Ancient whale may be largest animal ever

The blue whale has long held the title of largest animal of all time, an ocean giant reaching more than 100 feet in length and weighing more than 200 tons. But now, paleontologists have uncovered an immense cetacean that was shorter but possibly even heavier, a species that swam along the coast of ancient Peru more than 37 million years ago.

Named Perucetus colossus by University of Pisa paleontologist Giovanii Bianucci and colleagues, the prehistoric whale may have weighed more than 300 tons. The roughly 60-foot-long whale was described today in the journal Nature.

National University of San Marcos paleontologist and study co-author Mario Urbina found the partial skeleton 13 years ago among the rocks of the Ica Valley in southern Peru. The find was not immediately impressive, the bones so large that they looked more like boulders.

“At first, he had to convince the other members of the team that what he found was actually some fossil, because of the weird shape of it,” recounts Eli Amson, a co-author of the study and paleontologist at the State Museum of Natural History Stuttgart in Germany. But when researchers looked at thin sections of fragments from the site, the strange pieces turned out to be bones, and field teams spent the next ten years freeing Perucetus from the rock.

All told, the fossils excavated from the Ica Valley include 13 vertebrae, four ribs, and part of the hip. The anatomy of the bones, as well as the time period when the whale was swimming off the coast of South America, indicate that it was a relative of Basilosaurus, a fully aquatic whale that had a long snout full of piercing and cutting teeth. By comparing the known bones of Perucetus with the more complete skeletons of both living and fossil whales, Amson and colleagues were able to come up with size estimates for the giant.

While not remarkably long, Perucetus must have been a very heavy whale. The recovered fossils show signs of extensive pachyosteosclerosis, a condition marked by thick, dense bones seen in manatees as well as other early whales. Such dense bones help marine mammals remain heavy enough to stay submerged without being too heavy to come back to the surface.

“Too dense of a body means sinking to the bottom and having to consistently spend energy to move back up,” Amson says. To counter these heavy bones, marine mammals require enough buoyant tissues like muscles and fat to remain in a sweet spot where they can float easily in the water column without expending excess energy to swim up or down. “We used the ratio of skeletal tissue to all other tissues known in several living species of marine mammals to come up with overall body mass estimates for Perucetus,” Amson says, which place the whale somewhere between 93 tons and 370 tons.

If Perucetus was on the heavier end of these estimates, that would make it the largest known animal to ever live.

More fossil material is needed to determine exactly how large the ancient whale was. “The specimen is missing the top half of the skeleton, and so we’re missing a complete picture of this species’ entire size, which is a core challenge for estimating its weight,” says Nicholas Pyenson, a whale researcher at the National Museum of Natural History, who was not involved in the new study.

The fact that Perucetus belonged to an extinct group of whales that had different body proportions from living whales also complicates the weight estimates. These ancient whales were likely lighter for their length than modern whales are, Pyenson says. “The upper estimates of 300 tons seem incredulous to me, and even the lower estimate of 60 to 80 tons is spectacularly big,” he adds.

Even if Perucetus doesn’t turn out to be more massive than the blue whale, the cetacean was still a giant for its time. “What is clear is that this was an enormous animal, and indeed looks like its body mass was in and around that of the blue whale despite doing things in a completely different way,” says whale researcher Travis Park of the Natural History Museum London, who was not involved in the new study.

Until now scientists thought that whales began to evolve giant sizes around five million years ago, when changes in ocean circulation allowed huge filter feeders to grow, feeding on copious amounts of krill and other planktonic food. But Perucetus belonged to a group of whales that were active predators, generally munching on larger prey like fish.

“The find shows that we underestimated the late Eocene spike in body size for the earliest whales,” Pyenson says, which raises the question of how Perucetus caught enough food to fuel its enormous body.

While scientists have learned a lot about this colossal cetacean, it’s still unclear how exactly Perucetus lived. Such a big body undoubtedly required huge quantities of food, but what the whale ate is as-yet-unknown. “The head of Perucetus is a complete mystery,” Amson says.

Still, the animal’s extreme body mass points to a few possible diets. Unlike related basilosaurids that chased prey through the oceans, Percetus does not appear to have been an agile swimmer, Amson says, so it would not have hunted fast-moving prey like fish. It’s also unlikely that the whale was eating plants, as there are no known herbivorous whales.

The large marine mammal may have been a bottom feeder, searching the shallows for clams, crustaceans, and other morsels among the sand. A skull with teeth would go a long way to resolving the question, Park notes, because “if Perucetus was feeding on a lot of benthic, hard-shelled prey, then I would expect there to be adaptations to the skull and teeth for coping with this.”

For now, experts can only guess what the animal was eating in the ancient seas. “My personal favorite idea, though it is only speculation, is that Perucetus was a scavenger,” Amson says, “feeding on the carcasses of other large animals.”

Future finds will likely add new context to what the unusual whale was doing and why it evolved such a thickened skeleton. If one of these unusual whales has turned up, there are likely others.

Even as new fossil finds have outlined some of the major moments in whale evolution, Pyenson says, “we really haven’t yet discovered all the many ways of being a whale.”