Exactly why the animal was so big is unclear. To be sure, size has its advantages. Temperature control is one. Mammals must keep their body temperatures within a tight range. As they grow bigger, their surface area rises more slowly than their volume. That means bigger creatures have an easier time staying warm in the comparatively cold ocean.

But there are downsides, too. The most obvious is that a bigger body has a bigger appetite which is harder to satisfy. Biologists believe that whales attained their modern proportions only about 5m years ago, when climatic changes produced the rich feeding conditions necessary to support their enormous bodies. Since P. colossus lived millions of years before those changes, something else must explain its heft.

Working out how an extinct animal lived is even harder than deciding just how big it was. But there are clues. One striking feature of P. colossus’s bones is their density. All the bones the researchers found were unusually thick and heavy. Some lacked a medullary cavity, a hollow, marrow-filled section found in the middle of most mammals’ bones. These days, deep-diving marine mammals like the sperm whale, which can descend to well over 1,000 metres, tend to have comparatively light skeletons. It is those that live in shallower waters, such as manatees, which sport dense bones. For that reason, the researchers think that P. colossus was likewise a creature of the comparatively fertile coastal shallows.

Without a jaw or any teeth to study, exactly what it ate is unclear. One theory is that it was a herbivore, munching its way through prodigious quantities of seagrass and seaweed. It may have been a filter-feeder like modern baleen whales, straining small creatures from the water. The researchers speculate that it may even have been a scavenger, subsisting on the corpses of other animals. Definitive answers—and an update to the record books—may have to wait until a more complete skeleton is discovered. ■