Evolution of semiaquatic habits in hippos and their extinct relatives: insights from the ear region.

This article focuses on investigating the evolutionary history and aquatic adaptations of hippopotamoids through detailed morphological and morphometric analyses of the petrosal bone and cochlea in 12 extinct taxa and two extant species. The study finds that semiaquatic traits in hippopotamoids evolved convergently multiple times rather than being inherited from a common aquatic ancestor shared with cetaceans, as supported by phylogenetic signals and functional ear region characteristics. Early hippopotamoids exhibited terrestrial hearing, with increasing tegmen tympani inflation and cochlear modifications correlating with varying degrees of amphibious behavior across different clades and continents from the Oligocene to the Miocene. These findings align with molecular evidence indicating independent aquatic adaptations in hippopotamids and cetaceans and highlight the potential of ear morphology to clarify hippopotamoid phylogeny and paleoecology.