JOURNAL OF MORPHOLOGY 230:145-165 (1996) Jaw Muscles of Old World Squirrels RICHARD W. THORINGTON, JR., AND KAROLYN DARROW Department of Vertebrate Zoology, Smithsonian Institution, Washington, D.C. 20560 ABSTRACT The jaw, suprahyoid, and extrinsic tongue muscles were studied in 11 genera, belonging to five tribes, of Old World squirrels. Significant variation in most of the adductor muscles is evident. The most primitive state of sciuromorphy is seen in the African tree squirrels Paraxerus and Funisciu- rus, especially as reflected in the anterior deep masseter. A derived state of sciuromorphy is found in five genera of Old World squirrels and perhaps evolved independently in each. Reduction of the temporalis muscle was ob- served in three genera, distantly related to one another. A unique arrangement of the superficial masseter is reported in the Asian giant tree squirrels, Ratufa. The arrangement of the masseter in the African pygmy squirrel, Myosciurus, is very similar to that of the South American pygmy squirrel, Sciurillus. We present hypotheses about the functional significance of these differences. In the derived state of sciuromorphy, which is found in three cases in squirrels that feed extensively on hard fruits, the anterior deep masseter is well posi- tioned to increase the strength of the power stroke of the incisor bite. Among the pygmy squirrels, the position of the anterior deep masseter suggests that it plays a more significant role in molar chewing. © 1996 Wiley-Liss, Inc.* The jaw musculature of rodents has been the North American Miocene, the anterior the subject of study and basis for taxonomic deep masseter had moved forward onto the classification for more than 150 years. Brandt zygomatic plate, showing the initial stages of (1855) divided the rodents into three groups sciuromorphy (Korth, '87; Emry and Korth, based upon their masseter musculature and '96). the basic attributes of this classification are Among recent squirrels, features of cranial to be found in the review by Simpson ('45). morphology associated with the jaw muscula- One of these groups, the Sciuromorphs, is ture have played a significant role in subfam- characterized by the position of the origin of ilial classification (Moore, '59). This muscula- the anterior deep masseter muscle on the ture is well described for New World and anterior surface of the zygomatic plate. Sci- Holarctic squirrels (Toldt, '05; Bryant, '45; uromorphy evolved several times among the Turnbull, '70; Ball and Roth, '95) but is rodents, however, perhaps last among the practically unknown for tropical Old World squirrels themselves (Emry and Thorington, squirrels (Parsons, 1894). It is clear from '82). Therefore, this morphology is not an cranial morphology that jaw musculature var- indicator of common ancestry of all sciuro- ies among the Old World genera, and it ap- morphous rodents. pears as if various stages of sciuromorphy are In the earliest known fossil squirrel, Doug- represented among them. We document here lassia jeffersoni (formerly Protosciurus cf. the musculature associated with these stages jeffersoni, Emry and Korth, '96), of the late of sciuromorphy and attempt to assess the Eocene, 35 mybp, the jaw musculature was evolutionary trends and functional changes protrogomorphous (Emry and Thorington, they seem to represent. Additionally, it is '82). It is likely that sciuromorphy evolved in possible that these stages provide clues about the Sciuridae during the early Oligocene. In the times of divergence of the tribes of squir- the earliest European fossil squirrels, Pal- rels. aeosciurus goti and Palaeosciurus feignouxi, The function of jaw muscles has been well of the middle Oligocene, approximately 30 studied in Rattus (Hiiemae, '71b; Weijs and mybp, sciuromorphy was already evident (Vi- Dantuma, '75). In general, the jaw muscles of aney-Liaud, '74). In Protosciurus mengi of squirrels function similarly, although there © 1996 WILEY-LISS, INC. "This article is a US Govern- ment work and, as such, is in the public domain in the United States of America. 