The fossilised bones of an ancient creature that patrolled coastal waters 40m years ago belong to a newly discovered species that is a contender for the heaviest animal ever to have existed on Earth. Fossil hunters discovered remnants of the enormous and long-extinct whale in a rock formation in the Ica desert of southern Peru. Fully grown adults might have weighed hundreds of tonnes, researchers believe. “It was very unlike anything I had seen before,” said Alberto Collareta, a palaeontologist at the University of Pisa who worked on the fossils. It is “the heaviest skeleton as far as mammals are concerned, possibly the heaviest vertebrate ever”, he said. Until now, the blue whale was widely regarded to hold the record for the animal with the largest body size. The giant skeleton of Hope, the blue whale that looms over visitors to the Natural History Museum in London, is more than 25 metres long and weighs 4.5 tonnes. But the new species, named Perucetus colossus after the country of origin and its gigantic size, could have grown larger and heavier than the blue whale, the scientists said. Despite being a mere 20 metres long, the skeleton of P. colossus would have been two to three times heavier than that of a 25-metre blue whale because of its denser bones. Its body mass would have come in at 85 to 340 tonnes, they estimated. The researchers reached the conclusion after comparing 13 vertebrae, four ribs and a single hip bone from P. colossus with bones from close whale relatives. Beyond its vast size, P. colossus appears to have had forelimbs for walking on the seabed and little, vestigial hind limbs. “If we look at the most conservative estimate of 85 tonnes [for the newly discovered animal], we are definitely in the ballpark of the blue whale, which is the largest animal known to date,” said Eli Amson, a specialist in mammal fossils at the State Museum of Natural History in Stuttgart. “Maybe there’s some single individuals of blue whales that were much larger, but maybe from this new species there are also much larger individuals.” Writing in the journal Nature, Collareta and colleagues described how the fossils were discovered 13 years ago but took many years to prepare for study. Unlike most whales, which have relatively light skeletons and can prey on fast-moving fish and other marine creatures, P. colossus had heavier bones and was more likely a slow-swimming scavenger. Amson said the find offered important insights about gigantism in early cetaceans – animals that include dolphins, whales and porpoises – adding that while blue whales and their ancestors are usually found in open-sea environments, the newly discovered animal evolved in shallow coastal waters. Travis Park, a postdoctoral researcher and fellow at the Natural History Museum in London, who was not involved in the work, said the discovery suggested cetaceans achieved extreme body masses about 30m years earlier than previously thought. Other large basilosaurids, or ancient whales, that lived alongside P. colossus were probably more slender, serpentine, and flexible, making the animal very different from any cetacean previously known from the middle Eocene. “It shows us that we still have lots to learn about what cetaceans were capable of doing prior to the evolution of the modern groups,” Park said.