The largest animal on Earth swims through the depths of the oceans, but 50 million years ago whales walked on land on four legs.
A professor at Northeast Ohio Medical University reveals that the huge creatures are descendants of an ancient ‘little deer’ known as Indohyus.
Through research into the evolution of cetaceans, including hippos to whales, Hans Thewissen discovered a 47-million-year-old fossil in Pakistan containing a squat animal the size of a fox with an elongated body and a tail.
The bones trapped in a layer of mud reflect features of modern whales – a bone above the middle ear space and skull structure.
Thewissen and his team also determined that Indohyus waded in the water like a hippopotamus in search of food and as a means of avoiding predators, ultimately leading them to move from land to a fully aquatic lifestyle.
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The largest animal on Earth swims through the depths of the oceans, but 50 million years ago whales walked the surface on four legs. A professor at Northeast Ohio Medical University reveals that the huge creatures are descendants of an ancient ‘little deer’ known as Indohyus
Since Darwin, scientists have known that whales are descended from mammals that once walked on land, but which one had remained a mystery.
However, the missing link was confused when Thewissen and his team discovered the fossil in Pakistan.
It was cemented in a layer of mudstone found in the Kashmir region of India with hundreds of Indohyus bones.
The scientists describe the skeleton as “a mammal the size of a fox that looked something like a miniature deer.”
Through the researcher of the evolution of cetaceans, including hippos to whales, Hans Thewissen discovered that a 47-million-year-old fossil in Pakistan was that of a stocky animal with an elongated body and tail.
The bones trapped in a layer of mud reflect the characteristics of modern whales – a bone above the middle ear space and skull structure
After deeper analysis, researchers discovered similarities between the skull and ears of both the Indohyus and the whales.
They found that the bones of the Indohyus skeleton had a thick outer layer, much thicker than in other mammals of this size.
This trait is often seen in mammals that are slow water wading birds, such as the hippopotamus today.
“We think they were in the water waiting for their prey to drink, similar to crocodiles,” Thewissen told Discovery Magazine.
Indohyus’ water habits are further corroborated by the chemical makeup of their teeth, which revealed oxygen isotope ratios comparable to those of aquatic animals – all of which indicate that the creature spends much of its time in water.
Thewissen and his team also determined that Indohyus waded in the water like a hippopotamus in search of food and as a means of avoiding predators, ultimately leading them to move from land to a fully aquatic lifestyle.
Prior to these findings, whales were thought to be descended from carnivorous ancestors who moved to aquatic lifestyles to feast on ocean-dwelling fish.
The fossilized skull also had a bone above the middle ear space, which is also found in cetaceans.
And the eye sockets were towards the top of the Indohyus’ head – just like the placement of a whale’s eyes.
Thewissen’s team also looked at Indohyus’ teeth to find out what he was eating.
The levels of different carbon and oxygen isotopes in the tooth enamel of land animals differ from those in aquatic animals due to the different isotopic compositions in the food and water they ingest.
Indohyus’ teeth have higher levels of the carbon-13 isotope than is typical for water-foraging whales from the Eocene, suggesting that it fed on land-based plants.
“We would like to know in more detail what he ate,” Thewissen said. Isotopes in the teeth indicate that it was not submerged vegetation. We will study that in the future. ‘
Another clue as to how Indohyus lived can be found in his limb bones, which were thickened and heavy in the same way as a hippopotamus.
This suggests the animal was a wader, with heavy bones to keep it from floating.
Based on this evidence, Thewissen suggests that the ancestors of whales entered the water as a mechanism for avoiding predators, and did not develop specific aquatic feeding behaviors until much later.
Paleontologist Jonathan Geisler, of Georgia Southern University in Statesboro, had previously established a link between raoellids and whales, but his evidence was based only on tiny fragments of teeth. This new work strengthens the link, he says.
“What’s really important about these fossils is that they seem to support the hypothesis that the ancestor of cetaceans became semi-aquatic before teeth evolved that specialized in eating fish,” says Geisler.
The first ancestors of whales arose 42 million to 48 million years ago, which Thewissen describes as sea lions.
Then baleen whales followed, some 41 million years ago, including the ancestors of humpback and blue whales.
These were followed about seven million years later by toothed whales, which still swim in the oceans today.