Richard C. Fox

“ 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. ”

“ 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 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. ”

“ 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. ”

“ 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 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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 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.
Fig. 7. Thrinaxodon. Showing masseter and temporal muscles. Skull after Romer (1956) . Approx. × 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 [Pg 673] medial to the process. ”

“ According to the reasoning of this theory, temporal fenestration in all groups was [Pg 675] 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. ”