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New fossil whale might have been the world’s heaviest-ever animal
A new species of early whale might be the heaviest animal which has ever lived.
While its exact weight is a matter of debate, its unusual bones mean scientists can be certain that it was no ordinary cetacean.
Fossils found in Peru are redefining the history of whale evolution.
Named for its size and country of origin, a new paper estimates that Perucetus colossus could have weighed as much as 340 tonnes. If correct, the ancient whale would have weighed twice as much as the current record holder, the blue whale.
While researchers aren’t sure that the 40-million-year-old cetacean would have been quite this heavy, its extraordinary fossils point to it being unlike anything alive today.
Dr Travis Park, who researches whale evolution at the Museum and was not involved in the study, says, ‘Perucetus is the size of a whale but would have had a way of life more like a manatee or dugong.’
‘While it may not have been as heavy as 340 tonnes, it pushes back the occurrence of extreme body mass in the cetaceans by about 30 million years. It demonstrates just how important Peru is to study ancient marine fossils, and that the cetaceans have many more surprises left in store.’
The findings of the study were published in the journal Nature.
The bones of Perucetus are much denser than any from living whale. Image © Giovanni Bianucci.
The heaviest animal ever?
As soon as Perucetus’ fossils were discovered in Peru’s Ica Valley, it was immediately obvious that it was a very unusual species, even compared to the giant penguins and proto-whales which have previously been found in the region.
In total 18 bones were found, including 13 vertebrae, four ribs and part of the right hip. All of these bones are much larger than those of other whales from this time period, around 3.5 time bigger when compared to Cynthiacetus, one of its best-preserved relatives.
There are two main reasons for this difference. Firstly, the bones show a condition known as osteosclerosis, where cavities inside the bone have been filled in, while the bones are also pachyostotic, meaning extra bone has grown around the outside.
While it's possible these features may be pathologic, meaning the animal was suffering from disease, all of the bones were affected equally. These conditions are also found naturally in the bones of some modern animals such as dugongs and manatees, known together as sirenians.
Together, this suggests that the bones may have evolved to be extraordinarily dense. In sirenians, this is thought to be an adaptation to try and keep the large, buoyant marine mammals underwater as they feed, so it could have been similar for Perucetus.
To calculate just how much this unusual whale’s skeleton weighed, the researchers used the size and density of the bones in addition to comparisons to museum specimens such as Hope the blue whale. They estimate the skeleton weighed between 5.3 to 7.6 tonnes, the heaviest of any mammal and about the same as a fully grown African elephant.
Estimating how much the animal would have weighed when it was alive, however, is trickier. This is in part because Perucetus belongs to a group of extinct cetaceans known as basilosaurids, which were unlike almost any other living animal.
‘The body plan of large basilosaurids like Basilosaurus itself are almost serpentine, and there’s no living mammal like them today,’ Travis says. ‘Therefore, trying to use modern animals to estimate what Perucetus would have been like will never be completely accurate.’
The new species is related to Basilosaurus, an unusual cetacean which had an eel-like body. Image © Asmoth, licensed under CC BY-SA 4.0 via Wikimedia Commons.
To try and make an estimate, the researchers used a value known as the skeletal fraction (SF), which represents the amount of overall body mass made up of bones. This means that if you know the size and weight of a skeleton, it can be used to give an estimate of total body weight.
By using the SF of manatees and cetaceans, the researchers found that Perucetus’ overall weight was likely somewhere between 85 and 340 tonnes.
If this upper limit is correct then Perucetus would have been the heaviest animal ever to exist, but even at its lightest, it would still have been heavier than any land animal that’s ever lived, including the titanosaurs.
But regardless of being a potential record holder, Perucetus’ extreme size also means that it is rewriting the history of whale evolution.
‘In the overall history of cetacean body size, Perucetus is definitely an outlier,’ Travis says. ‘We knew that one of its relatives, Basilosaurus, was reaching around 20 metres in length but it was nowhere near as heavy.’
‘Other whales didn’t reach a similar weight until the evolution of baleen whales tens of millions of years later. Even if it’s not heavier than a blue whale, it is probably pretty close to the threshold of how big these animals can be.’
The closest living analogue to Perucetus is probably a manatee, which also live in shallow coastal waters. Image © Thierry Eidenweil/Shutterstock.
How did Perucetus live?
Due to the unusual characteristics of the skeleton and its incompleteness, the researchers can’t be completely certain how Perucetus would have lived. However, based on its relatives scientists have been able to make a few inferences.
‘As a basilosaurid, Perucetus was among the first cetaceans that were really adapted for oceanic life,’ Travis explains. ‘They were the first cetaceans to have a truly global reach and were generally adapted for coastal environments.’
The extremely dense bones suggest that Perucetus probably had a similar lifestyle to modern dugongs and manatees, with the combination of dense bone and lighter blubber allowing it to more easily control its buoyancy.
While the hip bones of Perucetus suggest it likely had small back legs, these were probably just evolutionary leftovers from its land-living ancestors and wouldn’t have been able to support its weight.
Like manatees and dugongs, it would have lived its whole life in the water, using its large tail to swim slowly through coastal waters, occasionally surfacing to breathe.
As for what it ate, the jury is out. Because of its size, it would have needed to eat a huge amount of food, and probably spent most of its day doing so. But without a skull it can only be speculated what it might have been eating.
‘The authors of the paper suggest that it might have eaten animals found on the seafloor, like molluscs and crustaceans,’ Travis says. ‘To eat so many of them each day, I would be surprised if it had a similar skull to its relatives, and instead it possibly would have had some kind of specialisation, perhaps suction or an expandable mouth cavity.’
Proving what it ate would mean finding more Perucetus fossils. If scientists are lucky, a more complete skeleton may be lying beneath the Peruvian plains, just waiting to resurface.