146 R.W. THORINGTON AND D.K. DARROW are clearly differences in details because of in our study the Indian striped squirrel, Fu- the differences in musculature. Squirrels lack nambulus, which was considered by Moore the infraorbital portion of the masseter found ('59) to be closely related to the other Funam- inRattus (Hiiemae, '71a). Most squirrels have bulini of Africa. Squirrels representing three a comparatively more massive anterior deep additional tribes were also examined: Cal- masseter originating in front of the zygo- losciurus and Tamiops are two genera of tree matic plate. In other respects, the anatomy of squirrels of the tribe Callosciurini which ex- the jaw muscles of squirrels and Rattus is hibits an extensive radiation of 13 genera in similar. In both, there are two basic ways Southeast Asia; Ratufa is the genus of giant that the jaw functions, in molar chewing and tree squirrels (some weighing in excess of 2 in incisor biting. During the power stroke of kg) found in India and Southeast Asia and chewing, the incisors do not occlude. The represents the tribe Ratufini; Xerus and At- food is ground between the upper and lower lantoxerus, of the tribe Xerini, are two gen- era of African ground squirrels. The 12 gen- molars with an upward and forward move- era we dissected represent all the tribes of ment of the mandible under the maxilla, tropical Old World squirrels with the excep- termed propalinal chewing. As in Rattus, the tion of the flying squirrels. temporo-mandibular joint is probably un- loaded (Hiiemae, '71b; Weijs and Dantuma, MATERIALS AND METHODS '75). During the power stroke of biting, the Specimens dissected are listed in the appen- mandible is protracted so that the incisors dix and their placement in Moore's ('59) clas- occlude and the molars do not. The mandibu- sification of squirrels is listed in Table 1. lar incisors are forcefully raised against the Dissectable museum specimens of these gen- maxillary incisors, and the temporo-mandibu- era are rare. Before undertaking this study lar joint is heavily loaded. Because of these we dissected specimens ofSciurus carolinen- similarities, the functions of the individual sis, and compared them in detail with the muscles of Old World squirrels are inter- preted as being like those of Rattus, except when anatomical differences suggest other- TABLE 1. Classification of the Sciuridae relevant to wise. The objective is to provide hypotheses this study1 about the functional significance of the mor- Family Sciuridae phological differences we have observed. Subfamily Sciurinae We present descriptions of the jaw muscu- Tribe Ratufini: Indo-Malayan giant squirrels lature of 12 genera of rodents. For reference Ratufa Gray, 1867 Tribe Protoxerini: African giant and sun squirrels to the primitive condition, protrogomorphy, Protoxerus Major, 1893 in which the masseter muscle takes origin Epixerus Thomas, 1909 only from the zygomatic arch, we include Heliosciurus Trouessart, 1880 Aplodontia, the mountain beaver of north- Tribe Funambulini: Indian and African tree and pygmy squirrels ern Oregon and Washington. We dissected all Subtribe Funambulina: Indian striped squirrel five genera of African tree squirrels, repre- Funambulus Lesson, 1832 senting two tribes (Moore, '59). These are Subtribe Funisciurina Funisciurus Trouessart, 1880 Paraxerus, a group of bush and tree squirrels Paraxerus Major, 1893 found throughout much of sub-Saharan Af- Subtribe Myosciurina: African pygmy squirrel rica, Funisciurus, a group of usually striped Myosciurus Thomas, 1909 squirrels, and Myosciurus, the African pygmy Tribe Callosciurini: Oriental squirrels Callosciurus Gray, 1867 squirrel (adults may weigh less than 20 g). Tamiops Allen, 1901 All of these were included by Moore in the Sundasciurus Moore, 1958 tribe Funambulini. The two other genera are Nannosciurus Trouessart, 1880 Heliosciurus, the sun squirrels with an exten- Dremomys Heude, 1898 Rhinosciurus Gray, 1843 sive range in sub-Saharan Africa, and Pro- Rubrisciurus Ellerman, 1954 toxerus, the African giant tree squirrel (weigh- Tribe Sciurini: Holarctic and Neotropical tree and ing approximately 700 kg) which is more pygmy squirrels restricted to the high tropical forests. These Sciurus Linnaeus, 1758 Tribe Xerini genera belong to the tribe Protoxerini. Be- Xerus Hemprich and Ehrenberg, 1832. Ethiopian cause specimens were not available, the only ground squirrel. genus of African squirrels that we did not Atlantoxerus Major, 1893. Barbary ground dissect was Epixerus, a primarily terrestrial squirrel. member of the Protoxerini. We also included 'Modified from Moore, '59. SQUIRREL JAW MUSCLES 147 morphology described by Ball and Roth ('95). process. Ratios