Wilga, C.D., R.E. Hueter, P.C. Wainwright and P.J. Motta. 2002. Evolution of upper jaw protrusion mechanisms in elasmobranchs. American Zoologist, 41:1248-1257.

Upper jaw protrusion is a prominent component of the feeding mechanism in most elasmobranchs and has received considerable attention over the years. In this paper, we review what is known of muscle activity during prey capture in elasmobranchs, particularly that of upper jaw protrusion, and evaluate the extent to which functional modifications have come about through changes in anatomy or patterns of muscle activity. To date, motor activity during feeding 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 muscles involved in cranial elevation, lower jaw depression and elevation show a conserved pattern of motor activity and function across species, other muscles show a more variable history. 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. During the evolution of feeding mechanisms of elasmobranchs, there have been several structural changes that incorporate  a pre-existing motor pattern to yield an unmodified kinematic profile. At least one instance of structural modification is accompanied by an alteration in the motor pattern leading to a change in movement pattern.