the two groups are unlikely to be closely related when extant and extinct artiodactyls are analyzed. Cetaceans originated about 50 million years ago in south Asia, whereas the family Hippopotamidae is only 15 million years old, and the first hippopotamids are only 6 million years old. Yet, analyses of fossil clades have not resolved the issue of cetacean relations. Another study has offered a suggestion that anthracotheres are part of a clade that also consists of entelodonts (and even Andrewsarchus) and that is a sister clade to other cetancodonts, with Siamotherium as the most basal member of the clade Cetacodontamorpha. References Piacenzian extinctions Eocene first appearances Prehistoric mammal families Taxa named by Joseph Leidy Paraphyletic groups