The partial skeleton of an ancient whale shows it could've weighed as much as 340 tons, making it one of the heaviest animals ever

• A new species of ancient whale, Perucetus colossus, was a giant of the ocean. 
• It may have weighed twice as much as a blue whale because of its dense bones.
• Researchers hypothesize that it was a slow swimmer that lived in coastal waters. 

Roughly 39 million years ago, a slow-moving giant glided in shallow, coastal waters, potentially feeding on mollusks and crustaceans.

In 2010, paleontologist Mario Urbina found fossilized remains of this ancient animal, a new species of basilosaurid whale called Perucetus colossus.

The remains were unusual, gigantic bones in the Pisco Basin in southwestern Peru. The partial skeleton included 13 vertebrae, four ribs, and a hip bone.

After excavating the bones, measuring them, and comparing them to other species, researchers have now estimated the ancient animal could have weighed twice as much as a blue whale, which typically weighs between 72 and 180 tons, thanks to its uncommonly thick bones. They described their findings in a new paper in the journal Nature. 

The discovery is significant because it shows that some early whales evolved to be gigantic millions differently, and much earlier, than the modern behemoths. For comparison, baleen whales like blue whales started expanding to huge proportions around 5 million years ago, roughly 34 million years after P. colossus.

"This giant basilosaurid lived a long time before the emergence of giant baleen whales, and if our ecological hypothesis is correct, in a completely different environment," Olivier Lambert, one of the authors of the paper, told Insider via email.

How to weigh an enormous skeleton 

To figure out what P. colossus may have weighed, the researchers scanned the bones' surface and examined their interior structure.

However, since it was only a partial skeleton, there were several estimations they had to make like relying on the proportions of a close relative, Cynthiacetus peruvianus, to determine the overall size.

They also used the mass-to-body ratio of manatees, beaked whales, and cetaceans to guesstimate the amount of blubber and soft tissue the ancient animal would have had to calculate its weight while alive.

The resulting values put the final tons between 85 and 340. Even at the low end, P. colossus would be on par with some blue whales. And at the high-end, it shatters records of how large we thought the largest animal was on Earth. But scientists will need to find a more-complete skeleton before dethroning the blue whale.

Other key features, like P. colossus's skull and teeth, are missing, so researchers can't say much about what it ate. But due to its size, it likely couldn't catch quick prey.

Additionally, researchers believe the whale's body swept to its final resting place and may not have lived in the environment where it was fossilized. That means the other fossils retrieved from the area may not offer clues to how P. colossus lived.

P. colossus had big bones and probably tiny limbs

Cetaceans are the aquatic mammal family that includes whales, dolphins, and porpoises. About 50 million years ago, their ancestors branched off from land ungulates, or hoofed mammals, and went back to the water.

The transition required adaptations. As some species acclimated to sea environments, they developed bones that were denser and less spongy than their modern counterparts. By the time P. colossus arrived, roughly 11 million years later, it was fully aquatic. 

At an estimated 65 feet, this P. colossus isn't as long as some blue whale skeletons, like the 72-foot specimen at the Santa Barbara Museum of Natural History. It's the colossus's bones themselves that add the suspected extra tonnage. 

The P. colossus skeleton shows evidence of pachyosteosclerosis, a condition where the bones thicken with extra layers and become denser with reduced internal cavities, helping to regulate buoyancy. 

"In addition to being very large, these vertebrae are extremely thickened, more than in any other marine mammal, and made of highly compact bone, which further participates to the surprisingly high skeletal weight," said Lambert. Each vertebra weighs over 220 pounds.

Its cumbersome weight would have made it difficult to move on land. The researchers propose the animal was fully aquatic and lived in shallow water. Similar to how some manatees move, the P. colossus likely swam by undulating its body. It also could have used its forelimbs and tail fluke to propel itself along the bottom of the seafloor, as sea cows do. 

Over the course of millions of years, cetaceans completely lost their hind limbs and their forelimbs became flippers. No limbs were found near the P. colossus skeleton, but fossilized evidence suggests it likely had both front and back legs. 

The innominate or hip bone shows evidence of where a femur was attached. "We can tell that a tiny hind limb was still present, as in other basilosaurids like Basilosaurus and Cynthiacetus," said Lambert.

Researchers based the limbs' appearance in the reconstruction on these other species. Whether this behemoth was bobbing along the bottom of the sea, munching on mollusks, it's all hypothetical until more specimens are found.