THE PROBLEM OF THE FUNCTIONAL SIGNIFICANCE OF CRANIAL KINESIS 



The phenomenon of cranial kinesis has been known for about 1 50 years, but one problem has 

 not been finally solved--what is the functional significance of this wide-spread complex adaptation 

 in vertebrates? In general, all are agreed that kinetism is correlated with feeding and chiefly 

 with the capture of food. But, in regard to the definite value of cranial kinesis, there are 

 several di fferent hypotheses. 



Frazzetta convincingly showed the inadequacy of Versluys' hypothesis, according to which cranial 

 kinesis served as a means of increasing the size of the gape of predatory animals, through a 

 greater spread between the upper and lower jaws. In the monokinetic metakinetic cranium, pro- 

 traction does not change the vertical range of the gape, and during amphikinesis or in the monokinetic- 

 mesokinetic skull, it decreases the gape. 



--Fig. 7- 



Bradley's hypothesis is also untenable; it proposed that kinesis, during the opening of the 

 mouth for grasping prey, permits adduction of the palatine elements for holding the prey between 

 them. The evolution of lizards has gone particularly in the direction of increasing the width of 

 the i n ter-pterygo i deal cavity IdepressionJ and the reduct'or. of the palatal teeth, that is, in a 

 direction diametrically opposed to that which would have been expected if such a kinetic function 

 were present. In addition, the protractor muscles are comparatively weak and cannot produce the 

 considerable compressive force between the left and right elements of the palatine arch necessary 

 for holding of prey between them. 



Romer's hypothesis on the amort izat ional role of kinesis ( 1937 ) on * ne impact of jaws cannot 

 be discarded so decisively. Frazzetta was inclined to display skepticism as regards this hypothesis, 

 since in the capacity of amortizators in the kinetic cranium, one must include the contraction of 

 the retractor muscles, i.e., the above-noted jaw adductors. The flexibility of the cranial bones 

 in the lower forms of lizards might, according to Frazzetta, with the same success play the 

 amort i zat ional role without the aid of the complex structure provided by kinesis. But, skull 

 kinesis is strongly developed even in small lizards. We can add that from the point of view of this 

 hypothesis incomprehensible is the role of the protractor muscles according to which, during the 

 opening of the mouth (before the clamping of the jaws) the maxillary segment of the skull is 

 elevated and thus bending the spring at the end. 



According to Frazzetta's hypothesis, cranial kinesis serves for attaining the simultaneous 

 closing of the jaws in seizing the prey. The gape is oriented so that the rising lower jaw and 

 lowering upper jaw are simultaneously applied to the prey, by which means the risk of the prey's 

 escape is lessened, as compared with the condition in the akinetic skull, where the prey is "caught' 

 by the lower jaw only. It seems to us, however, that Frazzetta lost sight of the fact that the 

 orientation of the jaws in relationship to seizing the object is not attained by kinet ic move- 

 ments, but by movements of the neck and by turning the head at the occ i p i to-cerv ical joint, i.e., 

 movements which take place also in the akinetic cranium. The simultaneity (or near simultaneity) 

 of contact with the prey by the upper and lower jaws might be attained in the akinetic skull by the 

 lowering of the head simultaneously with the raising of the lower jaw. It is unlikely in predators 

 with an akinetic skull (and of such there are very many, discounting the mammals), 



--p. 1407 — 



that, after contact of the prey by the upper and lower jaws, sufficient time elapses for the 

 escape of the prey. Even in rushing at concealed prey from ambush a predator discloses itself 

 sooner than its jaws (both together or one of them a split second earlier) grasp the prey. It 

 seems to us that kinesis is not of any essential benefit either in gaining a "moment of surprise" 

 in seizing prey nor for the catching of prey in general. 



In movements of the kinetic skull, the 

 and retraction is significant. The protrac 

 retractors are powerful jaw adductor muscle 

 load should likely be retraction. Strange 

 most scientists have sought the functional 

 hinted at the functional significance of th 

 the beak (according to his opinion) was onl 

 upper jaw. Kinesis, according to Marinelli 

 the place of principal pressure by the jaws 

 corner of the mouth. 



