Variation in the position of the jugal medial ridge among lizards (reptilia: squamata): its functional and taxonomic significance.

The course of the medial ridge in the lizard jugal shows considerable morphological variation. There are four basic configurations: (1) the medial ridge is located ventral to mid-height on the suborbital process and anterior to mid-length on the postorbital process; (2) the medial ridge is located ventrally on the suborbital process (as above), but posteriorly on the postorbital process; (3) the medial ridge is located dorsally on the suborbital process and anteriorly on the postorbital process; and (4) the medial ridge is centrally located along the entire length of the jugal. Ancestral character state reconstruction shows that type 1 is plesiomorphic for Squamata regardless of the broad-scale phylogenetic topology. Type 3 is present in chamaeleonids and convergently in Anolis barbatus. Type 3 is a synapomorphy of the chamaeleonids. Type 2 is considered plesiomorphic for Anguidae, Heloderma and Xenosaurus, although it is independently modified in some extant members. These taxa form a clade in molecular phylogenies of Squamata, and the course of the medial ridge of the jugal therefore provides some measure of morphological support for this arrangement. The course of the medial ridge may be best explained by the position of the eye and by the angle of the jugal; its relations with other bony orbital structures (supraocular osteoderms, palpebral, supraorbital flanges) and the posterior extent of the maxilla are also discussed.