Evolution of Central Auditory Pathways in Anamniotes

Abstract

An inner ear containing multiple sensory endorgans arose early in vertebrate history. Among jawed vertebrates the ear includes seven endorgans believed to be primitive for jawed fishes (otolithic endorgans: utricle, saccule, lagena; macula neglecta; three semicircular canals) as well as the various derived papular endorgans of anamniotes (amphibian and basilar papillae) and amniotes (basilar papilla or cochlear duct). What is remarkable about this collection of endorgans is that, with the exception of the semicircular canal cristae, each of the remaining endorgans has been implicated in hearing in one species or another. The classic notion that the otolithic endorgans do not contribute to hearing in land vertebrates has been recently disproven in amphibians (reviewed in Lewis et al. 1985), a discovery that challenges us to reexamine otolithic endorgan function in other vertebrates. Moreover, it has also been recently claimed that the evolution of the papillar endorgans, at least of anamniotes, predates the emergence of vertebrates onto land (Fritzsch 1987, Chapter 18), a view that requires us to reconsider the selective pressures that influenced the appearance of acoustic receptors.