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diff --git a/old/30321.txt b/old/30321.txt new file mode 100644 index 0000000..c91ce6b --- /dev/null +++ b/old/30321.txt @@ -0,0 +1,1389 @@ +The Project Gutenberg EBook of The Adductor Muscles of the Jaw In Some +Primitive Reptiles, by Richard C. Fox + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: The Adductor Muscles of the Jaw In Some Primitive Reptiles + +Author: Richard C. Fox + +Release Date: October 24, 2009 [EBook #30321] + +Language: English + +Character set encoding: ASCII + +*** START OF THIS PROJECT GUTENBERG EBOOK ADDUCTOR MUSCLES OF THE JAW *** + + + + +Produced by Chris Curnow, Joseph Cooper, Diane Monico, and +the Online Distributed Proofreading Team at +https://www.pgdp.net + + + + + + + + + + + +UNIVERSITY OF KANSAS PUBLICATIONS + +MUSEUM OF NATURAL HISTORY + + +Volume 12, No. 15, pp. 657-680, 11 figs. +May 18, 1964 + + +The Adductor Muscles of the Jaw +In Some Primitive Reptiles + + +BY + +RICHARD C. FOX + + +UNIVERSITY OF KANSAS +LAWRENCE +1964 + + +UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY + +Editors: E. Raymond Hall, Chairman, Henry S. Fitch, +Theodore H. Eaton, Jr. + + +Volume 12, No. 15, pp. 657-680, 11 figs. +Published May 18, 1964 + + +UNIVERSITY OF KANSAS +Lawrence, Kansas + + +PRINTED BY +HARRY (BUD) TIMBERLAKE, STATE PRINTER +TOPEKA, KANSAS +1964 + +30-1522 + + + + +The Adductor Muscles of the Jaw +In Some Primitive Reptiles + +BY + +RICHARD C. FOX + + +Information about osteological changes in the groups of reptiles that +gave rise to mammals is preserved in the fossil record, but the +musculature of these reptiles has been lost forever. Nevertheless, a +reasonably accurate picture of the morphology and the spatial +relationships of the muscles of many of these extinct vertebrates can +be inferred by studying the scars or other marks delimiting the origins +and insertions of muscles on the skeletons of the fossils and by +studying the anatomy of Recent genera. A reconstruction built by these +methods is largely speculative, especially when the fossil groups are +far removed in time, kinship and morphology from Recent kinds, and when +distortion, crushing, fragmentation and overzealous preparation have +damaged the surfaces associated with the attachment of muscles. The +frequent inadequacy of such direct evidence can be partially offset by +considering the mechanical demands that groups of muscles must meet to +perform a particular movement of a skeletal member. + +Both direct anatomical evidence and inferred functional relations were +used to satisfy the purposes of the study here reported on. The +following account reports the results of my efforts to: 1, reconstruct +the adductor muscles of the mandible in _Captorhinus_ and _Dimetrodon_; +2, reconstruct the external adductors of the mandible in the cynodont +_Thrinaxodon_; and 3, learn the causes of the appearance and continued +expansion of the temporal fenestrae among the reptilian ancestors of +mammals. + +The osteology of these three genera is comparatively well-known. +Although each of the genera is somewhat specialized, none seems to have +departed radically from its relatives that comprised the line leading +to mammals. + +I thank Prof. Theodore H. Eaton, Jr., for suggesting the study here +reported on, for his perceptive criticisms regarding it, and for his +continued patience throughout my investigation. Financial assistance +was furnished by his National Science Foundation Grant (NSF-G8624) for +which I am also appreciative. I thank Dr. Rainer Zangerl, Chief Curator +of Geology, Chicago Museum of Natural History, for permission to +examine the specimens of _Captorhinus_ and _Dimetrodon_ in that +institution. I am grateful to Mr. Robert F. Clarke, Assistant Professor +of Biology, The Kansas State Teachers College, Emporia, Kansas, for the +opportunity to study his specimens of _Captorhinus_ from Richard's +Spur, Oklahoma. Special acknowledgment is due Mr. Merton C. Bowman for +his able preparation of the illustrations. + + +Captorhinus + +The outlines of the skulls of _Captorhinus_ differ considerably from +those of the skulls of the primitive captorhinomorph _Protorothyris_. +Watson (1954:335, Fig. 9) has shown that in the morphological sequence, +_Protorothyris--Romeria--Captorhinus_, there has been flattening and +rounding of the skull-roof and loss of the primitive "square-cut" +appearance in transverse section. The quadrates in _Captorhinus_ are +farther from the midline than in _Protorothyris_, and the adductor +chambers in _Captorhinus_ are considerably wider than they were +primitively. Additionally, the postorbital region of _Captorhinus_ is +relatively longer than that of _Protorothyris_, a specialization that +has increased the length of the chambers within. + +In contrast with these dimensional changes there has been little shift +in the pattern of the dermal bones that roof the adductor chambers. The +most conspicuous modification in _Captorhinus_ is the absence of the +tabular. This element in _Protorothyris_ was limited to the occiput and +rested without sutural attachment upon the squamosal (Watson, +1954:338); later loss of the tabular could have had no effect upon the +origins of muscles from inside the skull roof. Changes in pattern that +may have modified the origin of the adductors in _Captorhinus_ were +correlated with the increase in length of the parietals and the +reduction of the supratemporals. Other changes that were related to the +departure from the primitive romeriid condition of the adductors +included the development of a coronoid process, the flattening of the +quadrate-articular joint, and the development of the peculiar dentition +of _Captorhinus_. + +The adductor chambers of _Captorhinus_ are large. They are covered +dorsally and laterally by the parietal, squamosal, postfrontal, +postorbital, quadratojugal and jugal bones. The chamber extends +medially to the braincase, but is not limited anteriorly by a bony +wall. The occiput provides the posterior limit. The greater part of the +adductor chambers lies mediad of the mandibles and thus of the +Meckelian fossae; consequently the muscles that arise from the dermal +roof pass downward and outward to their insertion on the