FRITZSCH, B; Creighton Univ., Omaha: Being all ears: The regressive and progressive evolution of the vertebrate ear.

The vertebrate ear shows a puzzling change in shape, number and distribution of sensory organs, pattern of innervation. Some of these features are associated with distinct vertebrate taxa and provide strong evidence for specific evolutionary affinities of disputed groups. One of those issues is the taxonomic position of sarcopterygian fishes with respect to tetrapods. Molecular data have been arguing for and against either Latimeria or lungfish to be the sister taxon of tetrapods. In the absence of conclusive molecular evidence it is worthwhile to reexamine the otic structures to provide further support for either relationship. The ear of Latimeria and lungfish has three canal cristae, a utricle, a saccule and a papilla neglecta. Lungfish utricles are in their own recess, like in elasmobranchs, whereas Latimeria has a utricle like tetrapods and teleost fish. Lungfish have a lagena epithelium but no lagena recess. In contrast, Latimeria has a lagenar recess comparable in position and innervation to those found in derived teleost fish, derived elasmobranches and most tetrapods. Latimeria, but not lungfish, has a sensory epithelium studded with numerous hair cells and covered by a tectorial membrane near the orifice of the lagena. Like the tetrapod basilar papilla, this structure is suspended across a periplymphatic space. Thus, topology, innervation and structural details suggest that Latimeria has a basilar papilla-like sensory epithelium suggesting affinity of this taxon with tetrapods. Given that all tetrapods share the basilar papilla as a unifying derived feature (secondarily lost in some) the most parsimonious explanation is the presence of such an epithelium in the aquatic ancestor, much like the epithelium found in Latimeria. Whether or not this epithelium plays a role in hearing or is a pressure receptor of a different function remains unclear.