The opercular apparatus of salamanders

The primitive structure of the salamander otic capsule (exemplified by Hynobius ) includes both operculum and columella in the fenestra vestibuli and connection of operculum with suprascapsular cartilage by an “opercularis” muscle formed by levator scapulae . Larvae of Hynobiidae, Ambystomatidae, and Salamandridae have early development of columella and stilus, which are used in sound transmission via the suspensorium until the development of the operculum and its muscular connection with the shoulder girdle at metamorphosis. Hynobiids and ambystomatids show parallel trends toward reduction or fusion of the operculum and widening of the attachment area of the levator scapulae on the suprascapular cartilage, and some members of both families have lost the operculum. The Salamandridae have lost columella and stilus as adult structures, but extend the attachment of levator scapulae ventrally by a division resulting in attachment to the scapulocoracoid, probably as an adaptation for better reception and transmission of sound vibrations in terrestrial environments. The Plethodontidae differ strikingly in otic structure from the other families in having an operculum present in the larval stage and fused to columella, and in utilization of part of the cucullaris major as an “opercularis” muscle. These changes are probably the result of changes in orientation of the fenestra vestibuli . In plethodontids with aquatic larval stages the columella is retained as a sound‐transmitting element, but in many specialized forms it is reduced or absent. Close correlation of otic structures with ecology limits their use as evolutionary indicators in salamanders. However, the general pattern of otic structures in salamanders, and the magnitude of differences in this region seen in plethodontids, indicates that the latter are probably not derived from the salamandrids, but from a primitive salamander stock.