Caecilians have evolved a unique, dual jaw-closing mechanism that consists of the ancestral component in which the M. adductor mandibulae pulls up on the lower jaw in the manner of a third-order lever, and a novel component in which the M. interhyoideus posterior pulls back and down on the retroarticular process of the lower jaw causing the anterior jaw ramus to pivot upward around the quadrate, as in a first-order lever. The novel component involves the acquisition of a new function for the M. interhyoideus posterior, which ancestrally is a ventral constrictor. The novel component is relatively poorly developed in rhinatrematids, a family of caecilians that is otherwise relatively primitive and is characterized by skulls with temporal fossae through which the two M. adductor mandibulae extend. The novel component is better developed in the other four caecilian families, all of which are thought to be relatively derived groups, and the members of which mostly have solidly roofed (stegokrotaphic) skulls. The skull of ancestral caecilians is thought to have had temporal fossae, which later became closed as the novel component of jaw-closure became better developed, perhaps in response to selection pressure for burrowing specialization. If this sequence is correct, then the stegokrotaphic skull of caecilians is secondarily derived, and the hypothesis that caecilians arose from Goniorhynchus-like microsaurs with stegokrotaphic skulls is incorrect. Lysorophid microsaurs, which have skulls with open temporal regions, are more likely ancestors of caecilians.