mandibular +rami. + + +_Mandible_ + +The mandibular rami of _Captorhinus_ are strongly constructed. Each +ramus is slightly convex in lateral outline. Approximately the anterior +half of each ramus lies beneath the tooth-row. This half is roughly +wedge-shaped in its lateral aspect, reaching its greatest height +beneath the short posterior teeth. + +The posterior half of each ramus is not directly involved in supporting +the teeth, but is associated with the adductor musculature and the +articulation of the ramus with the quadrate. The ventral margin of this +part of the ramus curves dorsally in a gentle arc that terminates +posteriorly at the base of the retroarticular process. The dorsal +margin in contrast sweeps sharply upward behind the teeth and continues +posteriorly in a long, low, truncated coronoid process. + +A prominent coronoid process is not found among the more primitive +members of the suborder, such as _Limnoscelis_, although the mandible +commonly curves upward behind the tooth-row in that genus. This area in +_Limnoscelis_ is overlapped by the cheek when the jaw is fully adducted +(Romer, 1956:494, Fig. 213), thereby foreshadowing the more extreme +condition in _Captorhinus_. + +The coronoid process in _Captorhinus_ is not oriented vertically, but +slopes inward toward the midline at approximately 45 degrees, +effectively roofing the Meckelian fossa and limiting its opening to the +median surface of each ramus. When the jaw was adducted, the coronoid +process moved upward and inside the cheek. A space persisted between +the process and the cheek because the process sloped obliquely away +from the cheek and toward the midline of the skull. The external +surface of the process presented an area of attachment for muscles +arising from the apposing internal surface of the cheek. + + +_Palate_ + +The palate of _Captorhinus_ is of the generalized rhynchocephalian type +(Romer, 1956:71). In _Captorhinus_ the pterygoids and palatines are +markedly arched and the relatively large pterygoid flange lies almost +entirely below the lower border of the cheek. The lateral edge of the +flange passes obliquely across the anterior lip of the Meckelian fossa +and abuts against the bottom lip of the fossa when the jaw is closed. + +The palatines articulate laterally with the maxillary bones by means of +a groove that fits over a maxillary ridge. This presumably allowed the +halves of the palate to move up and down rather freely. The greatest +amplitude of movement was at the midline. Anteroposterior sliding of +the palate seems impossible in view of the firm palatoquadrate and +quadrate-quadratojugal articulations. + +The subtemporal fossa is essentially triangular, and its broad end is +bounded anteriorly by the pterygoid flange. The fossa is lateral to +much of the adductor chamber; consequently muscles arising from the +parietals passed ventrolaterally, parallel to the oblique quadrate +ramus of the pterygoid, to their attachment on the mandible. + + +_Musculature_ + +These osteological features indicate that the adductor muscles of the +jaw in _Captorhinus_ consisted of two primary masses (Figs. 1, 2, 3). +The first of these, the capitimandibularis, arose from the internal +surface of the cheek and roof of the skull and inserted on the bones of +the lower jaw that form the Meckelian canal and the coronoid process. + +[Illustration: FIG. 1. _Captorhinus._ Internal aspect of skull, showing +masseter, medial adductor, and temporal muscles. Unnumbered specimen, +coll. of Robert F. Clarke. Richard's Spur, Oklahoma. x 2.] + +[Illustration: FIG. 2. _Captorhinus._ Internal aspect of skull, showing +anterior and posterior pterygoid muscles. Same specimen shown in Fig. +1. x 2.] + +The muscle was probably divided into a major medial mass, the temporal, +and a lesser, sheetlike lateral mass, the masseter. The temporal was +the largest of the adductors and arose from the lateral parts of the +parietal, the dorsal parts of the postorbital, the most posterior +extent of the postfrontal, and the upper parts of the squamosal. The +muscle may have been further subdivided, but evidence for subordinate +slips is lacking. The fibers of this mass were nearly vertically +oriented in lateral aspect since the parts of the ramus that are +available for their insertion lie within the anteroposterior extent of +the adductor chamber. In anterior aspect the fibers were obliquely +oriented, since the jaw and subtemporal fossa are lateral to much of +the skull-roof from which the fibers arose. + +The masseter probably arose from the quadratojugal, the jugal, and +ventral parts of the squamosal, although scars on the quadratojugal and +jugal are lacking. The squamosal bears an indistinct, gently curved +ridge, passing upward and forward from the posteroventral corner of the +bone and paralleling the articulation of the squamosal with the +parietal. This ridge presumably marks the upper limits of the origin of +the masseter from the squamosal. + +[Illustration: FIG. 3. _Captorhinus._ Cross-section of right half of +skull immediately behind the pterygoid flange, showing masseter, +temporal, and anterior pterygoid muscles. Same specimen shown in Fig. +1. x 2.] + +[Illustration: FIG. 4. _Captorhinus._ Internal aspect of left +mandibular fragment, showing insertion of posterior pterygoid muscle. +KU 8963, Richard's Spur, Oklahoma. x 2.8.] + +The masseter inserted on the external surface of the coronoid process, +within two shallow concavities separated by an oblique ridge. The +concavities and ridge may indicate that the muscle was divided into two +sheets. If so, the anterior component was wedge-shaped in +cross-section, and its thin posterior edge overlapped the larger mass +that inserted on the posterior half of the coronoid process. + +From a functional standpoint it is doubtful that a major component of +the adductors arose from the quadrate wing of the pterygoid, for when +the jaw is closed the Meckelian fossa is directly lateral to that bone. +If the jaw were at almost any angle but maximum depression, the +greatest component of force would be mediad, pulling the rami together +and not upward. The mediad component would increase as the jaw +approached full adduction. Neither is there anatomical evidence