were formed by dividing the In this and other recent studies (Thorington first measurement into each of the others. et al., '96a,b) we have examined the skulls of These ratios estimate the mechanical advan- all genera of squirrels with the exception of tage of the anterior and posterior deep masse- Rubrisciurus. During dissection, muscles ter, the temporalis, the superficial masseter, were bisected between origin and insertion, and the medial pterygoid muscles. and the mandible on one side was removed so that origins and insertions could be exam- RESULTS ined more carefully. We recorded our observa- Superficial masseter tions on camera lucida drawings made of the Aplodontia rufa appropriate skulls and mandibles in the col- The origin is a flat wide tendon originating lections of the National Museum of Natural on the ventral portion of the zygomatic arch History. just ventral to the infraorbital foramen (Fig. Anatomical nomenclature remains a prob- 1). The tendon is folded over on itself along lem. The Nomina Anatomica ('66) for hu- the ventral edge. Fibers insert on the medial mans is inappropriate. The Illustrated Veteri- corner of the angular process (Fig. 1), on its nary Anatomical Nomenclature (Schaller, '92) medial edge, and extensively on its dorsal does not include rodents. Some terms nor- surface, medial to the insertion of the medial mally used in comparative anatomy differ pterygoid. There is no insertion on the homo- from both. We follow the terminology for logue of the anterior deep masseter. muscles used by Ball and Roth ('95) to facili- tate comparison. Paraxerus The data presented in Table 2 are based on measurements of mandibles in the collection The superficial masseter (Fig. 2) originates of the National Museum of Natural History. from the maxillary bone ventral and poste- Measurements were taken from the middle rior to the infraorbital foramen via a flat wide of the mandibular condyle to the tip of the tendon with a strong ventral fold. On skulls, mandibular incisor, and to five other land- the origin can be seen as a line extending mark points: the anteriormost point on the from the anterior edge of the infraorbital masseteric ridge, the retromolar space where foramen to a point dorsal and just anterior to the temporalis inserts behind the third mo- the third premolar (Fig. 3a). Muscle fibers lar, to the tip of the coronoid process, the diverge from this tendon to insert onto the anteroventral edge of the angular process, lateral and medial surfaces of the mandible. and the posterodorsal corner of the angular Some dorsal fibers insert on the posterior TABLE 2. Ratios of mandibular measurements, lever arm/load arm (S.D.), for incisor bite1 Average length Species (n) of load arm (S.D.) A B C D E Aplodontia rufa (6) 52.9 (2.9) .40 (.004) .44 (.012) .55 (.011) .30 (.016) .17 (.016) African squirrels Paraxerus palliatus (6) 33.9 (0.4) .36 (.015) .49 (.011) .62 (.015) .37 (.004) .15 (.017) Funisciurus anaerythrus (8) 29.8 (1.4) .30 (.010) .47 (.026) .60 (.011) .35 (.014) .14 (.016) Protoxerus stangeri (6) 48.2 (2.0) .35 (.019) .51 (.011) .65 (.012) .40 (.015) .19 (.097) Heliosciurus rufobrachium (7) 33.7 (1.2) .36 (.033) .49 (.022) .65 (.026) .39 (.027) .16 (.005) Heliosciurus gambianus (6) 30.6 (1.2) .36 (.007) .52 (.017) .64 (.010) .39 (.016) .15 (.009) Myosciurus pumilio (5) 14.8 (0.3) .35 (.015) .45 (.018) .72 (.025) .41 (.026) .30 (.009) Atlantoxerus getulus (6) 30.8 (0.9) .34 (.022) .47 (.021) .63 (.010) .38 (.010) .19 (.013) Xerus erythropus (6) 38.2 (2.2) .31 (.013) .48 (.013) .64 (.018) .36 (.015) .22 (.009) Asian squirrels Funambulus pennanti (6) 25.4 (0.9) .34 (.017) .46 (.024) .67 (.012) .40 (.019) .26 (.016) Ratufa bicolor (6) 48.9 (2.1) .38 (.019) .54 (.010) .67 (.010) .38 (.008) .18 (.024) Callosciurus notatus (6) 35.3 (0.2) .33 (.008) .45 (.011) .63 (.010) .38 (.010) .21 (.015) Sundasciurus lowii (6) 27.5(0.1) .29 (.013) .44 (.011) .63 (.013) .38 (.005) .19 (.016) Sundasciurus hippurus (6) 42.8 (0.8) .32 (.007) .48 (.008) .67 (.008) .39 (.010) .23 (.004) Tamiops macclellandi (6) 21.6 (0.6) .33 (.017) .46 (.005) .63 (.009) .39 (.016) .18 (.023) Dremomys rufigenis (6) 35.7 (0.9) .31 (.017) .44 (.015) .63 (.011) .36 (.014) .17 (.004) Rhinosciurus laticaudatus (6) 39.5 (1.4) .27 (.011) .34 (.012) .54 (.016) .33 (.008) .24 (.015) iLoad arm: condyle to tip of mandibular incisor; lever arm A: condyle to posterodorsal point of angular process; lever arm B: condyle to anteroventral point of angular process; lever arm C: condyle to anteriormost point of masseteric ridge; lever arm D: condyle to retromolar pit; lever arm E: condyle to tip of coronoid process. 