THE SEA OTTER IN THE EASTERN PACIFIC OCEAN 43 



other carnivores. Two features of the temporalis, however, are noteworthy. 

 The superficial temporalis in most other carnivores arises from the entire 

 undersurf ace of the superficial aponeurosis and is not confined to the anterior 

 part as in Enhydra. Of the carnivores I have examined the only two that are 

 like Enhydra in this respect are the mink and river otter. 



The inferior head of the superficial temporalis is well developed in 

 Enhydra and distinct from the superficial head. In other carnivores this 

 muscle seems relatively smaller and blends with the superior head. The func- 

 tional significance of these differences is not clear. 



MANDIBULAR JOINTS 



Most carnivores have three jaw joints. The two temporomandibular joints 

 are obvious, but the third, the symphyseal joint has escaped the serious at- 

 tention of comparative morphologists. My studies indicate that the latter joint 

 has crucial functions in carnivore jaw mechanisms. 



The morphology of the temporomandibular joints in carnivores is distinctive. 

 The condyle of the lower jaw takes the form of a transversely oriented 

 cylinder that articulates more or less snugly in a trough-like glenoid fossa. 

 The temporomandibular joint of Enhydra does not appear significantly differ- 

 ent from that of other carnivores. 



The symphyseal joint of carnivores is more varied. This joint consistently 

 shows high mobility (i.e., high mobility for carnivores) in those large 

 carnivores that are powerful crushers. The mobile type of symphysis typically 

 has relatively flat articulating plates that are separated by a well-developed 

 fibrocartilagenous cushion along their anterosuperior borders. The symphysis 

 of Enhydra fits this model. Also in Enhydra, the articulating plates are 

 bound together by stout cruciate ligaments below and behind the fibrocartilage. 



The mobile type of symphysis seems to function to facilitate fitting the 

 teeth to the object that is to be crushed and the fibrocartilage cushion as a 

 shock absorber to reduce the biting force that is transmitted to the skull when 

 the resistance of the object is overcome (Scapino, 1965). The cruciate liga- 

 ments allow symphyseal movement, but stabilize the joint by preventing 

 direct lateral separation of the articulating plates. 



The teeth and symphysis of Enhydra appear to be primarily adapted for 

 crushing hard objects. The sea otter easily cracks the shells of mollusks with 

 its posterior teeth (Kirkpatrick et al, 1955). The combination of a loose 

 symphysis with bunodont teeth in this animal is also observed in other large 

 carnivores that are crushers. 



DENTITION 



The teeth of the sea otter, particularly the molars, are flattened 

 and rounded. They are not adapted to cut or shear flesh but to 

 crush invertebrates, the preferred food. Even the canines are 

 rounded, having a blunt point and no sharp edge (fig. 21). Be- 

 cause these are used to open the valves of such organisms as the 

 rock oyster (Pododesmus) the tips are often v^orn or broken. 

 Fisher (1941b) described the dentition of the sea otter on the 

 basis of fragmentary material (particularly from younger ani- 



