Ancient whale may be heaviest animal ever

Palaeontologist drilling into fossil
Bones of the new species were sampled with core drills to assess their inner structure. Credit: Giovanni Bianucci.

Whale fossils found in Peru dating back 39 million years may belong to the heaviest animal to have ever existed.

The question on every four-year-old’s lips is: what’s the biggest animal ever?

Outweighing every other animal, living or extinct, is the blue whale. These ocean behemoths commonly reach 24–33 metres in length and average 90–140 tonnes. The largest ever blue whale was a colossal 190 tonnes – equivalent to about 32 large male African elephants.

Blue whales are even heavier than the largest dinosaurs – the long-necked titanosaurs. The largest of these, Dreadnoughtus, would have weighed “only” 70–100 tonnes. For a full round up of the biggest animals, check out this episode of the podcast Huh? Science Explained.

But the new fossil discovery may yet see the blue whale toppled as the animal kingdom’s ultimate heavyweight.

Perucetus colossus is thought to be one of the largest animals on record and, according to a paper published in Nature, tip the scales on the blue whale. A type of basilosaurid whale, P. colossus would have had a long, serpentine body and powerful jaws. Its bone density supports the theory that basilosaurids were specialised to a coastal and shallow water lifestyle.

The ancient whale’s partial skeleton includes 13 vertebrae, 4 ribs and 1 hip bone, found at the Paracas Formation in the Ica valley of southern Peru. The whale lived 39 million years ago during the Eocene epoch (56–34 million years ago). The Eocene was a relatively warm, tropical time in Earth’s history. Mammals flourished in its millions of years after the extinction event 66 million years ago that saw the end of the “age of dinosaurs”.

Diagram ancient whale human size comparison with partial skeleton fossils
Preserved bones of the new species, Perucetus colossus. Credit: Giovanni Bianucci.

The first whale ancestors emerged in early Eocene. These mammals returned to the water in an evolutionary about-face hundreds of millions of years after their first terrestrial quadruped ancestors left the oceans.

Ambulocetus (meaning “walking whale”) is a genus of early amphibious cetacean that lived about 48 million years ago in what is now Pakistan. The vestiges of this terrestrial lifestyle (including non-functional hind limbs and air-breathing lungs) lives on even in modern whales and dolphins.

“Estimating body mass in basilosaurids is challenging,” the authors write. “For P. colossus, methods based on simple skeletal measurements would also probably be biased by the fact that its skeletal morphology starkly departs from that of other marine mammals.”

But, taking manatees and other living marine mammals as their guide, the researchers suggest minimum and maximum body mass for P. colossus.

The full Perucetus is estimated to have been about 17–20 metres long. However, bone density analysis suggests that the creature’s skeletal mass would have been 2–3 times that of a 25m long blue whale. Therefore, the authors estimate that P. colossus could have had a total body mass between 85 and 340 tonnes. Its estimated mean mass is 180 tonnes.

Three whale skeleton fleshed out with diver diagram on black background
Fleshed out models of the new species, P. colossus, along with smaller, close relative (Cynthiacetus peruvianus) and a blue whale. Credit: Florent Goussard for C. peruvianus’ skeleton model and staff in the Imaging and Analysis Centre at the Natural History Museum London for the blue whale’s skeleton model, copyright: Trustees of the Natural History Museum; Marco Merella for the Perucetus models.

This would comfortably place P. colossus in the same weight division as the blue whale, and even possibly challenge for the title of the heaviest animal to ever exist. Ocean animals such as whales have a certain advantage when it comes to growing big. Being in the water, rather than on land, means that animals can get heavier while having to contend with gravity far less than their land-locked counterparts.

But gigantism in cetaceans was previously believed to be a relatively recent development in whale evolution. The previous largest basilosaurine whale known to science, Basilosaurus cetoides (35–34 million years ago), could reach up to 20m in length, but probably didn’t weigh more than 10 tonnes.

P. colossus suggests that cetaceans had reached peak body mass about 30 million years earlier than previously thought – and only about 10 million years after returning to the oceans.

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