for an +adductor arising from the quadrate wing of the pterygoid. The bone is +smooth, hard, and without any marks that might be interpreted as muscle +scars. + +The internal adductor or pterygoid musculature in _Captorhinus_ +consisted of anterior and posterior components. The anterior pterygoid +arose from the lateral edge and the dorsal surface of the pterygoid +flange. The burred dorsal recurvature of the edge resembles that of the +flange of crocodiles, which serves as part of the origin of the +anterior pterygoid in those animals. In _Captorhinus_ the attachment of +the anterior pterygoid to the edge of the flange was probably +tendinous, judging from the extent of the development of the edge of +the flange. From the edge the origin extended medially across the +dorsal surface of the flange; the ridging of this surface is +indistinct, leading to the supposition that here the origin was more +likely to have been fleshy than tendinous. + +The anterior pterygoid extended obliquely backward and downward from +its origin, passed medial to the temporal muscle and inserted on the +ventral and medial surfaces of the splenial and angular bones beneath +the Meckelian fossa. The spatial relationship between the palate and +quadrate-articular joint indicate that the muscle was probably a minor +adductor in _Captorhinus_. + +When the jaw was adducted, the insertion of the anterior pterygoid was +in a plane nearly level with the origin. Contraction of the anterior +pterygoid when the jaw was in this position pulled the mandible forward +and did not adduct it. Maximum depression of the mandible produced +maximum disparity vertically between the levels of the origin and +insertion. The force exerted by the anterior pterygoid upon the +mandible when fully lowered most nearly approached the perpendicular to +the long axes of the mandibular rami, and the resultant force acting on +the mandible was adductive. + +The adductive component of force therefore decreased as the jaw swung +upward, with the result that the anterior pterygoid could only have +been active in initiating adduction and not in sustaining it. + +The evidence regarding the position and extent of the posterior +pterygoid is more veiled. On the medial surface of the mandible, the +prearticular and articular bones meet in a ridge that ventrally rims +the glenoid cavity (Fig. 4). The ridge extends anteriorly and curves +slightly in a dorsal direction and meets the Meckelian fossa. The +curved part of the ridge is made of the prearticular bone alone. A +small hollow above the ridge, anterior to the glenoid cavity, faces the +medial plane of the skull and is bordered by the articular bone behind +and above, and by the Meckelian fossa in front. + +The surfaces of the hollow and the prearticular-articular ridge bear +tiny grooves and ridges that seem to be muscle scars. The entire area +of the hollow and its bordering features was probably the area of +insertion of the posterior pterygoid. + +However, the area of insertion lies mostly ventral to the articulating +surface of the articular bone and extends but slightly in front of it. +Seemingly little lever effect could be exercised by an adductor +attaching in this position, namely, at the level of the fulcrum of the +mandibular ramus. + +The posterior pterygoid muscle probably arose from the anterior portion +of the pterygoid wing of the quadrate, from a ridge on the ventromedial +surface. From the relationship of the muscle to the articulation of the +jaw with the skull, it may be deduced that the muscle was limited in +function to the stabilization of the quadrate-articular joint by +keeping the articular surfaces in close contact with each other and by +preventing lateral slipping. + +Finally there is evidence for an adductor between the temporal and +masseter masses. The anterior dorsal lip of the Meckelian fossa +supports a small knob to which this muscle attached, much as in +_Sphenodon_ (Romer, 1956:18, Fig. 12). Presumably the muscle was +sheetlike and attached to the skull roof, medial to the attachment of +the masseter. + +A pseudotemporal may have been present, but evidence to indicate its +extent and position is lacking. The muscle usually arises from the +epipterygoid and nearby areas of the braincase and skull roof and +inserts in the anterior parts of the fossa of the jaw. In _Captorhinus_ +the lateral wing of the pterygoid cuts across the fossa, effectively +blocking it from the upper and medial parts of the skull, the areas of +origin for the pseudotemporal. + + +Dimetrodon + +The morphology of the skull of _Dimetrodon_ closely resembles that of +the primitive _Haptodus_ (Haptodontinae, Sphenacodontidae), and "hence +may be rather confidently described as that of the family as a whole" +(Romer and Price, 1940:285). The major differences between the two +genera are in the increased specialization of the dentition, the +shortening of the lacrimal, and the development of long vertebral +spines in _Dimetrodon_. The absence of gross differences in the areas +of the skull associated with the groups of muscles with which this +study is concerned, implies a similarity in the patterns of musculature +between the two groups. Romer and Price suggest that _Haptodus_, +although too late in time to be an actual ancestor, shows "all the +common features of the _Dimetrodon_ group on the one hand and the +therapsids on the other." The adductors of the jaw of _Dimetrodon_ were +probably little changed from those of the Haptodontinae and represent a +primitive condition within the suborder. + +_Dimetrodon_ and _Captorhinus_ differ in the bones associated with the +adductor mechanism; the area behind the orbit in _Dimetrodon_ is +relatively shorter, reducing the comparative longitudinal extent of the +adductor chamber. Furthermore, the dermal roof above the adductor +chamber slopes gently downward from behind the orbit to its contact +with the occipital plate in _Dimetrodon_. Temporal fenestrae are, of +course, present in _Dimetrodon_. + + +_Musculature_ + +The adductor musculature of the lower jaw in _Dimetrodon_ was divided +into lateral and medial groups (Figs. 5, 6). The lateral division +consisted of temporal and masseter masses. The temporal arose from the +upper rim of