Evolution of jaw depression mechanics in aquatic vertebrates: insights from Ghondrichthyes

The widely accepted phylogenctic position of Chondrichthyes as the sister group to all other living gnathostomes makes biomechanical analyses of this group of special significance for estimates of skull function in early jawed vertebrates. We review key findings of recent experimental research on the feeding mechanisms of living elasmobranchs with respect to our understanding of jaw depression mechanisms in gnathostome vertebrates. We introduce the possibility that the ancestral jaw depression mechanism in gnathostomes was mediated by the coracomandibularis muscle and that for hyoid depression by the coracohyoideus muscle, as in modern Chondrichthyes and possibly placoderms. This mechanism of jaw depression appears to have been replaced by the sternohyoideus (homologous to the coracohyoideus) coupling in Osteichthycs following the split of this lineage from Chondrichthyes. Concurrent with the replacement of the branchiomandibularis (homologous to the coracomandibularis) coupling by the sternohyoideus coupling as the dominant mechanism of jaw depression in Osteichthyes was the fusion and shift in attachment of the intcrhyoideus and intermandibularis muscles (producing the protractor hyoideus muscle, mistakenly refereed to as the geniohyoideus), which resulted in a more diversified role of the sternohyoideus coupling in Osteichthyes. The coracohyoideus coupling appears to have been already present in vertebrates where it functioned in hyoid depression, as in modern Chondrichthyes, before it acquired the additional role of jaw depression in Osteichthyes.