Cousin to the Whales
On the tree of life, whales dwell on a limb also occupied by pigs, giraffes camels and other hoofed members of the order Artiodactyla. Of the artiodactyls, hippopotamuses are the closest living relatives to whales.
The last common ancestor of whales and hippos is Andrewsarchus, which lived roughly 45 million years ago in what's now Mongolia. It's known only from this 3-foot-long skull, discovered in the Gobi desert in 1923.
Image: AMNH/R. Mickens