SUMMERS, A.P.; WAKE, M.H.; Univ. of California, Irvine; Univ. of California, Berkeley : Modeling the caecilian jaw: the importance of streptostyly and escaping lever constraints.

Caecilians are elongate, limbless amphibians with many adaptations for burrowing. They typically are sit-and-wait predators with a strong bite mechanism. They have extremely well ossified skulls. Nevertheless, the quadrate maintains a mobile connection with the rest of the skull, a condition called streptostyly. Streptostyly in reptiles is associated with increasing the gape and/or changing the effective angle of the jaw adductors. Caecilian quadrates do not move enough to effectively change the gape and the adductor muscles are small and poorly placed for force production. A major jaw closing muscle in caecilians is the interhyoideus posterior (IHP), a transverse muscle used in buccal respiration in other amphibians, which acts by pulling posteriorly at its insertion on an enlarged retroarticular process of the lower jaw. We propose a model of jaw function in which the force of the IHP is multiplied by the quadrate-pseudodentary linkage. This model requires streptostyly in order to multiply the IHP force by a factor of more than one. The model predicts that quadrate angle will have a very large effect on the force of jaw closure. The effect of changing angle and length of the retroarcticular process is far smaller than the effect of changing quadrate angle. Measured values from cleared and stained specimens indicate amplifications ranging from 2X ? 7X the IHP input force. There is a two-fold variation across ontogenetic stages of Dermophis mexicanus and a two-fold variation across three members of the genus Scolecomorphus.