the temporal opening, from the lateral wall of the skull +behind the postorbital strut, and from the dorsal roof of the skull. +The bones of origin included jugal, postorbital, postfrontal, parietal +and squamosal. This division may also have arisen from the fascia +covering the temporal opening (Romer and Price, 1940:53). The muscle +passed into the Meckelian fossa of the mandible and inserted on the +angular, surangular, prearticular, coronoid and dentary bones. +Insertion on the lips of the fossa also probably occurred. + +The lateral division arose from the lower rim of the temporal opening +and from the bones beneath. Insertion was in the Meckelian fossa and +on the dorsal surface of the adjoining coronoid process. + +[Illustration: FIG. 5. _Dimetrodon._ Internal aspect of skull, showing +masseter and temporal muscles. Skull modified from Romer and Price +(1940). Approx. x 1/4.] + +The reconstruction of the progressively widening masseter as it +traveled to the mandible follows from the progressively widening +depression on the internal wall of the cheek against which the muscle +must have been appressed. The depressed surface included the posterior +wing of the jugal, the whole of the squamosal, and probably the +anteriormost parts of the quadratojugal. Expansion of the muscle +rostrally was prevented by the postorbital strut that protected the +orbit (Romer and Price, 1940:53). + +The sphenacodonts possess the primitive rhynchocephalian kind of +palate. In _Sphenodon_ the anterior pterygoid muscle arises from the +dorsal surface of the pterygoid bone and from the adjacent bones. A +similar origin suggests itself for the corresponding muscle, the second +major adductor mass, in _Dimetrodon_. + +From the origin the muscle passed posterodorsad and laterad of the +pterygoid flange. Insertion was in the notch formed by the reflected +lamina of the angular, as suggested by Watson (1948). + +In _Dimetrodon_ the relationship of the dorsal surface of the palate +and the ventromedial surface of the mandible in front of the +articulation with the quadrate is unlike that in _Captorhinus_. When +the mandible of _Dimetrodon_ is at rest (adducted), a line drawn +between these two areas is oblique, between 30 and 40 degrees from the +horizontal. Depression of the mandible increases this angle. The +insertion of the anterior pterygoid is thus always considerably below +the origin, permitting the muscle to be active throughout the movement +of the mandible, from maximum depression to complete adduction. This +was a major factor in adding substantially to the speed and power of +the bite. + +The presence and extent of a posterior pterygoid is more difficult to +assess, because of the closeness of the glenoid cavity and the raised +ridge of the prearticular, and the occupancy of at least part of this +region by the anterior pterygoid. In some specimens of _Dimetrodon_ the +internal process of the articular is double (see Romer and Price, +1940:87, Fig. 16) indicating that there was a double insertion here. +Whether the double insertion implies the insertion of two separate +muscles is, of course, the problem. Division of the pterygoid into +anterior and posterior portions is the reptilian pattern (Adams, 1919), +and such is adhered to here, with the posterior pterygoid arising as a +thin sheet from the quadrate wing of the pterygoid and the quadrate, +and inserting by means of a tendon on the internal process of the +articular, next to the insertion of the anterior pterygoid. + +[Illustration: FIG. 6. _Dimetrodon._ Internal aspect of right cheek, +showing anterior and posterior pterygoid muscles. Skull modified from +Romer and Price (1940). Approx. x 1/4.] + +Watson (1948) has reconstructed the musculature of the jaw in +_Dimetrodon_ with results that are at variance with those of the +present study. Watson recognized two divisions, an inner temporal and +an outer masseteric, of the capitimandibularis, but has pictured them +(830: Fig. 4; 831: Fig. 5C) as both arising from the inner surface of +the skull roof above the temporal opening. But in _Captorhinus_ the +masseter arose from the lower part of the cheek close to the outer +surface of the coronoid process. Watson has shown (1948:860, Fig. 17B) +the same relationship of muscle to zygoma in _Kannemeyeria sp._ It is +this arrangement that is also characteristic of mammals and presumably +of _Thrinaxodon_. In view of the consistency of this pattern, I have +reconstructed the masseter as arising from the lower wall of the cheek +beneath the temporal opening. + +Watson's reconstruction shows both the temporal and masseter muscles as +being limited anteroposteriorly to an extent only slightly greater than +the anteroposterior diameter of the temporal opening. The whole of the +posterior half of the adductor chamber is unoccupied. More probably +this area was filled by muscles. The impress on the inner surface of +the cheek is evident, and the extent of both the coronoid process and +Meckelian opening beneath the rear part of the chamber indicate that +muscles passed through this area. + +Watson remarked (1948:829-830) that the Meckelian opening in +_Dimetrodon_ "is very narrow and the jaw cavity is very small. None the +less, it may have been occupied by the muscle or a ligament connected +to it. Such an insertion leaves unexplained the great dorsal production +of the dentary, surangular and coronoid. This may merely be a device to +provide great dorsal-ventral stiffness to the long jaw, but it is +possible and probable that some part of the temporal muscle was +inserted on the inner surface of the coronoid. Indeed a very +well-preserved jaw of _D. limbatus?