Evolution of upper jaw protrusion mechanisms in elasmobranchs

Abstract

SYNOPSIS. Upper jaw protrusion is a prominent component of the feeding mech-anism 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 evolved through changes in anat-omy 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 in-volved in cranial elevation and lower jaw depression and elevation show a con-served pattern of motor activity and function across species, other muscles show a more variable history. Our observations of elasmobranch upper jaw protrusion mechanisms suggest 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 elasmo-branchs, there have been two structural changes incorporating a pre-existing mo-tor pattern to yield an unmodified kinematic profile, the original preorbitalis and the descendent preorbitalis. One additional instance of structural modification is accompanied by an alteration in the motor pattern leading to a change in move-ment pattern, the levator palatoquadrati