin and, in associationwith this, the insertion of the cleido-occipita-lis on the nuchal line is more extensive thanin other squirrels (Fig. 1). The cranial fibersof trapezius in tree squirrels probably func-tion both to assist in head movements and inprotraction of the shoulder joint. The ab-sence of these fibers in flying squirrels doesnot greatly affect the protraction of thescapula, but it does eliminate the use of thetrapezius in cranial movement. The moreextensive insertion of the cleido-occipitalisin flying squirrels presumably assumes thisfunction. Occipitoscapularis has a restrictedlateral origin on the nuchal line and tends toinsert on the vertebral border and not on thespine of the scapula. The insertion of atlanto-scapularis dorsalis extends to or onto thevertebral border of the scapula (Fig. 3). Boththese muscles are positioned to have betterleverage than in tree squirrels for rotatingthe scapula during retraction of the limb. Inlarger flying squirrels, the lateral head oftriceps has a muscular origin, and in thegiant flying squirrel Petaurista, it has anextensive muscular origin, contrasting withthe tendinous origin in other squirrels (Fig.8). As a result of this, there appears to be anincrease in the number of long muscle fibersin these flying squirrels. A very importantaspect of flying squirrel locomotion is safelanding after a glide. The triceps absorbs theimpact, and the lateral head may play aspecial role in this if it is composed of fasttwitch fibers, as in other mammals (Arm-strong, ’81). The radial and ulnar heads of


the forearm flexors have more restricted ori-gins than usual for squirrels, reflecting theelongation of the radius and ulna for gliding,and the pronator quadratus is greatly re-duced or absent. Flying squirrels exhibitgreatly reduced mobility between the distalends of radius and ulna, yet they pronateand supinate their forearms readily. Prona-tion and supination occur at the elbow andare effected by pronator teres and supinatormuscles, thereby reducing the function ofthe pronator quadratus (Thorington, ’84).The flying squirrels are characterized by thederived morphology of the abductor pollicis,the flexor carpi ulnaris, and the palmarislongus, associated with the extension andretraction of the styliform cartilage and thewing tip of the plagiopatagium (Thoringtonet al., in press).

Derived features of forelimb musculaturein squirrels may characterize groups of gen-era or tribes, like the clavobrachialis (Fig. 7)and the scapular insertion of subclavius (Fig.11), or they may evolve independently inseveral tribes, like the loss of the atlanto-scapularis dorsalis. No derived myologicalfeatures of the forelimb appear to be synapo-morphies of two or more tribes. This clearlysuggests that tribal divergence occurredprior to this evolutionary differentiation.Adapting to burrowing and terrestrial loco-motion appears to require little modificationof the forelimb musculature from that of treesquirrels. Only a few modifications are re-quired in the evolution of pygmy squirrels aswell. We see the greatest divergence in fore-limb musculature from that of tree squirrelsamong the flying squirrels.

This survey of forelimb musculature raisesa number of questions. Is there a change offunction of the subclavius muscle in Ratufawhich parallels the additional insertion onthe supraspinatus? Is there a difference inthe ways that tree squirrels and flying squir-rels bound up vertical trunks, especially inthe degree of retroflexion of the shoulderjoint, which correlates with the prominentteres fossa in tree squirrels and its absencein flying squirrels? Is scapular rotation dif-ferent in flying squirrels from in tree squir-rels? Does the lateral head of triceps play animportant role in absorbing the impact oflanding after a glide? In fact, is there anydifference in landing speeds and momentumbetween jumping tree squirrels and glidingflying squirrels? How does the clavobrachia-lis function in ground squirrels? Does it actat the same time as brachialis, or does itfunction differently? The atlantoscapularisdorsalis is absent in some and present in

other closely related species of Spermophi-lus. How does the absence of this muscleaffect scapular rotation? Does the absence ofpronator quadratus in pygmy tree squirrelsreflect a difference in how they pronate andsupinate their forearms, as it does in flyingsquirrels? We conclude this study with morequestions, but more focused questions thanbefore, about the functional anatomy of theseinteresting animals.

ACKNOWLEDGMENTS

We thank Lawrence R. Heaney andWilliam Stanley of the Field Museum ofNatural History (FMNH), Eric Yensen of theO.J. Smith Museum of Natural History atAlbertson College of Idaho (OJSMNH), andDuane Schlitter and Suzanne McLaren ofthe Carnegie Museum (CM) for the loan ofspecimens. Peter Zahler provided us withthe Eupetaurus forelimb. Advice and com-ments on the manuscript were provided byRobert Hoffmann and Brian Stafford. Fund-ing for A.D.K.B. was provided by the Re-search Training Program and the MinorityInternship Program of the Smithsonian In-stitution.

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APPENDIX. Specimens dissected

Family SciuridaeSubfamily Sciurinae: Tree, ground, and pygmy squirrels

Tribe Ratufini: Indo-Malayan giant squirrelsRatufa affinis: USNM 522980Ratufa bicolor: USNM 546334Ratufa indica: USNM 548661

Tribe Protoxerini: African giant and sun squirrelsProtoxerus stangeri: USNM 481817, 481821Heliosciurus gambianus: USNM 381419, 481830Heliosciurus rufobrachium: USNM 463538, 463544, 463545, 541537

Tribe Funambulini: Indian and African tree and pygmy squirrelsSubtribe Funambulina: Indian striped squirrels

Funambulus palmarum: USNM 448821, 448824Subtribe Funisciurina: African striped squirrels and tree squirrels

Funisciurus anerythrus: USNM 463536Paraxerus ochraceus: USNM 251930Paraxerus palliatus: USNM 548034

Subtribe Myosciurina: African pygmy squirrelsMyosciurus pumilio: USNM 514360, 220760

Tribe Callosciurini: Oriental squirrelsCallosciurus notatus: USNM 521141, USNM uncatalogued (IMR 84702, 86338, 86351)Nannosciurus melanotis: USNM 154407

Tribe Sciurini: Holarctic and neotropical tree and pygmy squirrelsSciurus carolinensis: USNM 396002, 497249, 497250, 522976

Tribe Marmotini: Holarctic ground squirrelsSpermophilus columbianus: OJSMNH uncatalogued (EY821)Spermophilus richardsonii: USNM 398236Spermophilus variegatus: OJSMNH 414Sciurotamias davidianus: USNM 541398

Tribe Xerini: African ground squirrelsXerus rutilus: CM 86231Atlantoxerus getulus: USNM 477053, 477054

Subfamily Pteromyinae: Flying squirrelsBelomys pearsoni: USNM 359595Eoglaucomys fimbriatus: FMNH 140501Eupetaurus cinereus: USNM uncatalogued forelimbGlaucomys volans: USNM 457978, 457979Hylopetes spadiceus: USNM uncatalogued (IMR 89902, 89799)Petaurista philippensis: USNM 334352, 334359Pteromys volans: USNM 547926


Comparative myology of the forelimb of squirrels (Sciuridae)

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