_ (R. 105: Pl. I, Fig. 2) bears a +special depressed area on the outer surface of the extreme hinder end +of the dentary which differs in surface modelling from the rest of the +surface of the jaw, has a definite limit anteriorly, and may represent +a muscle insertion. The nature of these insertions suggests that the +muscle was already divided into two parts, an outer masseter and an +inner temporalis." But, unaccountably, Watson's illustration (1948:830, +Fig. 4) of his reconstruction limits the insertion of the temporal to +the anterior limit of the Meckelian opening and a part of the coronoid +process above it. No muscle is shown entering the Meckelian canal. It +seems more likely that the temporal entered and inserted in the canal +and on its dorsal lips. The masseter inserted lateral to it, over the +peak of the coronoid process, and overlapping onto the dorsalmost +portions of its external face, as Watson has illustrated (Plate I, +middle fig.). + +I am in agreement with Watson's reconstruction of the origins for both +the anterior and posterior pterygoid muscles. On a functional basis, +however, I would modify slightly Watson's placement of the insertions +of these muscles. Watson believed that the jaw of _Dimetrodon_ was +capable of anteroposterior sliding. The articular surfaces of the jaws +of _Dimetrodon_ that I have examined indicate that this capability, if +present at all, was surely of a very limited degree, and in no way +comparable to that of _Captorhinus_. The dentition of _Dimetrodon_ +further substantiates the movement of the jaw in a simple up and down +direction. The teeth of _Dimetrodon_ are clearly stabbing devices; they +are not modified at all for grinding and the correlative freedom of +movement of the jaw that that function requires in an animal such as +_Edaphosaurus_. Nor are they modified to parallel the teeth of +_Captorhinus_. The latter's diet is less certain, but presumably it was +insectivorous (Romer, 1928). With the requisite difference in levels of +origin and insertion of the anterior pterygoid in _Dimetrodon_ insuring +the application of force throughout the adduction of the jaws, it would +seem that the whole of the insertion should be shifted downward and +outward in the notch. If this change were made in the reconstruction, +the anterior pterygoid would have to be thought of as having arisen by +a tendon from the ridge that Watson has pictured (1948:828, Fig. 3) as +separating his origins for anterior and posterior pterygoids. The +posterior pterygoid, in turn, arose by tendons from the adjoining +lateral ridge and from the pterygoid process of Romer and Price. +Tendinous origins are indicated by the limitations of space in this +area, by the strength of the ridges pictured and reported by Watson, +and by the massiveness of the pterygoid process of Romer and Price. + + +Discussion + +A comparison of the general pattern of the adductor musculature of +_Captorhinus_ and _Dimetrodon_ reveals an expected similarity. The +evidence indicates that the lateral and medial temporal masses were +present in both genera. The anterior pterygoid aided in initiating +adduction in _Captorhinus_, whereas in _Dimetrodon_ this muscle was +adductive throughout the swing of the jaw. Evidence for the presence +and extent of a pseudotemporal muscle in both _Captorhinus_ and +_Dimetrodon_ is lacking. The posterior division of the pterygoid is +small in _Captorhinus_. In _Dimetrodon_ this muscle has been +reconstructed by Watson as a major adductor, an arrangement that is +adhered to here with but slight modification. + +The dentition of _Captorhinus_ suggests that the jaw movement in +feeding was more complex than the simple depression and adduction that +was probably characteristic of _Dimetrodon_ and supports the +osteological evidence for a relatively complex adductor mechanism. + +In _Captorhinus_ the presence of an overlapping premaxillary beak +bearing teeth that are slanted posteriorly requires that the mandible +be drawn back in order to be depressed. Conversely, during closure, the +jaw must be pulled forward to complete full adduction. The +quadrate-articular joint is flat enough to permit such anteroposterior +sliding movements. The relationship of the origin and insertion of the +anterior pterygoid indicates that this muscle, ineffective in +maintaining adduction, may well have acted to pull the mandible +forward, in back of the premaxillary beak, in the last stages of +adduction. Abrasion of the sides of the inner maxillary and outer +dentary teeth indicates that tooth-to-tooth contact did occur. Whether +such abrasion was due to contact in simple vertical adduction or in +anteroposterior sliding is impossible to determine, but the evidence +considered above indicates the latter probability. + +Similarities of _Protorothyris_ to sphenacodont pelycosaurs in the +shape of the skull and palate already commented upon by Watson (1954) +and Hotton (1961) suggest that the condition of the adductors in +_Dimetrodon_ is a retention of the primitive reptilian pattern, with +modifications mainly limited to an increase in size of the temporalis. +_Captorhinus_, however, seems to have departed rather radically from +the primitive pattern, developing specializations of the adductors that +are correlated with the flattening of the skull, the peculiar marginal +and anterior dentition, the modifications of the quadrate-articular +joint, and the development of the coronoid process. + + +Thrinaxodon + +The evidence for the position and extent of the external adductors of +the lower jaw in _Thrinaxodon_ was secured in part from dissections of +_Didelphis marsupialis_, the Virginia opossum. Moreover, comparison of +the two genera reveals striking similarities in the shape and spatial +relationships of the external adductors. These are compared below in +some detail. + +The sagittal crest in _Thrinaxodon_ is present but low. It arises +immediately in front of the pineal foramen from the confluence of +bilateral ridges that extend posteriorly and medially from the base of +the postorbital bars. The crest diverges around the foramen, reunites +immediately behind it, and continues posteriorly to its junction with +the supraoccipital crest (Estes, 1961). + +In _Didelphis_ the sagittal crest is high and dorsally convex in +lateral aspect, arising posterior to and medial to the orbits, reaching +its greatest height near the midpoint, and sloping down to its +termination at the supraoccipital crest. Two low ridges extend +posteriorly from the postorbital process to the anterior end of the +sagittal crest and correspond to ridges in similar position in +_Thrinaxodon_. + +The supraoccipital crest flares upward to a considerable extent in +_Thrinaxodon_ and slopes posteriorly from the skull-roof proper. The +crest extends on either side downward to