There are two main hypotheses for the relationships of the mammalian order Cetacea (comprising whales, dolphins and porpoises). The first hypothesis, mainly supported by DNA sequence data1,2, is that one of the groups of artiodactyls (for example, the hippopotamids) is the closest extant relative of whales and that Artiodactyla are paraphyletic if Cetacea are excluded from it. The second hypothesis, mainly supported by palaeontological data3,4, identifies mesonychians, a group of extinct archaic ungulates, as the sister group to whales. These two hypotheses are not mutually exclusive, because mesonychians and cetaceans could be sister groups, and this combined clade (Cete) could be the sister group to a group of artiodactyls.