A contribution to the morphology of visceral jaw-opening muscles of urodeles (Amphibia: Caudata)

The morphology and probable evolution of the depressor muscles of the lower jaw, innervated by the seventh cranial nerve (VII;M. Ceratomandibularis, M. Depressor Mandibulae), in larval, neotenic, and metamorphosed Urodela were investigated. Observations on innervation and skeletal connections of the muscles were included, especially the relations of form and extension of the squamosal to proximal attachments of the M. Depressor Mandibulae. The M. Ceratomandibularis is purely premetamorphic, but present in the Amphiumidae. Concerning the M. Depressor Mandibulae, homologies of pre- and postmetamorphic muscles were evaluated, including the "M. Levator Hyoidei" of Dipnoi, Sirenidae, and other Tetrapoda, which is the homologue of the posterior M. Depressor Mandibulae of salamanders. The concept of "M. Depressor Mandibulae Anterior" and "M. Depressor Mandibulae Posterior" (Baird ['51] Univ. Kansas Sci. Bull. 34:221-265) was extended to nonbolitoglossine salamanders. The probable ancestral state of the M. Depressor Mandibulae for urodeles and possible plesiomorphic and apomorphic conditions obtaining in various taxa of different ranks were determined. In an attempt to reconcile results with previously published hypotheses of certain sister-group relationships by various authors, holophyly of (Dicamptodontidae + Ambystomatidae), Proteidae, and Bolitoglossini is supported. No synapomorphies for the taxa (Sirenidae + Salamandridae) or (Amphiumidae + Plethodontidae) emerge from the present study. J Morphol 233:77-97, 1997.