Static biting in lizards: functional morphology of the temporal ligaments

In lizards, the basic lepidosaurian bauplan of the skull is modified by the reduction of one or more skull elements such as the lower temporal bar (e.g. iguanids), the upper temporal bar (e.g. varanids) and the postorbital bar (e.g. geckos). Skull modifications in Plocederma stellio, Uromastix acanthinurus, Corucia zebrata and Tiliqua scincoides are restricted to the reduction of the lower temporal arcade (the plesiomorph condition for lizards; i.e. possessing a ‘basic’ skull). In spite of the potential mobility (streptostyly) of the quadrate, none of these species shows streptostylic movements during the static power phase of biting. It is assumed that the temporal ligaments fulfil the role of the lower temporal arcade in stabilizing the quadrate. This function is supported by the results of mathematical simulations, which demonstrate that static biting loads the ligament in tension. Apparently, these basic lizard skulls show a shift of the attachment site of the temporal ligament from the quadrate to the lower jaw. This does not affect its quadrate-stabilizing function. However, analyses of forces occurring in the quadratomandibular joint suggest that skull configurations with a jugomandibular ligament confine the possible orientations of the joint forces to a narrow range, more aligned with the quadrate. In this way, optimization of the joint morphology to resist loads imposed by biting might more easily be reached. This is assumed to be important for lizards with a robust skull built for forceful biting.