its confluence with the +zygomatic bar. The area of the crest that is associated with the +temporal musculature is similarly shaped in _Didelphis_. + +The zygomatic bar in each genus is stout, laterally compressed, and +dorsally convex on both upper and lower margins. At the back of the +orbit of _Thrinaxodon_, the postorbital process of the jugal extends +posterodorsally. At this position in _Didelphis_, there is but a minor +upward curvature of the margin of the bar. + +In _Thrinaxodon_ the dorsal and ventral postorbital processes, arising +from the postorbital and jugal bones respectively, nearly meet but +remain separate. The orbit is not completely walled off from the +adductor chamber. The corresponding processes in _Didelphis_ are +rudimentary so that the confluence of the orbit and the adductor +chamber is complete. + +The adductor chamber dorsally occupies slightly less than half of the +total length of the skull of _Thrinaxodon_; in _Didelphis_ the dorsal +length of the chamber is approximately half of the total length of the +skull. + +[Illustration: FIG. 7. _Thrinaxodon._ Showing masseter and temporal +muscles. Skull after Romer (1956). Approx. x 7/10.] + +The coronoid process in _Thrinaxodon_ sweeps upward posterodorsally at +an angle oblique to the long axis of the ramus. Angular, surangular and +articular bones extend backward beneath and medial to the process. The +process extends above the most dorsal point of the zygomatic bar, as in +_Didelphis_. The mandibular ramus is ventrally convex in both genera. + +The relationships described above suggest that _Thrinaxodon_ and the +therapsids having similar morphology in the posterior region of the +skull possessed a temporal adductor mass that was split into major +medial and lateral components (Fig. 7). The more lateral of these, the +masseter, arose from the inner surface and lower margin of the +zygomatic bar and inserted on the lateral surface of the coronoid +process. + +The medial division or temporal arose from the sagittal crest and +supraoccipital crest and the intervening dermal roof. The muscle +inserted on the inner and outer surfaces of the coronoid process and +possibly on the bones beneath. + +_Thrinaxodon_ represents an advance beyond _Dimetrodon_ in several +respects. The zygomatic bar in _Thrinaxodon_ extends relatively far +forward, is bowed outward and dorsally arched. Consequently, the +masseter was able to extend from an anterodorsal origin to a posterior +and ventral insertion. The curvature of the jaw transforms the +anterodorsal pull of the muscle into a dorsally directed adductive +movement regardless of the initial angle of the jaw. This is the +generalized mammalian condition. + +With the development of the secondary palate the area previously +available for the origin of large anterior pterygoid muscles was +reduced. The development of the masseter extending posteroventrally +from an anterior origin presumably paralleled the reduction of the +anterior pterygoids. The therapsid masseter, as an external muscle +unhindered by the crowding of surrounding organs, was readily available +for the many modifications that have been achieved among the mammals. + +In the course of synapsid evolution leading to mammals, the temporal +presumably became the main muscle mass acting in adduction of the lower +jaw. Its primacy is reflected in the phyletic expansion of the temporal +openings to permit greater freedom of the muscles during contraction. +In the synapsids that lead to mammals, there is no similar change in +the region of the palate that can be ascribed to the effect of the +pterygoid musculature, even though these adductors, like the temporal, +primitively were subjected to severe limitations of space. + + +Didelphis + +Dissections reveal the following relationships of the external +adductors of the jaw in _Didelphis marsupialis_ (Fig. 8). + + 1. MASSETER + + Origin: ventral surface of zygomatic arch. + + Insertion: posteroventral and lateroventral surface of + mandible. + + 2. EXTERNAL TEMPORALIS Origin: sagittal crest; anteriorly + with internal temporalis from frontal bone; posteriorly with + internal temporalis from interparietal bone. + + Insertion: lateral surface of coronoid process of mandible. + + 3. INTERNAL TEMPORALIS + + Origin: sagittal crest and skull roof, including posterior + two-thirds of frontal bone, whole of parietal, and + dorsalmost portions of squamosal and alisphenoid. + + Insertion: medial surface of coronoid process; dorsal edge + of coronoid process. + +[Illustration: FIG. 8. _Didelphis marsupialis._ Showing masseter and +temporal muscles. Skull KU 3780, 1 mi. N Lawrence, Douglas Co., Kansas. +x 3/5.] + + +Temporal Openings + +In discussions of the morphology and functions of the adductor +mechanism of the lower jaw, the problem of accounting for the +appearance of temporal openings in the skull is often encountered. Two +patterns of explanation have evolved. The first has been the attempt to +ascribe to the constant action of the same selective force the openings +from their inception in primitive members of a phyletic line to their +fullest expression in terminal members. According to this theory, for +example, the synapsid opening appeared _originally_ to allow freer +expansion of the adductor muscles of the jaw during contraction, and +continued selection for that character caused the openings to expand +until the ultimately derived therapsid or mammalian condition was +achieved. + +The second course has been the attempt to explain the appearance of +temporal openings in whatever line in which they occurred by the action +of the same constant selective force. According to the reasoning of +this theory, temporal fenestration in all groups was due to the need +to decrease the total weight of the skull, and selection in all those +groups where temporal fenestration occurs was to further that end. + +Both of these routes of inquiry are inadequate. If modern views of +selection are applied to the problem of explaining the appearance of +temporal fenestrae, the possibility cannot be ignored that: + + 1. Selective pressures causing the inception of temporal + fenestrae differed from those causing the continued + expansion of the fenestrae. + + 2. The selective