Wilga, C.D., R.E. Hueter P.C. Wainwright and P.J. MOTTA. Evolution Of Upper Jaw Protrusion Mechanisms In Elasmobranchs. American Zooloigist, In Review. From the Symposium Evolution of feeding motor patterns in vertebrates to be presented at the Annual Meeting of the Society for Integrative and Comparative Biology, January 2001, at Chicago, Illinois.

Upper jaw protrusion is an important component of the feeding mechanism in most elasmobranchs. In this paper, we review what is known of muscle activity during prey capture, particularly that of upper jaw protrusion, in elasmobranchs and evaluate the extent to which evolutionary modifications have come about through changes in anatomy or patterns of muscle activity. To date motor activity has been documented in only four species of elasmobranchs, although they represent the three major elasmobranch groups: Galea (typical sharks); Squalea (dogfish sharks); and Batoidea (skates and rays). Our efforts show that while certain muscles show a common pattern of motor activity among species during feeding, that of cranial elevation and lower jaw depression and retraction, other muscles are functionally and evolutionarily plastic, those involved with upper jaw protrusion and retraction. Our observations of elasmobranch upper jaw protrusion mechanisms suggests a mosaic of character changes over the course of evolution that involve anatomical changes in all cases and modifications of muscle activation patterns in some cases. The muscles involved in upper jaw retraction also show modulation in some species and may be active during jaw opening as well as in jaw retraction. The primary jaw closing muscle in a durophagous shark exhibits an extended double-burst pattern of motor activity during crushing of hard-shelled prey. Within the evolution of feeding mechanisms of elasmobranchs, there are several structural changes that retain a conserved motor pattern and behavior. At least one instance of structural modification is accompanied by an alteration in the motor pattern which led to a change in behavior. Finally, several instances of plasticity in activation of certain muscles exist. Despite numerous hypotheses and many studies on the mechanics of upper jaw protrusion in elasmobranchs, the functions still elude us. At least one function appears to be decreasing the time necessary for closing of the mouth, which may affect prey capture.