Fossilized four-legged whale discovered in Peru

A whale ancestor with four legs has been discovered from fossils in Peru. The missing link between land and sea animal has provided clues as to how whales returned to living in the oceans after millions of years above the surface.

PP
An artists interpretation of Peregocetus pacificus (by Alberto Gernari)

Although whales and dolphins have been swimming around our oceans for millions of years, their ancestors were very much at home on the land. Now one of those ancestors, Peregocetus pacificus, has been revealed from fossils discovered by researchers in Peru. The four legged creature was a perfect mix between land and ocean dweller and sheds some light on how whales evolved into the beautiful animals we see today. Despite looking more like a dinosaur than a whale these fascinating creatures are now believed to be the first mammal to revert back to living underwater. It is also believed that these whacky whales made amazing journeys from Africa to South America without flippers or flukes. So how did whales transition back into the water? What have we learnt? And what were these ancestors really like?

From the sea to land, and back again

Despite living in the world’s oceans for millions of years, whales and dolphins are still babies compared to other marine animals like sharks, octopuses and crustaceans. That is because they split off from other ocean creatures to live on the land hundreds of millions of years ago, like all the animals we see around us today. The difference is that after evolving to live above the surface cetaceans re-evolved to live back below it. That is why all cetaceans from the blue whale to the bottlenose dolphin continue to breathe air rather than water today. But this evolutionary u-turn has made it very hard to track whales’ evolutionary history above the surface. That is because when they were running around on land they were indistinguishable from other mammals and it was only when they developed aquatic features that we could begin to identify fossils as whale ancestors.

Blue whale skeleton
Whilst lot is known about modern cetaceans less is known about their ancestors

The missing link

It was previously believed that the last precursor to whales were small deer-like mammals discovered in India and Pakistan. Researchers thought that these ancestors slowly developed whale like properties after millions of years of living in the shallows. But the discovery of fossils in Peru have provided evidence that there were whales walking around on the Earth before there were any swimming in the sea. Peregocetus pacificus was discovered in 2011 by a group of seven palaeontologists led by Olivier Lambert and is believed to have originated around 42 million years ago. What stands it apart from other whale ancestors discovered in Asia, apart from its physiology, is that it was found in South America. Therefore it must have been capable of making the long swim from Africa. It is therefore likely that P. pacificus was the last whale ancestor to fully be able to live above the surface and a key piece in the evolutionary puzzle of all cetaceans.

Bones
The bones found in Peru that have helped bring P. pacificus to life

Physiology & behaviour

Peregocetus pacificus was perhaps the perfect mix between land and sea animal. Its four legs were all of equal length, fused to the spine at the hips and shoulder, and its digits ended in tiny flattened hooves meaning it would have been comfortable walking on land. But at the same time its streamlined body shape, long powerful tail and most likely webbed feet would have also made it a competent swimmer. However it is likely that the technique it used to swim was more like that of an otter, undulating up and down, than a modern day whale. Its teeth and jaw shape suggest it was carnivorous and resembles modern day predators with canines, pre-molars and complex cusp molars. This means it most likely hunted on land whereas most modern day whales have simple peg-like teeth if they swallow their underwater prey whole or baleen brushes if they feed on plankton.

Fluke
The fluke of a whales tail shows just how far they have come

Over millions of years of evolution caused by an increasingly aquatic lifestyle its physiology and behaviour slowly morphed into what we see today. The pelvic bones uncoupled from the spine to enable more efficient swimming. As more time was spent in buoyant conditions there was less need for strong weight bearing legs. The front legs became flippers and the vestigial hind legs shrunk and disappeared altogether. Their heads became more flattened and its teeth rounded out or disappeared with the shift in diet. Finally the tail developed a fluke to allow for deep dives whilst internally respiratory and circulatory systems developed to deal with a lack of oxygen and crushing pressures of the deep. The result was the incredible array of whales and dolphins we see in our oceans today.

Filling in the gaps

Apart from being very interesting to learn about the discovery of an animal such as Peregocetus pacificus highlights that there is still so much we don’t know about the evolutionary history of our planet. When writing ‘On the Origin of Species’ Charles Darwin even named a chapter ‘On the Imperfection of the Geological Record’. This was seen as something of a flaw in his theory at the time but he was completely right in realising there were still many ancestral species waiting to be discovered. As well as better revealing how modern day cetaceans evolved this discovery also stirs up our fossilferous imagination about an animal we didn’t realise ever existed. Who knows what we will find next. But as we better understand what came before us it gives us a greater appreciation of where we are now.


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