When Alberto Collareta first laid eyes on the boulder-sized vertebrae of the extinct animal, he couldn’t believe what he was seeing. He wondered how a creature that large could even move around.

“I was in front of something unlike anything I had ever seen,” said Collareta, a University of Pisa researcher and co-author of a study published Wednesday in the journal Nature describing the freshly unearthed species of giant prehistoric whale.

Dubbed by its discoverers Perucetus colossus, or simply P. colossus, the titanic animal may not be just a record setter. P. colossus is also compelling scientists to reconsider their ideas about how animals are able to grow to gigantic sizes.

“This is another way in which you can get big,” said Hans Thewissen, a paleontologist and whale evolution expert at Northeast Ohio Medical University. With a body that looked vaguely like that of a manatee rather than a blue whale’s, it clearly did something different than other whales to maintain its huge mass.

But not everyone is convinced this colossus, while undoubtedly big, is truly more massive than a blue whale. The research team acknowledges their estimates for the animal’s body mass range widely, from 85 tons all the way up to 340 tons. The team exhumed only a partial skeleton without a skull, leading some scientists to say more fossils are needed before anyone names a new heavyweight champion of the animal kingdom.

“I don’t think we know enough about this group of whales to really weigh in on which interpretation on its body weight is the right one,” said Nick Pyenson, a paleontologist at the Smithsonian Institution. “I’m really skeptical of these high-end estimates.”

But he added: “Clearly, it is really big.”

Why animals get gigantic

In the animal kingdom, it’s usually good to be big. It’s easier to deter predators, care for young and move about when an animal is large and in charge.

But there are many factors that weigh against an animal’s growth. On land, one of the biggest is gravity itself. Legs can only be so strong to support a heavy frame.

In water, buoyancy helps aquatic animals balloon over eons to gargantuan sizes. Blue whales and their relatives only evolved to their current size in the past few million years, after a sudden rise in ocean upwelling provided them with abundant krill, their favorite meal, which helped fuel their growth.

With ribs over a yard long and vertebrae weighing over 200 pounds each, the 39 million-year-old fossilized remains of the P. colossus required multiple field campaigns to exhume from the foot of a mountain in southern Peru’s Ica Valley after its discovery 13 years ago. The animal’s scientific name means “colossal whale from Peru.” The specimen today is housed at the Natural History Museum in Lima.

Its bones were thick and compact, more like a hippopotamus than a blue whale, suggesting it did not pursue a fast-moving prey such as krill. It must have done something different to sustain its weight.

The research team instead said P. colossus may have fed off the seafloor, munching on sea grass, feasting on bottom-dwelling animals or scavenging on carcasses.

There are a few issues with some of those hypotheses. For one, no whale is known to feed on plants. And it would take a lot of dead animals to sustain a scavenger as big as P. colossus. “I have a hard time thinking how many carcasses would be needed to sustain this animal,” Thewissen said.

Weighty questions about an extinct whale

However it fed, the discovery reveals there are multiple ways whales can evolve into giants. “It’s clear that Perucetus shows there are many, many other ways to be a whale,” Pyenson said. “And we haven’t really discovered all those ways yet. So that’s really exciting.”

The research team admits their ideas about their discovery’s diet are speculative. And they acknowledge there is a wide range in their estimates of the whale’s size, due to the skeleton’s many missing pieces and to the uncertainly about how best to put flesh to bone in 3D models.

“We have been extremely conservative in our approach and do not provide one single estimate but a range of values,” said Eli Amson, a researcher at the Stuttgart State Museum of Natural History in Germany who also co-wrote the paper.

Noting that the lower estimate of 85 tons is still “larger than some adult blue whales,” he said his team cannot be definitive about whether P. colossus or the blue whale is heavier.

“But we can claim with a great degree of certainty that its weight was in the ballpark of that of the blue whale,” he added.

The only way to get a better idea of what P. colossus’s life — and girth — were like is to find more fossils. The research team plans to continue roaming the Peruvian desert for bones.

Near the top of the wishlist is a skull, which would help solve the riddle of what exactly P. colossus ate to get so big.

“We really do need a skull,” Collareta said.