pressures both for the inception and + continued expansion of the fenestrae differed from group to + group. + + 3. Selection perhaps involved multiple pressures operating + concurrently. + + 4. Because of different genotypes the potential of the + temporal region to respond to selective demands varied from + group to group. + +[Illustration: FIG. 9. _Captorhinus._ Diagram, showing some +hypothetical lines of stress. Approx. x 1.] + +[Illustration: FIG. 10. _Captorhinus._ Diagram, showing areas of +internal thickening. Approx. x 1.] + +[Illustration: FIG. 11. _Captorhinus._ Diagram, showing orientation of +sculpture. Approx. x 1.] + +Secondly, the vectors of mechanical force associated with the temporal +region are complex (Fig. 9). Presumably it was toward a more efficient +mechanism to withstand these that selection on the cheek region was +operating. The simpler and more readily analyzed of these forces are: + + 1. The force exerted by the weight of the skull anterior to + the cheek and the distribution of that weight depending + upon, for example, the length of the snout in relation to + its width, and the density of the bone. + + 2. The weight of the jaw pulling down on the suspensorium + when the jaw is at rest and the compression against the + suspensorium when the jaw is adducted; the distribution of + these stresses depending upon the length and breadth of the + snout, the rigidity of the anterior symphysis, and the + extent of the quadrate-articular joint. + + 3. The magnitude and extent of the vectors of force + transmitted through the occiput from the articulation with + the vertebral column and from the pull of the axial + musculature. + + 4. The downward pull on the skull-roof by the adductor + muscles of the mandible. + + 5. The lateral push exerted against the cheek by the + expansion of the mandibular adductors during contraction. + + 6. The necessity to compensate for the weakness in the skull + caused by the orbits, particularly in those kinds of + primitive tetrapods in which the orbits are large. + +The distribution of these stresses is further complicated and modified +by such factors as: + + 1. The completeness or incompleteness of the occiput and the + location and extent of its attachment to the dermal roof. + + 2. The size and rigidity of the braincase and palate, and + the extent and rigidity of their contact with the skull. + +The stresses applied to the cheek fall into two groups. The first +includes all of those stresses that ran through and parallel to the +plane of the cheek initially. The weight of the jaw and snout, the pull +of the axial musculature, and the necessity to provide firm anchorage +for the teeth created stresses that acted in this manner. The second +group comprises those stresses that were applied initially at an +oblique angle to the cheek and not parallel to its plane. Within this +group are the stresses created by the adductors of the jaw, pulling +down and medially from the roof, and sometimes, during contraction, +pushing out against the cheek. + +It is reasonable to assume that the vectors of these stresses were +concentrated at the loci of their origin. For example, the effect of +the forces created by the articulation of the jaw upon the skull was +concentrated at the joint between the quadrate, quadratojugal, and +squamosal bones. From this relatively restricted area, the stresses +radiated out over the temporal region. Similarly, the stresses +transmitted by the occiput radiated over the cheek from the points of +articulation of the dermal roof with the occipital plate. In both of +these examples, the vectors paralleled the plane of the cheek bones. +Similar radiation from a restricted area, but of a secondary nature, +resulted from stresses applied obliquely to the plane of the cheek. The +initial stresses caused by the adductors of the jaw resulted from +muscles pulling away from the skull-roof; secondary stresses, created +at the origins of these muscles, radiated out over the cheek, parallel +to its plane. + +The result of the summation of all of those vectors was a complex grid +of intersecting lines of force passing in many directions both +parallel to the plane of the cheek and at the perpendicular or at an +angle oblique to the perpendicular to the plane of the cheek. + +Complexities are infused into this analysis with the division of +relatively undifferentiated muscles into subordinate groups. The +differentiation of the muscles was related to changing food habits, +increased mobility of the head, and increase in the freedom of movement +of the shoulder girdle and forelimbs (Olson, 1961:214). As Olson has +pointed out, this further localized the stresses to which the bone was +subjected. Additional localization of stresses was created with the +origin and development of tetrapods (reptiles) that were independent of +an aquatic environment and were subjected to greater effects of gravity +and loss of bouyancy in the migration from the aqueous environment to +the environment of air. The localization of these stresses was in the +border area of the cheek, away from its center. + +What evidence is available to support this analysis of hypothetical +forces transmitted through the fully-roofed skull of such an animal as +_Captorhinus_? + +It is axiomatic that bones or parts of bones that are subject to +increased stress become thicker, at least in part. This occurs +ontogenetically, and it occurs phylogenetically through selection. Weak +bones will not be selected for. Figure 10 illustrates the pattern of +the areas of the skull-roof in the temporal region that are marked on +the internal surface by broad, low thickened ridges. The position of +these ridges correlates well with the position of the oriented stresses +that were presumably applied to the skull of _Captorhinus_ during life. +It can be seen from Figure 10 that the central area of the cheek is +thinner than parts of the cheek that border the central area. The +thickened border areas were the regions of the cheek that were +subjected to greater stress than the thin central areas. + +External evidence of stress may also be present. The pattern of +sculpturing of _Captorhinus_ is presented in Figure 11. The longer +ridges are arranged in a definite