Could this massive whale from 40 million years ago have been the world's heaviest animal?

Move over, blue whale, there is a new contender for the heaviest animal in Earth's history.

Scientists have described fossils of an early whale unearthed in Peru called Perucetus colossus that lived about 38-40 million years ago during the Eocene epoch. 

It was a creature built like a manatee that may have been heavier than the blue whale, which has long been considered the heftiest animal on record.

Italian researchers who published their findings the scientific journal, Nature, estimated Perucetus, or "colossal Peruvian whale", was about 20 metres long and weighed up to 340 tonnes, a mass greater than any known animal, including the blue whale and the largest dinosaurs.

"The main feature of this animal is certainly the extreme weight, which suggests that evolution can generate organisms that have characteristics that go beyond our imagination," University of Pisa palaeontologist Giovanni Bianucci said.  

The minimum mass estimate for Perucetus was 85 tonnes, with an average estimate of 180 tonnes.

The biggest-known blue whale weighed around 190 tonnes, though it was longer than Perucetus at 33.5 metres.

A long-necked, four-legged herbivore known as Argentinosaurus, that lived about 95 million years ago in Argentina and was ranked in a study published in May as the most-massive dinosaur, was estimated to weigh about 76 tonnes.

Part of Perucetus's skeleton was excavated in a coastal desert of southern Peru — a region rich with whale fossils — with 13 vertebrae, four ribs and one hip bone unearthed.

The bones were extremely dense and compact.

This characteristic, called pachyosteosclerosis, is absent in living cetaceans — including whales, dolphins and porpoises — but present in sirenians, another marine mammal group that includes manatees and dugongs.

Its skeletal mass alone was between 5 and 8 tonnes, at least twice the weight of the blue whale.

"Its fat, bloated body may have been more like that of a sirenian than of any living whale," Mr Bianucci said

"Among sirenians, due to its giant size and probable similar lifestyle, it could recall Steller's sea cow, discovered in 1741 and exterminated by humans a few years later."

'Peaceful giant'

No cranial or tooth remains were found, making it difficult for researchers to understand its diet and lifestyle.

They suspect Perucetus lived like sirenians as an animal that fed near the bottom of shallow coastal waters, rather than an active predator. 

Palaeontologist Olivier Lambert from the Royal Belgian Institute of Natural Sciences in Brussels said the mammal would have moved slowly due to its size. 

"Because of its heavy skeleton and, most likely, its very voluminous body, this animal was certainly a slow swimmer," he said.

"This appears to me, at this stage of our knowledge, as a kind of peaceful giant, a bit like a super-sized manatee.

A history of gigantism

Mr Bianucci said it was unlikely Perucetus was a filter-feeder like today's baleen whales including the blue whale.

"Perhaps it was herbivorous like the sirenians, but this would be the only case among cetaceans [or] it fed on small molluscs and crustaceans in sandy bottoms like the gray whale. Or it could have been a scavenger on vertebrate carcasses, similar to some extant large-body sharks," he said.

Whales evolved over 50 million years ago from hoofed, land-dwelling mammals as big as a medium-sized dog and Perucetus still had vestigial back limbs to prove this. 

Skeletal traits indicate Perucetus was related to Basilosaurus, another early whale that was similar in length but not as big an a predator, boasting a streamlined body, powerful jaws and large teeth.

Mr Bianucci said Perucetus indicates to them that "cetaceans developed gigantism at least twice".

"In relatively recent times, with the evolution of the large baleen whales, and some 40 million years ago, with the radiation of the Basilosaurus relatives of which Perucetus is the most extraordinary representative," he said.

Reuters