Meeting Abstract

P1.98  Thursday, Jan. 3  Mechanical factors in the evolution of chondrichthyan jaw suspension mechanisms HUBER, D.R.*; MOTTA, P.J.; University of Tampa; University of South Florida dhuber@ut.edu

Jaw suspension mechanisms vary greatly in cartilaginous (chondrichthyan) fishes. In the two extant lineages of chondrichthyans the upper jaw is either fused to the cranium (autostyly in holocephalans) or articulates indirectly with the posterior cranium via hyomandibular cartilages, accompanied by various anterior cranio-palatine articulations (amphistyly, euhyostyly, hyostyly, and orbitostyly in elasmobranchs). The evolutionary trend towards enhanced jaw kinesis in elasmobranchs was facilitated by reduction in size and number of these articulations. Jaw suspension mechanics were biomechanically modeled in an autostylic holocephalan, amphistylic elasmobranch, and three hyostylic elasmobranchs. The jaw joints of each switched from compressive to tensile loading when mechanical advantage exceeded 1.0. Suspensorial loading was substantially higher in hyostylic (anterior = 37 N, hyomandibular = 22 N) than in amphistylic elasmobranchs (anterior = 5 N, hyomandibular = 1 N), owing to higher muscular forces acting between the jaws and cranium. For species in which the anterior articulation remains intact during biting, this articulation is loaded in compression while the hyomandibulae are in tension. A reversal of loading regime occurs when the jaws protrude far enough to dissociate the anterior cranio-palatine articulation, as in some galeoid and batoid elasmobranchs. This suggests that as elasmobranchs have evolved from amphistyly to euhyostyly, hyostyly, and orbitostyly, suspensorial loading increased and the hyomandibulae changed from tensile to compressive elements to support the highly kinetic jaws. Alternatively, autostyly in holocephalans provides an akinetic, high leverage feeding mechanism for processing hard prey.