Leviathan mellvillei's skull and jawbone, diagrammed above, boasted large teeth in both the top and bottom jaws—and enough room to accommodate a modern man.
By contrast, today's sperm whales have smaller teeth, which are mostly in their lower jaws—an adaption for hunting squid by suction.
Leviathan shows that "raptorial feeding had gotten to a really extreme size," said Jonathan Geisler, a whale evolution expert at the New York Institute of Technology, who was not involved in the discovery.
The idea that Leviathan fed on other whales is a "good inference," Geisler said. "But to really pin it down, we're going to need to find some whale bones with [Leviathan's] bite marks."
Some of those marked fossils, he added, might already be sitting in museum collections.
(Also see "Early Whales Gave Birth on Land, Fossils Reveal.")