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 a.s.sociated 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 a.s.sistance 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 inst.i.tution. I am grateful to Mr. Robert F. Clarke, a.s.sistant 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 ill.u.s.trations.

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 pos...o...b..tal 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 dent.i.tion of _Captorhinus_.

The adductor chambers of _Captorhinus_ are large. They are covered dorsally and laterally by the parietal, squamosal, postfrontal, pos...o...b..tal, 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 pa.s.s 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 a.s.sociated 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 s.p.a.ce 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 f.l.a.n.g.e lies almost entirely below the lower border of the cheek. The lateral edge of the f.l.a.n.g.e pa.s.ses 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 f.l.a.n.g.e. The fossa is lateral to much of the adductor chamber; consequently muscles arising from the parietals pa.s.sed 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 ma.s.ses (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 ca.n.a.l and the coronoid process.

[Ill.u.s.tration: FIG. 1. _Captorhinus._ Internal aspect of skull, showing ma.s.seter, medial adductor, and temporal muscles. Unnumbered specimen, coll. of Robert F. Clarke. Richard's Spur, Oklahoma. 2.]

[Ill.u.s.tration: FIG. 2. _Captorhinus._ Internal aspect of skull, showing anterior and posterior pterygoid muscles. Same specimen shown in Fig.

1. 2.]

The muscle was probably divided into a major medial ma.s.s, the temporal, and a lesser, sheetlike lateral ma.s.s, the ma.s.seter. The temporal was the largest of the adductors and arose from the lateral parts of the parietal, the dorsal parts of the pos...o...b..tal, 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 ma.s.s 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 ma.s.seter 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, pa.s.sing 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 ma.s.seter from the squamosal.

[Ill.u.s.tration: FIG. 3. _Captorhinus._ Cross-section of right half of skull immediately behind the pterygoid f.l.a.n.g.e, showing ma.s.seter, temporal, and anterior pterygoid muscles. Same specimen shown in Fig.

1. 2.]

[Ill.u.s.tration: FIG. 4. _Captorhinus._ Internal aspect of left mandibular fragment, showing insertion of posterior pterygoid muscle.

KU 8963, Richard's Spur, Oklahoma. 2.8.]

The ma.s.seter 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 ma.s.s 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 f.l.a.n.g.e. The burred dorsal recurvature of the edge resembles that of the f.l.a.n.g.e 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 f.l.a.n.g.e was probably tendinous, judging from the extent of the development of the edge of the f.l.a.n.g.e. From the edge the origin extended medially across the dorsal surface of the f.l.a.n.g.e; 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, pa.s.sed 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 ma.s.seter ma.s.ses. The anterior dorsal lip of the Meckelian fossa supports a small k.n.o.b 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 ma.s.seter.

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 dent.i.tion, 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 a.s.sociated 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 a.s.sociated 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_.