148 R.W. THORINGTON AND D.K. DARROW Aplodontia | superficial masseter posterior deep masseter QJ| Q] anterior deep masseter temporalis [A\\] zygomaticomandibularis 5 mm ant. deep mass. sup. mass, post, deep mass. Fig. 1. Origins and insertions of the jaw muscles of Aplodontia rufa. the superficial masseter insert on this aponeu- portion of the tendon of insertion of the rosis and others insert in the most anterior anterior deep masseter. The other dorsal fi- bers insert on the ventral and posterior edges portion of the pterygoid fossa of the man- dible. of the angular process (Fig. 3a). The muscle fibers of the posterior deep masseter and Funisciurus those of the superficial masseter are not sepa- rable at this insertion. The more ventral fi- The muscle is similar to that of Paraxerus bers wrap around the ventral edge of the except in the following particulars. The ori- mandible and insert on the medial side of the gin is a fiat wide tendon which is thicker at its angular process, ventral to the insertion of ventral edge but not folded over on itself. In the medial pterygoid muscle (see Fig. 6). The Funisciurus anaerythrus and Funisciurus dorsal edge of this insertion appears to be lemniscatus fibers insert musculously both delimited by a ridge, separating it from the on the posterior edge of the angular process insertion of the medial pterygoid. This ridge and on the tendon of insertion of the poste- is the point of insertion of the medial aponeu- rior deep masseter at the dorsal corner of the rosis of the medial pterygoid. Some fibers of angular process. The superficial masseter of SQUIRREL JAW MUSCLES 149 lateral temporalis anterior deep masseter medial temporalis posterior deep masseter superficial masseter Fig. 2. Typical arrangement of the jaw muscles of squirrels, as seen in Funisciurus pyrropus. Funisciurus pyrropus, by contrast, did not on the tendon of the anterior deep masseter insert all the way to the dorsal corner of the is like that in Paraxerus. In Heliosciurus angular process. gambianus an insertion on this tendon is lacking. Myosciurus Protoxerus The origin bears a different relationship to the infraorbital foramen because of the reduc- The muscle differs from Paraxerus in sev- tion of the infraorbital canal and the poste- eral respects. The tendon originates from a rior location of the foramen in this species. roughened area on the maxillary bone, ven- The muscle originates from a small area near tral and anterior to the infraorbital foramen the ventral edge of the rostrum just caudal to (Fig. 3a). This different position results from the maxillary-premaxillary suture and well the retreat of the infraorbital foramen to the in front of the infraorbital foramen (Fig. 3a). zygomatic plate and the absence of an infraor- The tendon of origin is a long, flat, wide bital canal in Protoxerus. The tendon is folded ribbon that is not folded over on itself. On the over on itself on its ventral edge. Fibers of lateral surface of the mandible, the muscle the superficial masseter insert extensively on inserts on only the ventral edge of the angu- the tendon of insertion of the anterior deep lar process (Fig. 3a). On the medial surface of masseter, along the ventral masseteric ridge the mandible, it inserts extensively in a well- posterior to the insertion of the anterior deep defined fossa ventral and anterior to the inser- masseter, and on the ventral half of the pos- tion of the medial pterygoid muscle. terior edge of the angular process as well as on its medial surface (Fig. 3a). In Protoxerus Heliosciurus the insertion of the superficial masseter is The muscle is similar to Paraxerus except more closely associated with the anterior deep that the tendon of origin comes only from a masseter than with the posterior deep masse- protuberance, the masseteric knob, at the ter. ventral edge of the infraorbital foramen (Fig. Funambulus 3a). The tendon is more rope-like than ribbon- like, and the ventral edge is not folded over. The muscle differs from that of Para- In Heliosciurus rufobrachium the insertion xerus in the following ways. The tendon 150 R.W. THORINGTON AND O.K. DARROW Paraxerus Heliosciurus Myosciurus PH anterior deep masseter 1 5 mm ^ superficial masseter 1 zygomaticomandibularis |#| posterior deep masseter |~~J sup. mass, ant. deep mass.