Last August, a team of paleontologists announced that they had discovered the fossilized bones of a gigantic ancient whale. Perucetus, as they named it, might have weighed over 200 tons, which would make it the heaviest animal that has ever lived.
But in a study published Thursday, a pair of scientists have challenged that bold claim. “The numbers don’t make any sense,” said Nicholas Pyenson, a paleontologist at the Smithsonian National Museum of Natural History and one of the authors of the new study.
In their new analysis, Dr. Pyenson and Ryosuke Motani, a paleontologist at the University of California, Davis, concluded that Perucetus probably weighed 60 to 70 tons, which would have made it about the size of a sperm whale.
They also analyzed fossils of blue whales and provided a new estimate of the weight of that species. They concluded that blue whales weigh up to 270 tons — much more than previous estimates, of up to 150 tons — which would make them far and away the heaviest known species in the history of the animal kingdom.
Perucetus first came to light in 2010, when Mario Urbina, a paleontologist at the Museum of Natural History at the National University of San Marcos in Lima, Peru, spotted a bone in a desert in southern Peru. He and his colleagues excavated 13 vertebrae, four ribs, and part of a pelvis.
The bones had many hallmarks of whales’ bones. But they were also astonishingly large and heavy. Dr. Urbina and his colleagues reconstructed the full skeleton of Perucetus by studying the much smaller whales that lived at the same time. They also drew inspiration from living manatees, which have dense skeletons that let them stay underwater to graze on sea grass.
Dr. Urbina and his colleagues ended up with a reconstruction of a bizarre animal. It had an enormous éclair-shaped trunk, a tiny head, flippers and vestigial hind legs.
But Dr. Motani, an expert on reconstructing the bodies of extinct marine animals, was puzzled by their conclusions. “I thought, how could it be? How can you pack that mass into that volume?” he said.
Dr. Motani contacted Dr. Pyenson, an expert on whale fossils. They both felt that modeling Perucetus after manatees was a mistake, since only whales have evolved to truly gigantic sizes.
“It’s really important to compare apples to apples,” Dr. Pyenson said.
For their own study, Dr. Pyenson and Dr. Motani took a fresh look at living whales. Since no one can haul a live blue whale onto a scale, no one has ever made a precise measurement of its weight. Dr. Pyenson and Dr. Motani dredged up data collected by Japanese whaling ships in the 1940s, and used that as the basis for a new estimate.
They also created a three-dimensional model of the blue whale, and used it to make a model of Perucetus. With this approach, they estimated that Perucetus weighed 60 to 70 tons, much less than the other research team had concluded.
Eli Amson, an expert on bone tissue at the State Museum of Natural History in Stuttgart, Germany and an author on the original study, disagreed with the new approach. “This extinct whale had a very different biology than that of recent whales,” he said.
Dr. Amson said that he and his colleagues are now making their own three-dimensional model of the ancient species. They are finding that Perucetus was even more manatee-like than they originally believed, strengthening their conclusion that it rivaled or surpassed the blue whale in weight, he said.
Dr. Pyenson said Perucetus remains a major discovery, despite the smaller size he and Dr. Motani are suggesting. Paleontologists have long believed that whales evolved to huge sizes only in the past few million years. Even at 60 tons, Perucetus would have been a giant among early whales.
“Whales were clearly exploring big sizes,” Dr. Pyenson said.
