Whale that lived 39 million years ago changes what we thought about evolution
A massive whale that lived 39 million years ago and weighed up to 340 tonnes is thought to have been the heaviest animal to ever exist.
Newly discovered Perucetus colossus is believed to rival the blue whale for sheer size, but with a skeletal mass estimated at two to three times that of an average blue whale, is predicted to have grown heavier than its modern relative.
Despite its massive size, Perucetus is thought to have had small front fins, a small pointed head and rounded fluke (tail fin) similar to a manatee.
The name Perucetus colossus names comes from its country of origin, Peru, and its massive size.
The animal is modelled from a partial skeleton, including 13 vertebrae, four ribs and one hip bone, all discovered in the south of the country. Discovered by a team from the University of Pisa and State Museum of Natural History Stuttgart in Germany led by Dr Eli Amson, the estimated complete skeletal mass is thought to be bigger than any animal ever known on land or sea.
The specimen was transported to the Natural History Museum in Lima, where they found that the vertebra weighed well over 100kg (220lb) and the ribs reached a length of 1.4m (4ft 7in).
The effect of such large bones on buoyancy suggests the whale lived in coastal waters, and may have led to its extinction at the end of the Eocene period when shallower environments became less productive.
Instead, Perucetus’s relatives, the ancestors of present day whales and dolphins, will have thrived in deeper waters.
Writing in the journal Nature, the authors note: ‘Although uncertainties remain regarding the precise lifestyle and diet of P. colossus, it was a slow-swimming, most likely coastal, benthic [bottom of the sea] consumer.’
While life on Earth is believed to have originated in the sea, the ancestors of marine mammals first evolved on land, before returning underwater.
The extensive fossil record of cetaceans – aquatic mammals including dolphins, whales and porpoises – provides an important understanding of this evolutionary history. A move towards gigantism and higher body masses is a common trend as species adapted to an aquatic environment, but peak body mass is thought to have been a relatively recent diversification.
However, the discovery of Perucetus suggests the opposite, and that peak body mass occurred around 30 million years earlier than previously thought.
‘The most important implication for the discovery of P. colossus concerns the evolution of cetaceans, and more generally that of extreme gigantism,’ said Dr Amson.
‘So far, extreme gigantism in cetaceans, as seen in the baleen whales, has been regarded as a relatively recent event – around five to 10 million years ago.
‘Thanks to P. colossus, we now know that gigantic body masses have been reached 30 million years before previously assumed, and in a coastal environment.’
During the same time, the second half of the Eocene period, peak body mass was already observed in other mammalian families on land.
The Natural History Museum in Lima has organised a temporary exhibition of Perucetus to coincide with the publication of the research.
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