Prehistoric whale could have been the heaviest animal of all time

Prehistoric whale could have been the heaviest animal of all time

This is what the primeval whale Perucetus colossus might have looked like. © Alberto Gennari

So far, the blue whale is considered the heaviest animal on our planet – not even the dinosaurs reached its enormous body mass. But now Peruvian paleontologists have uncovered the fossil of an ancient whale that may have surpassed even this giant in size and mass. The primeval whale, dubbed Perucetus colossus, lived around 39 million years ago and had unusually heavy, large bones. Its skeleton was two to three times that of a blue whale, and the total mass of Perucetus colossus may have ranged from 85 to 340 tons – more than any other known animal. The paleontologists suspect that the massive giant lived in shallower water, similar to today’s manatees, and was a rather slow swimmer. What he fed on is still unknown.

Although whales are now perfectly adapted to aquatic life, they did not evolve in the sea. Instead, they descended from terrestrial mammals that reverted to aquatic life over time. Primeval even-toed ungulates of the genus Indohyus are considered possible ancestors of whales. These roughly raccoon-sized animals lived on the Indian subcontinent at the beginning of the Eocene around 50 million years ago. Similar to today’s whales, they had thickened ear bones and also very heavy bones overall, similar to hippos and manatees. Then, as the Eocene progressed, the progenitors of whales reduced their limbs and converted them into flippers. “During this global greenhouse era, whales specialized more and more in aquatic life and also rapidly developed larger bodies,” report Giovanni Bianucci from the University of Pisa and his colleagues. The Basilosaurus, which lived around 40 million years ago and is around 18 meters long, is considered the first large whale. However, the largest and heaviest whale of all is the blue whale (Balaenoptera musculus). Today’s giant of the seas reaches a length of up to 33 meters and a mass of up to 200 tons.

Skeleton twice as heavy as that of the blue whale

But now a fossil discovered in southern Peru could overtake the blue whale. The find consists of 13 huge vertebrae, four ribs and parts of the hip that were found in a rock formation in southern Peru around 39 million years old. Each vertebra of the fossil weighs well over 100 kilograms and the ribs reach a length of 1.40 meters. From the characteristics of the bones, Bianucci and his colleagues conclude that it must be a primeval whale from the group of basilosaurids. When alive, this animal was probably about the size of a blue whale. The scientists gave it the name Perucetus colossus – “the colossal whale from Peru”. For their study, they examined the morphology of the bones and scanned them to measure their volume. They also performed core drilling to assess the internal bone structure. Completely preserved skeletons of close relatives were also included in the analysis.

Examination of the fossil whale bones revealed that Perucetus colossus was not only very large but also unusually heavy. “With an estimated mass of 5.3 to 7.6 tons, its skeleton alone weighed 2 to 2.9 times that of a 25-meter-long blue whale,” report Bianucci and his team. The reason: Unlike the early basilosaurids, the bones of all modern whales are relatively porous on the inside and therefore lighter. This helps the animals get more buoyancy. “This osteoporotic structure is typical of open-ocean and secondary aquatic tetrapods that swim actively,” the paleontologists explain. As a result, the skeleton of whales only accounts for 2.2 to 5.1 percent of the total body mass, compared to four to ten percent of land animals. But Perucetus colossus lacks the whale-typical light bones. Instead, they are extremely massive due to the accumulation of additional bone mass on their outside and a particularly high bone density. This primeval whale combined an enormous size with the heaviest bones known to date.

(Video: Nature)

Was Perucetus colossus the heaviest known animal?

To reconstruct the total mass of the prehistoric whale, the paleontologists used various ratios of soft-tissue to skeletal mass known from living marine mammals. According to these calculations, Perucetus colossus could have weighed between 85 and 340 tons. It was possibly even heavier than the blue whale. “This early whale drastically shifts the previously known upper limit of skeletal mass in mammals and aquatic vertebrates. It may also be the heaviest animal ever described,” says senior author Eli Amson from the Stuttgart State Museum of Natural History. The new find demonstrates that whales reached gigantic sizes around 30 million years earlier than previously thought. “For us, one of the key findings of our work is that the transition to true gigantism in whales evolved much earlier in Earth’s history than we previously thought. Combining gigantic size with extremely high bone weight, Perucetus colossus lived as early as 39 million years ago,” says Amson.

“Discoveries of such extreme body dimensions are an opportunity to reevaluate our understanding of animal evolution,” comment non-study scientists JGM Thewissen and David A. Waugh of Northeast Ohio Medical University. “It seems we still have a hard time understanding how amazing size can be in whales.” Bianucci and his colleagues suggest that the ancient colossus, with its heavy bones and low buoyancy, probably wasn’t a particularly active, dynamic swimmer, and tended to swim in shallower ones sea ​​areas – similar to today’s manatees. “The extra weight helps these animals regulate their buoyancy and stay underwater, much like a scuba diver’s weight belt,” explains Amson. Because of its relatively long, massive vertebrae, Perucetus colossus likely swam by a slow, undulating up-and-down movement of its body. How the primeval whale fed is still unclear because of the missing head.

Source: Giovanni Bianucci (University of Pisa) et al., Nature, doi: 10.1038/s41586-023-06381-1

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