. Bulletin. Natural history; Natuurlijke historie. MAMMALIAN ORDER TAENIODONTA 137 ^ M genioglossus. Fig. 48.—Continued. See legend on p. 135. vorelike) condition. In Stylinodon, as in Ailuropoda (Davis 1964, p. 51), the glenoid fossa is shallow and somewhat expanded transversely to allow some side- to-side motion of the teeth, whereas the postglenoid processes are medially set, allowing the condyles to pivot around them laterally (such that, when pivoted, the sagittal axis of the mandible was at an acute angle to the sagittal axis of the skull). However, these adaptations in Stylinodon appear
. Bulletin. Natural history; Natuurlijke historie. MAMMALIAN ORDER TAENIODONTA 137 ^ M genioglossus. Fig. 48.—Continued. See legend on p. 135. vorelike) condition. In Stylinodon, as in Ailuropoda (Davis 1964, p. 51), the glenoid fossa is shallow and somewhat expanded transversely to allow some side- to-side motion of the teeth, whereas the postglenoid processes are medially set, allowing the condyles to pivot around them laterally (such that, when pivoted, the sagittal axis of the mandible was at an acute angle to the sagittal axis of the skull). However, these adaptations in Stylinodon appear to be poorly developed and perhaps were fairly inefficient. The biomechanics of the jaw-closing mechanism of Stylinodon can be analyzed using the bifulcral model of Bramble (1978). In this model (Fig. 50), the bite point is considered an independent occlusal fulcrum along with the traditional joint fulcrum, and under analysis the lower jaw is considered to rotate about this point as well as about the craniomandibular joint. Thus, there are vertical ro- tational forces at the bite point (B) and also secondary rotational forces at the condyle of the jaw (r). Whereas the forces applied at the bite point by the jaw musculature are always positive (, they drive the jaws together), at the cranio- mandibular joint they may be positive (, driving the condyle against the glenoid fossa), negative (driving the condyle away from the glenoid fossa) or zero. There are also horizontal translational components to the forces generated by the jaw- closing muscle groups. These forces may be positive (driving the mandible an- teriorly), negative (driving the mandible posteriorly) or zero. The external morphology of the craniomandibular joint in Stylinodon consists of a relatively heavy bony roof vertically above the mandibular condyle and a relatively small postglenoid process. There is no postglenoid "hook," as found in some carnivores (see Bramble 1978) and used to passively
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