El Perucetus pudo haber sido el animal más pesado de la historia

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Paleontologists presented on Wednesday the fossilized bones of one of the strangest whales in history. This 39 million-year-old leviathan, called Perucetus, possibly weighed around 200 tons, as much as a blue whale, which until now was the heaviest animal on record.

While blue whales are elegant and fast divers, Perucetus was a very different beast. Researchers suspect that it lazily sailed through shallow coastal waters, like a manatee, propelling its sausage-shaped body with a paddle-like tail.

Some experts warned that more bones would have to be discovered before a rigorous estimate of Perucetus’ weight could be made. But they all agreed that the strange find would change the way paleontologists viewed the evolution of whales from land mammals.

“This is a strange and amazing fossil, no doubt,” said Nicholas Pyenson, a paleontologist at the Smithsonian National Museum of Natural History, who was not involved in the study. “This discovery shows that there are many other ways to be a whale that we haven’t discovered yet”.

Mario Urbina, a paleontologist at the National University of San Marcos Museum of Natural History in Lima, Peru, first encountered Perucetus in 2010. He was walking through a desert in southern Peru when he noticed a rocky protrusion sticking out of the sand. When he and his colleagues finished uncovering it, the lump turned out to be a gigantic vertebra.

As they continued to excavate, researchers found a total of 13 vertebrae, along with four ribs and part of a pelvis. Except for the pelvis, all the fossils were very dense and unusually thick, making it difficult to determine what kind of animal they belonged to.

Only the pelvis accurately revealed what the scientists had found. Unlike the other bones, the pelvis was small and delicately formed. It had crests and other distinctive features that indicated it was from a whale, specifically from an early branch of the whale evolutionary tree.

Whales evolved from dog-sized land mammals about 50 million years ago. Some of the early species had short limbs and likely lived like seals, hunting fish and crawling onto the shore to reproduce.

These early whales disappeared after a few million years. They were replaced by a group of entirely aquatic whales called basilosaurids. These elusive animals could grow as long as a school bus but retained vestiges of their land life, such as tiny hind legs with fingers.

Basilosaurids dominated the oceans until about 35 million years ago. When they became extinct, another group of whales emerged that gave rise to the ancestors of present-day whales.

The largest whales today, such as the blue whale and the fin whale, only reached their gigantic size in the past few million years. Changes in ocean currents supported vast populations of krill and other invertebrates near the poles. The whales grew enormously by scooping up these prey in rapid dives.

Perucetus’ pelvis revealed that it was a basilosaurid, but the whale had evolved into a distinct species from any previously found. Eli Amson, a bone tissue expert at the State Museum of Natural History in Stuttgart, Germany, discovered that its ribs and backbone had additional layers of external bone, giving them swollen shapes.

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

“It would just spark,” he said.

Amson and his colleagues 3D scanned the fossilized bones to reconstruct the complete skeleton of the whale. They compared Perucetus to other basilosaurids that have been preserved from head to tail.

If the rest of Perucetus was a denser, thicker 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, with bones twice as heavy as those of a blue whale.

This bulky skeleton also suggests that Perucetus had a barrel-shaped body. Although it was only two-thirds the length of a blue whale, Amson and his colleagues suspect it weighed about the same.

“It is definitely in the range of blue whales,” said Amson.

Pyenson, the researcher at the Smithsonian National Museum of Natural History, believes it is premature to make that estimate. “Until we find the rest of the skeleton, I think we should put the heavyweight issue aside,” he said.

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

The fossil suggests that Perucetus achieved such a large size without feeding like blue whales do. Analysis of its bones suggests that it lived more like a colossal manatee.

Manatees feed on seagrass on the ocean floor. Their lungs are filled with air, and their intestines produce gas by fermenting their food. To stay underwater, manatees have developed dense bones that act as a counterweight.

The structure of Perucetus’ spine is similar to that of a manatee. Amson imagined the whale swimming like a manatee, slowly raising and lowering its tail.

Based on the rocks where the fossils were found, Amson and his colleagues suspect that Perucetus moved slowly through coastal waters no more than 45 meters deep. But how these gigantic bodies fed remains a mystery.

Regardless of how it managed to survive, Perucetus is evidence that whales did not have to wait until recent times to become enormous. “The most important message is not that we can enter the Guinness World Records,” said Amson. “It is that there is another path to gigantism”.

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

Amson believes that Perucetus could have even lived as a scavenger, scavenging corpses.

On the other hand, Thewissen leans towards the idea that these whales collected mud from the seafloor to eat the worms and shellfish it contained, something that gray whales do today.

The head of Perucetus would have adaptations for whatever form of life it pursued. “I would love to see this guy’s skull,” Thewissen said.

Regardless of how it managed to survive, Perucetus is evidence that whales did not have to wait until recent times to become enormous. “The most important message is not that we can enter the Guinness World Records,” said Amson. “It is that there is another path to gigantism”.

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