 Today , the closest living relatives of cetaceans are the hippopotamuses ; these share a semi @-@ aquatic ancestor that branched off from other artiodactyls some 60 mya . Around 40 mya , a common ancestor between the two branched off into cetacea and anthracotheres ; nearly all anthracotheres became extinct at the end of the Pleistocene 2 @.@ 5 mya , eventually leaving only one surviving lineage - the hippo . 