pattern. Their position and direction +correlates well with the thickened border of the cheek, the region in +which the stresses are distinctly oriented. For example, a ridge is +present on the internal surface of the squamosal along its dorsal +border. Externally, the sculptured ridges are long and roughly +parallel, both to each other and to the internal ridge. + +The central area of the cheek is characterized by a reticulate pattern +of short ridges, without apparent orientation. The thinness of the bone +in this area indicates that stresses were less severe here. The random +pattern of the sculpture also indicates that the stresses passed in +many directions, parallel to the plane of the cheek and obliquely to +that plane. + + +_Possible Explanation for the Appearance of Temporal Openings_ + +Bone has three primary functions: support, protection and participation +in calcium metabolism. Let us assume that the requirements of calcium +metabolism affect the mass of bone that is selected for, but do not +grossly affect the morphology of the bones of that mass. Then selection +operates to meet the needs for support within the limits that are set +by the necessity to provide the protection for vital organs. After the +needs for protection are satisfied, the remaining variable and the one +most effective in determining the morphology of bones is selection for +increased efficiency in meeting stress. + +Let us also assume that bone increases in size and/or compactness in +response to selection for meeting demands of increased stress, but is +selected against when requirements for support are reduced or absent. +Selection against bone could only be effective within the limits +prescribed by the requirements for protection and calcium metabolism. + +We may therefore assume that there is conservation in selection against +characters having multiple functions. Since bone is an organ system +that plays a multiple role in the vertebrate organism, a change in the +selective pressures that affect one of the roles of bone can only be +effective within the limits set by the other roles. For example, +selection against bone that is no longer essential for support can +occur only so long as the metabolic and protective needs of the +organism provided by that character are not compromised. If a character +no longer has a positive survival value and is not linked with a +character that does have a positive survival value, then the metabolic +demands for the development and maintenance of that character no longer +have a positive survival value. A useless burden of metabolic demands +is placed upon the organism because the character no longer aids the +survival of the organism. If selection caused, for example, muscles to +migrate away from the center of the cheek, the bone that had previously +provided support for these muscles would have lost one of its +functions. If in a population of such individuals, variation in the +thickness of the bone of the cheek occurred, those with thinner bone in +the cheek would be selected for, because less metabolic activity was +diverted to building and maintaining what is now a character of reduced +functional significance. A continuation of the process would eliminate +the bone or part of the bone in question while increasing the metabolic +efficiency of the organism. The bone is no longer essential for +support, the contribution of the mass of bone to calcium metabolism and +the contribution of this part of the skeleton to protection have not +been compromised, and the available energy can be diverted to other +needs. + +The study of _Captorhinus_ has indicated that the central area of the +cheek was subjected to less stress than the border areas. A similar +condition in basal reptiles may well have been present. A continued +trend in reducing the thickness of the bone of the cheek in the manner +described above may well have resulted in the appearance of the first +reptiles with temporal fenestrae arising from the basal stock. + +Such an explanation adequately accounts for an increased selective +advantage in the step-by-step thinning of the cheek-wall prior to the +time of actual breakthrough. It is difficult to see the advantage +during such stages if explanations of weight reduction or bulging +musculature are accepted. + +After the appearance of temporal fenestrae, selection for the classical +factors is quite acceptable to explain the further development of +fenestration. The continued enlargement of the temporal fenestrae in +the pelycosaur-therapsid lineage undoubtedly was correlated with the +advantages accrued from securing greater space to allow increased +lateral expansion of contracting mandibular adductors. Similarly, +weight in absolute terms can reasonably be suggested to explain the +dramatic fenestration in the skeletons of many large dinosaurs. + + +Literature Cited + +ADAMS, L. A. + + 1919. Memoir on the phylogeny of the jaw muscles in recent + and fossil vertebrates. Annals N. Y. Acad. Sci., + 28:51-166, 8 pls. + +ESTES, R. + + 1961. Cranial anatomy of the cynodont reptile _Thrinaxodon + liorhinus_. Bull. Mus. Comp. Zool., 125(6):165-180, + 4 figs., 2 pls. + +HOTTON, N. + + 1960. The chorda tympani and middle ear as guides to origin + and development of reptiles. Evolution, 14(2):194-211, + 4 figs. + +OLSON, E. C. + + 1961. Jaw mechanisms: rhipidistians, amphibians, reptiles. + Am. Zoologist, 1(2):205-215, 7 figs. + +ROMER, A. S. + + 1928. Vertebrate faunal horizons in the Texas Permo-Carboniferous + redbeds. Univ. Texas Bull., 2801:67-108, 7 figs. + + 1956. Osteology of the reptiles. Univ. Chicago Press, xxii + + 772 pp., 248 figs. + +ROMER, A. S. and PRICE, L. I. + + 1940. Review of the Pelycosauria. Geol. Soc. Amer. Special + Papers, No. 28, x + 538 pp., 71 figs., 46 pls. + +WATSON, D. M. S. + + 1948. _Dicynodon_ and its allies. Proc. Zool. Soc. London, + 118:823-877, 20 figs., 1 pl. + + 1954. On _Bolosaurus_ and the origin and classification of + reptiles. Bull. Mus. Comp. Zool., 111(9):200-449, + 37 figs. + + _Transmitted December 5, 1963._ + + +30-1522 + + + + + + + + +End of the Project Gutenberg EBook of The Adductor Muscles of the Jaw In +Some Primitive Reptiles, by Richard C. 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