How A Palaeontologist Studies Whale Evolution?

It was a cool November morning in 1973, and palaeontologist Vijay Prakash Mishra knocked around for fossils along the flat-topped hills in Kutch, Gujarat.

It was a cool November morning in 1973, and palaeontologist Vijay Prakash Mishra knocked around for fossils along the flat-topped hills in Kutch, Gujarat.

“There had been reports that there were large skulls but nobody in India, in fact, had identified them,” said 78-year-old Palaeontologist, Ashok Sahni, the sensei of Indian palaeontology and Mishra’s teacher who had chalked out this detective mission.

Mishra spent days trodding around the silvery, salt-crusted desert, trying to spot ancient remains. Finally, he stumbled upon some abnormally large fossils.

“I kept searching and searching,” said Mishra. “Then in some marine rocks I found teeth and bones. But they weren’t of reptiles. These were distinctly mammalian but far more primitive than similar fossils found elsewhere.”

The discovery was the first of its kind in India. These were ancestral remains of the biggest animal on our planet – those belonging to the order of aquatic mammals called cetaceans, comprising whales, dolphins and porpoises.

Skeletons tumble

“The ‘real’ whale story begins at about 47-48 million year old rocks in Pakistan and India,” said Sunil Bajpai, a palaeontologist at IIT Roorkee who followed the tracks of Sahni and Mishra. “It documents the transition of a whale-like mammal from land to water.”

Over three decades of finds, this fascinating tale of mammalian evolution wove itself through discoveries that threw up eye-popping facts.

For example, when Bajpai began his study in the 1980s, geneticists had just traced whales to hoofed mammals. It turned out that dolphins and whales were more closely related to hippopotamuses and cows than to sharks and sardines. In fact, fossil digs in northern Pakistan and Kashmir and Kutch in India confirmed these ideas.

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Through several trips that decade, Bajpai and his collaborator Hans Thewissen, a Dutch-American palaeontologist, poked around bright red and yellow rocks in Kutch belonging to a geological epoch called the Eocene – a time of balmy global weather.

This was about 50 million years ago. The Indian subcontinent was drifting closer to Asia, squeezing out the Tethys sea along its northern fringes. The habitat, with its leafy riverbanks, was a cozy set-up for a cat-sized, deer-like hoofed mammal. This was the Indohyus – the Indian pig.

While Thewissen and Bajpai scouted for fossils in Kutch in the 1980s, a territorial Indian geologist, A. Ranga Rao, scooped truckloads of fossils from Kalakot in Kashmir, including that of Indohyus. In 2005, an Indohyus fossil fell on Thewissen’s lap through Rao’s widow.

Back in the US, Thewissen’s fossil handler accidentally knocked off a walnut-shell-like bone while chipping at the Indohyus relic. It was an involucrum, a bowl-shaped ear bone that conclusively identified whales.

“Only whales hear that way, using that structure,” said Thewissen over a phone call.

The hoofed, land-dwelling herbivore, which looked nothing like streamlined, new-age whales, stuck as the first page of the whale fossils’ casebook. Indohyus was perhaps one of the earliest, four-legged whale ancestors to dive into water, to avoid predators or to look for food.

Next in the cetacean evolutionary queue was the sharp-toothed Pakicetus. The fossil of this wolf-faced primitive whale was found in 1981, this time in northern Pakistan by an American palaeontologist named Philip Gingerich.

It signalled yet another amazing transformation. In this version of primitive whales, the eye sockets had migrated from the sides of the head to the top of its skull. Like a crocodile, this feature may have allowed it to spot prey at a river’s edge while staying submerged – a valuable evolutionary feature as this whale ancestor preferred water.

“For any animal or organism to change its habit or how it derives its source of nourishment or where it is living there has to be an ecological stress,” Sahni said. “This ancient ocean, the Neotethys that separated India from Asia, was becoming shallow and narrow. It created ecological opportunities.”

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Next in line was the clumsy, otter-like Ambulocetus. Thewissen unearthed this fossil in 1991 in Pakistan. Ambulocetus, literally ‘walking whale’, was amphibious. It had a strong muscular tail and possibly webbed walrus-like feet, and was cozy in riverine environments lacing salty seas.

Over 10 million years, archaic whales flipped from being like the terrestrial, deer-like Indohyus to the amphibious, webbed-feet ambulocetus.

The evolutionary compass was now steering towards the first truly marine ancestral whale: Remingtonocetus. Sahni found this fossil in the 1980s. This crocodile-headed ancestor probably splashed about muddy lagoons and had small eyes, which meant it used its sense of smell to catch fish.

Sahni had labeled the aquatic, short-limbed Remingtonocetus after Remington Kellogg, a celebrated American palaeontologist. Ironically, in the 1930s, Kellogg had dismissed the existence of whales in the Indian Ocean during the Eocene epoch. The earliest fossil whales were then known from the Faiyum basin in Egypt, a place that is now called Wadi-Al-Hitan, or ‘whale valley’.

Four decades later, cetacean remains were bursting out of the banks of the Indus river in Pakistan, the rocky outcrops of the Himalayas and the barren Kutch desert in India. The region became tagged as the cradle and graveyard of Earth’s early whales.

“The earliest ones really looked more like wolves than like whales,” said Thewissen, sculpting an image of ancestral whales. “Shortly thereafter, there were ones that looked like crocodiles. Then there were whales that looked more like seals or sea lions and otters.”

Under the sea

In the next stage of evolution, aquatic whales shed their limbs – be it hoofed or webbed feet – for fins and paddles. The more-spindle-shaped predators, their bodies streamlined for swimming better, now followed their prey into deeper seas, which explains why these fossil finds are not limited to India.

“Protocetids are part of the early whale story but they are rather cosmopolitan,” said Bajpai.

Fossils of the marine, flat-tailed protocetids were found across many continents – Africa, Europe, North America and South America. They probably wore flukesThe tail of cetaceans – flat instead of vertical[/footnotes] and were about 10 feet long. Their nostrils had migrated from the tip of the snout to halfway up.

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“Only when they become pursuit predators are they able to cross bigger bodies of water,” said Thewissen. “So by the time you come to Basilosaurus, they are basically all over the world.”

Basilosaurus is Latin for ‘king lizard’. The 19th century discoverer of these fossils had initially misidentified them as a giant sea serpent. The misnomer stuck.

This primitive whale had a narrow body, almost as long as a school bus. It wore front paddles like modern seals with tiny hind limbs. The nostrils, or blowhole, were now at the top of the head.

Like modern whales, basilosaurus had facial bones that supported echolocation, which meant that they could use sound echoes in water to navigate or find prey. It had thick tissues in its ears to withstand pressure, allowing it to dive deeper and for longer.

These are features we recognise in some of the largest animals found today – the baleen whales and the toothed whales.

More than 50 million years ago, a cat-sized, hoofed terrestrial animal originated around a riverbank neighbouring the Tethys sea. It evolved to zigzag through seas and reached foreign shores, and eventually transformed into an aquatic mammal and the largest creature on Earth.

“The beauty of the story is that you had a land animal that was able to completely adapt to life in the water,” said Sahni. “There was no big predator in the ocean at that time. The sea monsters were done with the dinosaurs. There were big sharks but no big mammal that was the top predator. So it was an opportunity for a medium sized animal to try and exploit this ecological niche.”

Originally published at the wire

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