Four-Legged Whale Ancestors Reached South America in an Otter-Like Swimming Style

Artistic reconstruction of two individuals of Peregocetus, one standing along the rocky shore of nowadays Peru and the other preying upon sparid fish. The presence of a tail fluke remains hypothetical. (A. Gennari)
Four-Legged Whale Ancestors Reached South America in an Otter-Like Swimming Style
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Reinout Verbeke

A four-legged whale from Peru indicates that early whales crossed the South Atlantic before 42.6 million years ago and may have propelled like otters: with a robust tail and webbed fingers on their long feet.

In 2011, an international team of palaeontologists excavated a well-preserved skeleton of a four-legged whale ancestor in Playa Media Luna, in Peru's desert-like Pisco Basin. The new find, described in the journal Current Biology, is about 43 million years old and illustrates a key phase in the evolution and dispersal of early whales: it provides clues about when, through which path and with which anatomical adaptations early whales reached the New World.

Oldest Four-legged Whale of the New World

Whales evolved more than 50 million years ago in southern Asia from four-legged hoofed mammals no larger than a wolf. Some gradually adapted to water, but could still move over land. Fossil finds show that these amphibious whales have spread from nowadays India and Pakistan to North Africa.

The fossil skeleton of Peregocetus pacificus - literally: “the travelling whale that reached the Pacific Ocean”, shows what happened next. For a long time this phase remained a mystery due to fragmentary and disputable fossil evidence. The new four-legged whale ancestor, 3.4 to 4 metres long, is 42.6 million years old and the oldest fossil of a whale in the New World. It is also the most complete skeleton of a four-legged whale outside India and Pakistan.

Like An Otter

“The animal could carry its own weight and crawl about on land”, says palaeontologist Olivier Lambert of the Royal Belgian Institute of Natural Sciences, who excavated and described the skeleton together with colleagues from Peru, Italy and France. “We can see this, among other things, because the pelvis is firmly attached to the sacrum. And the front and hind legs are very similar to those of Peregocetus's ancestors from India and Pakistan. You can even see marks of small hooves on the toes and fingers.”

But Peregocetus must also have been a good swimmer. The excavation team has not found the last tail vertebrae, so we don't know if the animal had a tail fluke. “But the anatomy of the first vertebrae of the tail resembles that of amphibious mammals such as otters and beavers”, Lambert says. “So we think the animal propelled through the water by wave-like movements of the posterior part of the body, including the tail, and by moving its large feet and long toes that were most likely webbed.”

Going With the flow

The researchers think its ancestors crossed the Atlantic Ocean between North Africa and the north of South America. At that time, the distance between the two continents was half what it is today. The surface current from Africa to South America must have helped these animals to reach the other side. Later, relatives of Peregocetus would spread further north, to the east coast of North America.

Some of those four-legged whales will evolve into the basilosaurids, with highly reduced hind legs. Basilosaurids were fully aquatic and mainly used their tail fluke to propel themselves forward. From this family will emerge the two groups of whales we still know today: baleen whales (Mysticeti), such as the humpback whale and blue whale, and toothed whales (Odontoceti), such as dolphins and sperm whales.

Peru’s Pisco Basin is a hotspot for palaeontologists studying fossil whales. In 2017, the international team with Olivier Lambert found, 200 metres away from the spot where Peregocetus pacificus was excavated, a 36.4 million year old descendant of the basilosaurids, identified as the oldest known member of the mysticete group: Mystacodon selenensis.

This study of Peregocetus was supported by the Muséum National d’Histoire Naturelle, Paris, and the Italian Ministero dell’Istruzione dell’Università e della Ricerca.

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