But even before the move, this lineage was setting size records. One ancient cousin to modern whales and hippos, called Andrewsarchus mongoliensis, ranks as the largest mammal known to have stalked the land as a predator. A skull from this creature, the only fossil found so far from this beast, greets visitors on their way into a new exhibit on whales here at the American Museum of Natural History.

"It's odd to have a big predator in this hoofed plant-eating mammal group," said John Flynn, co-curator of the exhibit, referring to the group to which whales and the now-extinct Andrewsarchus belonged. "But if you think about it, some of the other relatives like pigs and peccaries are pretty ferocious and will eat just about anything."

In an artist's rendering, the 45-million-year-old Andrewsarchus has a profile not unlike a giant feral pig with a more streamlined snout. This 6-foot-tall creature lived solely on land, but its relatives began taking to the water and eventually left land completely. [Whale Gallery: Giants of the Deep]

The "first whale," a creature whose lifestyle (living on land but eating fish from the nearby sea) represented the early stage of this transition into the water, was a wolf-size fish eater that lived about 50 million years ago on the edges of the ancient Tethys Sea, according to the exhibit.