Wilga, C.D. A functional analysis of jaw suspension in elasmobranchs. In Review.

The morphology of the jaw suspension, upper jaw, and hyoid is quantified and related to upper jaw protrusion in five elasmobranchs possessing four divergent jaw suspension types: 1) broadnose sevengill shark, Notorynchus cepedianus, orbitostylic with postorbital articulation; 2) spiny dogfish, Squalus acanthias, orbitostylic without postorbital articulation; 3) bonnethead shark, Sphyrna tiburo, and 4) lemon shark, Negaprion brevirostris, both hyostylic; and 5) Atlantic guitarfish, Rhinobatos lentiginosus, euhyostylic. The results of this study indicates that jaw suspension type is not a good predictor of jaw mobility as measured by upper jaw protrusion. Instead, the best morphological predictor of jaw mobility appears to be either a long ethmopalatine ligament or no ligament, both of which allow the upper jaw to move farther away from the cranium. The morphology of the palatoquadrate-cranial connections are critical in determining the mobility of the jaws and indicates that descriptions of palatoquadrate morphology should be included in descriptions of jaw suspension states. Protrusion of the upper jaw significantly reduces the mouth closing distance during feeding in those elasmobranchs with relatively greater upper jaw protrusion. Therefore, one advantage of upper jaw protrusion for feeding appears to be a reduction in the time to jaw closure by closing the mouth dorsally by upper jaw protrusion as well as ventrally by lower jaw elevation. A table of jaw suspension types is provided to illustrate the phylogenetic differences among gnathostome groups. Jaw suspension types mapped onto a gnathostome phylogeny supports the evolution of holostyly and hyostyly from an autodiastylic ancestral condition.