The evolution of cranial design and performance in squamates: Consequences of skull-bone reduction on feeding behavior

Abstract

The evolution of cranial design in lepidosaurians is characterized by a general trend toward the loss of cranial elements. The evolution of relatively lighter skulls in squamates appears tightly coupled to a reduction in relative mass of the jaw adductor, implying functional consequences for bite force and feeding behavior. Interestingly, among squamates the postorbital bar was reduced or lost at least twice independently and taxa characterized by the loss of these cranial elements (e.g., geckos and varanids) are generally reported as having a mobile skull. In Gekkotans, the loss of the postorbital bar was accompanied by a reduction of the supratemporal bar, resulting in a pronounced cranial kinesis. Our data show that having a kinetic skull has functional consequences and results in a reduction in bite force. The lower bite force may in turn be responsible for the decreased feeding efficiency as reflected in the longer duration of intra-oral transport cycles. Gekkotans, however, appear to exploit their intracranial mobility in ways that increases the velocity of jaw movement during opening and closing, which may allow them to capture more elusive prey. The morphological changes observed in the evolution of the cranial system in squamates appear tightly linked to functional and constructional demands on the skull, making squamate skull evolution a model system to investigate the consequences of morphological changes in a complex integrated system of performance, behavior, and ecology.