Amphibia: A Case of Diversity and Convergence in the Auditory Region

Abstract

The ears of extant amphibians are remarkably diverse and when fossil taxa are considered, the picture becomes even more complicated. Anurans have a differentiated stapes inside a middle ear cavity associated with a eustachian tube and tympanum. Instead, salamanders and caecilians have rudimentary stapes connected to the cheek or jaw articulation, and they lack the tympanum and middle ear cavity. At the same time, batrachians (salamanders and frogs) share a second ear ossicle, the batrachian operculum, whereas all lissamphibians have a second receptor in the inner ear, the amphibian papilla. The largest fossil clade and probable stem group of Lissamphibia, the temnospondyls, had a stapes similar to that of anurans, consistent in the possession of a ventral process and an elongate and slender distal shaft that probably attached to a tympanum. The evolutionary sequence of ear types forms a puzzle with several of the major groups each sharing features that others lack. The primitive condition is exemplified by the temnospondyl ear, especially that of dissorophoids. We argue that the loss of the tympanic system was an evolutionary option only available after the batrachian operculum had evolved.