First insights on otolith macroevolution

Otoliths are calcified structures located in fishes’ inner ear involved in both hearing and equilibrium functions. Although their shapes are known to be species-specific, we still lack evidence of the evolutionary forces driving their disparity, i.e. their morphological diversity. To better understand the pattern of their shape diversification, we collected otoliths in 697 species from 68 orders and 309 families. We used geometric morphometric methods to quantify the size and the shape of both mesial and dorsal views. Phylogenetic information was recovered from pruning the consensus time-tree from Betancur et al. (2013). First, we found significant but low phylogenetic signal on shape and size variation as well as size being a poor predictor of otolith shape variation across taxa. Next, we revealed strong morphological covariation between lateral and dorsal views. Surprisingly, we also found high covariation between lateral otolith and its sulcus outline suggesting their evolution mirror each other. Finally, we found the accumulation of otolith disparity is unrelated to the age of the taxonomic group when we highlighted a higher lateral shape and dorsal size disparity in clades evolving at faster rates. Our analyses at a very large phylogenetic scale demonstrate how the diversification of otoliths is not random and is probably driven by various morpho-functional constraints. We ask for further studies focusing on eco-morphological hypotheses to reveal more macroevolutionary drivers of inner ear functional morphology.