Why did solid otoliths evolve in the ears of modern bony fishes?

Modern bony fishes (Teleostei), which comprise almost one-half of all extant vertebrate species, show a remarkable diversity in ear morphology, including otolith shape. Fish bioacousticians are still puzzling over the question of why solid calcareous otoliths with species-specific shapes evolved, whereas most “non-teleost” vertebrates possess numerous tiny otoconia. This question is linked to when and how often a switch from loosely aggregated material towards a solid structure—and from apatite to calcium carbonate—occurred during the vertebrate radiation and whether those character shifts are related to altered hearing. In a recent study, we constructed a hypothetical framework for otolith evolution by compiling the available information on the structure of otoliths and otoconia in > 160 species covering all major vertebrate groups. We concluded that solid teleost otoliths may have initially evolved as a selectively neutral by-product of other key innovations, and that the teleost-specific genome duplication event may have facilitated their subsequent diversification. Differences in otolith mass and shape might have enabled the perception of different ranges of acoustic information and may have evolved during the diversification of auditory abilities. In some teleost groups, otolith morphology may have co-evolved with ancillary hearing structures, especially if ears are closely connected to a gas bladder.