| | Atlantoxerus Protoxerus Fig. 3. Origins and insertions of the masseter muscles in (a) African squirrels and (b) Asian squirrels. Scale bars are all 5 mm. of origin does not fold over on itself at ficial masseter are separate from the fibers the ventral edge, nor does it insert on the of the posterior deep masseter at the aponeurosis of the anterior deep mass- posterior edge of the angular process (Fig. eter. At its insertion, the fibers of the super- 3b). SQUIRREL JAW MUSCLES 151 Callosciurus Tamiops Ratufa Funambulus Figure 3 (Continued) Callosciurus and Tamiops deep masseter. As in Paraxerus, but not Heli- The muscle differs from that of Paraxerus osciurus, the tendon of origin is folded over in that the tendon of origin lacks a ventral on itself at its ventral edge. fold and muscle fibers do not insert onto the anterior deep masseter. In Callosciurus but Ratufa not in Tamiops, fibers of insertion are exten- The superficial masseter is very different sively intermingled with fibers of the poste- in this genus. The tendon of origin is very rior deep masseter at their insertion onto the broad and extends from the ventral edge of angular process. Further, some of the more the infraorbital foramen to the dorsal edge of dorsal muscle fibers of the superficial masse- the zygomatic plate (Fig. 3b). It has a thick ter insert deep to the fibers of the posterior ventral fold. The muscle fibers are not sepa- deep masseter on the posterior edge of the angular process. rable from those of the anterior deep masse- ter. The superficial fibers attributable to the Xerus and Atlantoxerus superficial masseter insert in the usual pat- The muscle differs from that in Paraxerus tern on the posterior edge and medial side of in that the tendon originates from a masse- the angular process. Fibers of insertion of the teric knob, as in Heliosciurus (Fig. 3a), and superficial masseter are not separable from the muscle does not insert onto the anterior those of the posterior deep masseter. 152 R.W. THORINGTON AND O.K. DARROW Anterior deep masseter Myosciurus Aplodontia rufa The anterior deep masseter has an exten- The most anterior fibers of the lateral layer sive origin from the zygomatic plate and from of the masseter are partially separable into a the whole surface of the maxillary bone on bundle that appears to be the homologue of the lateral surface of the rostrum (Fig. 3a). the anterior deep masseter. These fibers origi- The origin extends to, but not across, the nate from a pit on the ventral surface of the maxillary-premaxillary suture. The fibers ex- zygomatic arch, ventral and slightly lateral tend vertically and slightly posteriorly to in- to the infraorbital foramen and just posterior sert on the ventral masseteric ridge and in a to the origin of the tendon of the superficial distinct fossa on the lateral surface of the masseter (Fig. 1). Some fibers originate from mandible (Fig. 3a). The posterior portion of the deep side of the aponeurosis of origin of the anterior deep masseter muscle is a nar- the posterior deep masseter. The insertion of row, thin layer of fibers originating from the this "anterior deep masseter" is musculous ventral edge of the zygomatic arch, deep to on the lateral surface of the mandible and the posterior deep masseter. It inserts on the aponeurotic on a ridge which appears to be lateral surface of the mandible, posterior to the homologue of the ventral masseteric ridge the fibers originating from the zygomatic on the mandible of squirrels. Posteriorly, the plate. insertion of this muscle is inseparable from the insertion of fibers that originate more Heliosciurus posteriorly on the zygomatic arch and that would appear to be homologous to the fibers On the zygomatic plate and the side of the of the posterior deep masseter of squirrels. rostrum, the origin extends further dorsally and more anteriorly than in Paraxerus and Paraxerus and Funisciurus in Funisciurus. The dorsal edge is at the level of the lacrimal bone and the anteriormost The anterior deep masseter (Fig. 2) origi- fibers take origin from the vicinity of the nates from the zygomatic plate on the ante- premaxillary-maxillary suture on the side of rior surface of the zygoma and from the the rostrum (Fig. 3a). The insertion in Heli- lateral surface of the maxillary bone (Fig. osciurus is restricted to the ventral portion of 3a), in front of the zygoma. This origin does the masseteric ridge of the mandible (Fig. not extend as high as the lacrimal bone and 3a). The insertion on the masseteric ridge is does not reach the premaxillary-maxillary exclusively aponeurotic. The posterior por- suture. The muscle inserts aponeurotically tion of the anterior deep masseter is a layer along the ventral edge of the masseteric ridge of musculature on the deep surface of the (Fig. 3a). The aponeurosis does not extend to aponeurosis of origin of the posterior deep the dorsal portion of the masseteric ridge. masseter, similar to that seen in Paraxerus In Funisciurus and in Paraxeruspalliatus, palliatus. but not in Paraxereus ochraceus, there is a layer of muscle fibers originating from the Protoxerus deep surface of the medial aponeurosis of origin of the posterior deep masseter. This The origin extends further to the side of deep muscle layer is the posterior portion of the rostrum than in Heliosciurus, with the the anterior deep masseter, as recognized by most anterior fibers taking origin from the Ball and Roth ('95). The anterior fibers of premaxillary bone (Fig. 3a). The dorsal edge this posterior portion are continuous with of the origin is at the level of the lacrimal the fibers of the anterior portion of the ante- bone. The muscle inserts aponeurotically on rior deep masseter, parallel it, and insert the ventral masseteric ridge and for a short musculously on the lateral surface of the distance on the dorsal masseteric ridge, but mandible and onto the ventral masseteric not as far as the base of the coronoid process ridge posterior to the aponeurotic insertion (Fig. 3a). There is an extensive, thick poste- of anterior deep masseter. The posterior fi- rior portion of the anterior deep masseter, bers of this deep layer become progressively originating from the deep surface of the me- more parallel to the fibers of the posterior dial aponeurosis of the posterior deep masse- deep masseter and gradually become indistin- ter (Fig. 4), as in Paraxerus. The medial guishable from it. This deep layer of muscle aponeurosis is thicker and more extensive fibers inserts on the lateral surface of the than that of Paraxerus and maintains the mandible. distinction between the muscle layers. SQUIRREL JAW MUSCLES 153 zygomatic arch mandible superficial aponeurosis of origin medial aponeurosis of origin — posterior portion of anterior deep masseter posterior deep masseter aponeurosis of insertion superficial masseter Fig. 4. Vertical cross-section of the masseter muscles as seen in Protoxerus stangeri. The section lies slightly posterior to M3. We did not have specimens of Epixerus to portion of the anterior deep masseter is a dissect but we examined skulls. The deep thin layer of muscle fibers originating from fossa on the zygomatic plate and the promi- the deep surface of the medial aponeurosis of nence of the masseteric ridge suggest that the posterior deep masseter. The aponeuro- Epixerus has an anterior deep masseter that sis does not extend the entire length of the is more massive and inserts more strongly on zygomatic arch and the deep muscle layer the mandible than it does in Protoxerus. The tapers off toward its posterior end. anterior deep masseter of Rheithrosciurus, a Callosciurus and Tamiops large terrestrial squirrel in Borneo, must be more remarkable yet, judging from the ex- In Callosciurus there is extensive origin traordinarily deep fossa on the side of the from the side of the rostrum anterior to the rostrum and the prominence of the masse- premaxillary-maxillary suture. In Tamiops teric ridges. the origin extends only slightly beyond the premaxillary-maxillary suture. In both, the Funambulus insertion on the mandible is almost exclu- The origin extends to the side of the ros- sively aponeurotic on the ventral edge of the trum, anterior to the premaxillary-maxillary masseteric ridge (Fig. 3b). The insertion does suture and dorsally as high as the lacrimal not extend to the dorsal masseteric ridge. bone. The muscle inserts aponeurotically on The posterior portion of the anterior deep the ventral masseteric ridge and on the dor- masseter is an extensive layer of muscle fi- sal masseteric ridge half way to the base of bers originating from the