Intracranial joint movements in the Agamid lizard Amphibolurus barbatus

Abstract

Movement at the quadrate‐articular, quadrate‐squamosal, frontoparietal, and basipterygoid joints of the lizard Amphibolurus barbatus were measured from biplanar cinefluorographs taken at 60 fps while the animals were feeding on crickets. Resolution of the cinefluorographs was enhanced by placing tantulum pins into the lizard's skull at selected sites. The X, Y, and Z coordinates of the pins in each film frame were recorded using a computer assisted digitizing tablet. A computer program then calculated for each frame the angular position of the skeletal elements of each of the four joints.The results from 29 chewing cycles show that in Amphibolurus: 1) During the opening and closing phases of most chewing cycles quadrate movement is negatively correlated with movement at the mandibular joint. Thus, as the jaws open, the quadrate rotates anteriorly tending to reduce gape and protracting the lower jaw. 2) Movement of the quadrate is independent of movement at the other intracranial joints. 3) The amount of movement at the frontoparietal joint is inconsistent with Frazzetta's ('62) model of lizard cranial kinesis. 4) In many chewing cycles, the fleshy tongue holds the prey against the upper jaws as the lower jaw is depressed. 5) During the stationary phase the quadrate tends to rotate anteriorly.These results suggest that in Amphibolurus quadrate movement serves two functions: 1) food transport during the opening and closing phases as suggested by Robinson ('66), and 2) during the stationary phase the movement serves to increase mechanical advantage of the pterygoideus muscle as suggested by Smith ('80).