Hippopotamus and whale phylogeny.

Thewissen et al. describe new fossils from India that apparently support a phylogeny that places Cetacea (that is, whales, dolphins, porpoises) as the sister group to the extinct family Raoellidae, and Hippopotamidae as more closely related to pigs and peccaries (that is, Suina) than to cetaceans. However, our reanalysis of a modified version of the data set they used differs in retaining molecular characters and demonstrates that Hippopotamidae is the closest extant family to Cetacea and that raoellids are the closest extinct group, consistent with previous phylogenetic studies. This topology supports the view that the aquatic adaptations in hippopotamids and cetaceans are inherited from their common ancestor.