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Illustration of the reconstruction of Perucetus colossus in its coastal habitat with an estimated body length of 20 metres.

The researchers who found the bones of Perucetus colossus think it might have been about 20 metres long and probably lived in shallow waters (artist’s reconstruction).Credit: Alberto Gennari

It is a big news day for palaeontologists, as well as for kids who memorize science facts. The blue whale (Balaenoptera musculus) is no longer indisputably the heaviest animal to have ever lived. A newly described fossilized whale named Perucetus colossus, dating to roughly 38 million years ago, might have been heavier than a blue whale, even if it was not as long.

Blue whales, which are endangered, weigh about 100 to 150 tonnes, although some might be as heavy as 200 tonnes1. Perucetus colossus was between 85 and 340 tonnes, according to the scientists who found and described the remains: 13 vertebrae, 4 ribs and a bit of pelvis (see ‘A whale of a whale’). Their best-guess estimate is that the whale was around 180 tonnes. This mind-boggling mass is the result of its bones, which were big and dense — an evolutionary adaptation that helped it to dive. The researchers report the find today in Nature2.

Deep-diving whales have evolved the ability to completely empty their lungs when plunging into the abyss, but P. colossus probably lived in shallow coastal areas, based in part on what researchers know about ancient oceans. This means that it would have dived with air in its lungs — typical behaviour for shallow divers, the researchers say. For P. colossus, however, having all that air in its body would have made it difficult to stay at the bottom of the sea floor without some ballast, which is why it might have evolved such heavy bones. During the Eocene epoch, most marine resources would have been at the bottom of the ocean, says co-author Rodolfo Salas-Gismondi, a palaeontologist at Cayetano Heredia University in Lima, Peru. But whether the ancient whale ate seagrass, molluscs, the carcasses of other animals or something else entirely remains a mystery.

Other lineages have evolved dense and heavily built bones for the same reason — notably manatees, which the researchers think P. colossus might have resembled. Unlike evolving a new organ or respiratory system, it seems comparatively easy for animals to add or remove bone tissue over time, says study co-author Eli Amson, a palaeontologist at the State Museum of Natural History Stuttgart in Germany. The whale’s bones were so enlarged that they look swollen, almost like inflated balloons.

A heavy lift

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When the remains were first found years ago, in a fossil-rich deposit called the Pisco Formation, in the Ica Desert, a coastal area of Peru, palaeontologist and co-author Mario Urbina at the National Major University of San Marcos in Lima and his team weren’t even sure that they were bones. Urbina invited Salas-Gismondi, a longtime friend and colleague, out to see partially excavated vertebrae around 2012. “Mario said to me, ‘I found a thing — a giant thing — and maybe it’s important but I don’t know.’ He showed me the fossil in the wall of a hill.” The bones lacked the porous, spongey structure of typical bones. They looked “like rocks”, Salas-Gismondi says. It wasn’t until they were analysed under a microscope that the team felt confident they were actually bones.

A WHALE OF A WHALE. Graphic shows fossilized bones of Perucetus colossus. The whale could be largest animal that ever lived.

Source: Ref. 2

Extracting the behemoth ribs and vertebrae from the hard sediment took years. “They did like 20 expeditions,” Salas-Gismondi says. “They were collecting two vertebra each time because they were so heavy and large.” The fossil is now part of the collection of the Natural History Museum in Lima.

The museum has become a hub for Peruvian palaeontology, according to Salas-Gismondi. “Before we were in the museum, foreign palaeontologists just collected fossils and took them to their countries,” he says. “Now we have a bigger team of Peruvian palaeontologists and funding, and fossils can stay here.”

Geerat Vermeij, a palaeobiologist at the University of California, Davis, calls the research “extremely cool”. He adds: “To find something as big and as important as that in this day and age, when we’ve already done so much palaeontology, is amazing to me.”

Rewriting history?

Nicholas Pyenson, a palaeontologist at the Smithsonian Institution in Washington DC, agrees that P. colossus is an awe-inspiring beast, calling it “stupendous and very weird”. He says that it will re-write the story of the evolution of gigantism in whales. But he’s not ready to give P. colossus the blue whale’s crown as the heaviest animal of all time. “The crown belongs to a blue whale,” he says. “Figuring out body weight for an extinct species is really, really hard.” The skeletons are incomplete, Pyenson adds, “and we don’t really know how to put meat on bone for them”.

Perucetus colossus belongs to a group of extinct whales known as the basilosaurids, at least one of which Pyenson describes as a “a long slinky animal”. He therefore thinks Perucetus was probably lighter than a blue whale. “Count me as a sceptic,” he says.

A scientist drills into a huge bone to take a sample.

Researchers drilled into the bones of P. colossus to study their density and inner structure.Credit: Giovanni Bianucci

Amson and his colleagues are not claiming that P. colossus was the heaviest animal of all time. Amson wants to be clear about that: “We say it’s a challenger.” Many steps are involved in extrapolating the weight of an entire living being from that of a fossil. In this case, the shape of the head is unknown, because its skull has not been found — yet. The animal probably had quite a bit of blubber, but exactly how much? The researchers can only guess.

The team chose to create a visual reconstruction of what the whale might have looked like, basing the head on skulls of related basilosaurid species, but they caution that some of the details are speculative. It could have been skinnier. But it also could have been quite a bit longer or fatter, Amson says.

So what does Amson personally think P. colossus — possibly the heaviest animal that ever lived — would have looked like in the flesh? “Probably like a giant sausage,” he says.

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