deep surface of the the coronoid process (Fig. 3b). The posterior medial aponeurosis of the posterior deep mas- 154 R.W. THORINGTON AND O.K. DARROW seter. It inserts on the lateral surface of the Paraxerus and Funisciurus mandible dorsal to the ventral masseteric The posterior deep masseter (Fig. 2) origi- ridge. nates from the lateral surface of the zygo- matic arch, the superficial aponeurosis, and Xerus and Atlantoxerus the medial aponeurosis arising from the ven- The origin extends as far dorsal as the tral edge of the zygomatic arch (Fig. 4). The lacrimal bone and anterior to the premaxil- medial aponeurosis is robust anteriorly and thinner toward the posterior end of the zygo- lary-maxillary suture. It extends slightly be- yond the suture in Atlantoxerus and exten- matic arch. Where the medial aponeurosis ends, ventrally and posteriorly, separate lay- sively anterior to the suture in Xerus. The ers of the anterior and posterior deep masse- zygomatic plate in both genera is concave with a strong lateral edge. The insertion of ters are indistinguishable. The insertion of the posterior deep masseter is both aponeu- the anterior deep masseter is restricted to rotic and musculous. The aponeurosis of in- the ventral edge of the masseteric ridge (Fig. sertion extends from the dorsal corner of the 3a). There is a thin posterior portion of ante- angular process, ventrally along the inser- rior deep masseter which takes origin from tion of the superficial masseter, and anteri- the medial aponeurosis of the posterior deep orly along the ventral masseteric ridge. The masseter and inserts on the lateral surface of musculous insertion is dorsal to the aponeu- the mandible and on the ventral masseteric rotic insertion and covers a broad area on the ridge. lateral surface of the angular process. Ratufa Myosciurus The origin extends dorsal to the lacrimal The zygomatic arch is short in this species bone and anterior to the maxillary-premaxil- so the posterior deep masseter is narrow. It lary suture (Fig. 3b). The fibers are not sepa- takes origin from the zygomatic arch and rable from those of the superficial masseter. inserts tendinously on the posterior angle of Fibers originating on the zygomatic plate the angular process and musculously for sev- insert aponeurotically on the ventral masse- eral millimeters more ventrally on the angu- teric ridge and musculously on the lateral lar process. surface of the mandible. The posterior por- tion of the anterior deep masseter is a thin Heliosciurus layer of muscle fibers originating from the deep aponeurosis of the posterior deep masse- The muscle is similar to that in Paraxerus. ter, paralleling the fibers of the anterior fi- In Heliosciurus rufobrachium there is a dis- bers of the same muscle. The posterior por- tinct tendinous insertion on the posterior tion inserts on the lateral surface of the corner of the angular process and the groove mandible and on the ventral masseteric ridge. adjoining the insertion of the superficial mas- seter. In Heliosciurus gambianus the inser- tion is tendinous only on the posterior corner Posterior deep masseter Aplodontia rufa of the angular process. This muscle originates from the ventrolat- Protoxerus eral surface of the zygomatic arch (Fig. 1) The muscle is similar to that in Paraxerus, and from a thick aponeurosis which forms its with a distinct aponeurotic insertion on the deep surface. It inserts musculously on the posterior corner of the angular process and whole ventral surface of the angular process in the groove that extends ventrally along the except for its medial corner, posterior to the anterior edge of the insertion of the superfi- insertion of the anterior deep masseter and cial masseter. The aponeuroses of origin and ventral to the insertion of the zygomatico- insertion are stronger and more extensive mandibularis (Fig. 1). A tendon of insertion than in Paraxerus (Fig. 4). is lacking. A line of separation between the insertions of the posterior deep masseter and Funambulus zygomaticomandibularis can be seen on the mandible. At their insertion, there is a very The muscle is similar to that in Paraxerus, slight mingling of the dorsal fibers of the with a distinct tendinous insertion on the superficial masseter and the adjoining fibers posterior corner of the angular process but of the posterior deep masseter. with a musculous insertion more ventrally.