Cribriform plate size documents loss of olfactory receptor genes in the early evolution of whales, dolphins, and porpoises

Having transitioned from a terrestrial lifestyle to an aquatic one over evolutionary time, cetaceans are prime subjects for the study of reduced olfaction. Here, we investigated the size of the cribriform plate in extant Artiodactyla and fossil whales. We also investigated the number of olfactory receptor (OR) genes encoded in artiodactyl genomes and found that the size of the cribriform plate is positively correlated with the number of OR genes among extant species. We extended this finding to fossil whales and estimated that a loss of genes occurred during the early/middle Eocene, before cetaceans became obligately aquatic. Extant toothed whales have reduced olfaction, but two Miocene toothed whales are predicted to have as many or more OR genes than extant baleen whales. Therefore, some fossil toothed whales might have had some olfaction. For their body size, extant baleen whales have cribriform plates that are similar in size to that of the dromedary camel, which is consistent with the growing evidence that baleen whales have retained some olfaction. As the first quantitative, morphological analysis of cetacean olfaction across the land-to-water transition, our study facilitates further study of olfactory neuroanatomy and the link between olfactory phenotype and genotype.