Paleontologists on Wednesday unveiled the fossilized bones of one of the strangest whales in history. The 39-million-year-old leviathan, called Perucetus, may have weighed about 200 tons, as much as a blue whale — by far the heaviest animal known, until now.

While blue whales are sleek, fast-swimming divers, Perucetus was a very different beast. The researchers suspect that it drifted lazily through shallow coastal waters like a mammoth manatee, propelling its sausage-like body with a paddle-shaped tail.

Some experts cautioned that more bones would have to be discovered before a firm estimate of Perucetus’s weight could be made. But they all agreed that the bizarre find would change the way paleontologists saw the evolution of whales from land mammals.

“This is a weird and stupendous fossil, for sure,” said Nicholas Pyenson, a paleontologist at the Smithsonian National Museum of Natural History, who was not involved in the study. “It’s clear from this discovery that there are so many other ways of being a whale that we have not yet discovered.”

Mario Urbina, a paleontologist at the Museum of Natural History at the National University of San Marcos in Lima, Peru, first set eyes on Perucetus in 2010. He was walking across a desert in southern Peru when he noticed a rocky bump bulging out of the sand. When he and his colleagues finished digging it out, the lump proved to be a gigantic vertebra.

Digging further, the researchers found 13 vertebrae in total, along with four ribs and part of a pelvis. Except for the pelvis, all the fossils were remarkably dense and strangely thickened, making it hard to figure out what kind of animal they belonged to.

Only the pelvis revealed exactly what the scientists had found. Unlike the other bones, the pelvis was small and delicately formed. It had crests and other distinctive features that revealed it to be a whale’s — in particular, from an early branch of the evolutionary tree of whales.

Whales evolved from dog-sized land mammals about 50 million years ago. Some of the earliest species evolved short limbs and most likely led a seal-like existence, hunting for fish and then hauling themselves onto the shore to reproduce.

Those early whales disappeared after a few million years. They were replaced by a group of entirely aquatic whales called basilosaurids. These slinky beasts could grow as long as a school bus but retained vestiges of their life on land — including tiny hind legs, complete with toes.

Basilosaurids dominated the oceans until about 35 million years ago. As they became extinct, another group of whales emerged, giving rise to the ancestors of living whales.

Today’s biggest whales, like blue whales and fin whales, only reached their gargantuan sizes in the past few million years. Shifts in ocean currents supported vast populations of krill and other invertebrates near the poles. The whales could grow immense by scooping up these prey on lunging dives.

The pelvis of Perucetus revealed it to be a basilosaurid, but the whale had evolved into a basilosaurid unlike any found before. Eli Amson, an expert on bone tissue at the State Museum of Natural History in Stuttgart, Germany, found that its ribs and spine had extra layers of outer bone, giving them bloated shapes.

A typical bone is full of pores, which make it lighter without sacrificing strength. Dr. Amson observed that the bones of Perucetus were solid throughout. The fossil is so hard in parts that it would be impossible to drive a nail into it with a hammer.

“It would make nothing but sparks,” he said.

Dr. Amson and his colleagues made three-dimensional scans of the fossil bones in order to reconstruct the whale’s full skeleton. They compared Perucetus to other basilosaurids that have been preserved from head to tail.

If the rest of Perucetus were a denser, thickened version of these whales, its complete skeleton would weigh between 5.8 and 8.3 tons. That would mean Perucetus had the heaviest skeleton of any mammal — bones that were twice as heavy as a blue whale’s.

That bulky skeleton also suggests that Perucetus had a thick, barrel-like body. Even though Perucetus was only about two-thirds the length of a blue whale, Dr. Amson and his colleagues suspect that it weighed about the same.

“It’s definitely in the blue whale ballpark,” Dr. Amson said.

Dr. Pyenson, of the Smithsonian National Museum of Natural History, thought it was premature to make such an estimate. “Until we find the rest of the skeleton, I think we should shelve the heavyweight-contender issue,” he said.

But Hans Thewissen, a paleontologist at Northeast Ohio Medical University, who was not involved in the study, said the estimate was reasonable. “I agree with the excitement around the weight,” he said.

The fossil suggests that Perucetus reached such a big size without feeding as blue whales do. The analysis of its bones suggests it lived more like a gargantuan manatee.

Manatees graze on sea grass on the ocean floor. Their lungs are full of air, and their guts produce gas as they ferment their food. To stay underwater, manatees have evolved dense bones as ballast.

The structure of Perucetus’s spine is similar to that of a manatee. Dr. Amson envisioned the whale swimming in a manatee style, slowly raising and lowering its tail.

Based on the rocks where the fossils were found, Dr. Amson and his colleagues suspect that Perucetus moved slowly through coastal waters no deeper than 150 feet. But how they fueled their giant bodies is still a mystery.

Dr. Amson said it was possible that Perucetus also fed on sea grass, but that would make it the first herbivorous whale known to science. “We deem it unlikely, but who knows?” he said.

Dr. Amson even imagines Perucetus possibly living as a scavenger, picking over carcasses.

By contrast, Dr. Thewissen favored the idea that these whales scooped up mud from the sea floor to eat the worms and shellfish it contained — something that gray whales do today.

The head of Perucetus would have adaptations for whichever way of life it pursued. “I would love to see the skull of this guy,” Dr. Thewissen said.

However it made a living, Perucetus is proof that whales did not have to wait until recently to get huge. “The most important message is not that we can enter the Guinness Book of World Records,” said Dr. Amson. “It’s that there’s another path to gigantism.”

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