How Did Whales Reach the Americas? A Four-Legged Fossil Offers New Clues

Dubbed Peregocetus pacificus, the newly-described species was adapted to life both in and out of the water

Artistic reconstruction of two individuals of Peregocetus, one standing along the rocky shore of nowadays Peru and the other preying upon fish. The presence of a tail fluke remains hypothetical. A. Gennari

The story of modern whales begins around 50 million years ago, in what is today India and Pakistan, with four-legged hoofed mammals about the size of a wolf. As whales’ ancient ancestors became increasingly adapted to aquatic environments, they dispersed to North Africa and then to the Americas, eventually losing their hind legs and gaining flippers. But due to fragmentary fossil evidence, the outlines of this journey are fuzzy. Scientists are not entirely sure when these early whales migrated, what route they took, or how they adapted along the way.

As Leila Miller reports for the Los Angeles Times, a newly-described, four-legged whale fossil from Peru is helping fill in some of these gaps. Discovered in 2011 in Peru’s Pisco Basin, the fossil dates to 43 million years ago and points to a creature that stretched about 13 feet long and had four legs strong enough to support its weight on land. The whale also had a robust tail, which it likely used to move through water. Writing in the journal Current Biology, an international team of researchers reveal that the fossil represents “the first record of an amphibious whale for the whole Pacific Ocean.”

The researchers have dubbed the species, which was previously unknown to scientists, Peregocetus pacificus, or “the travelling whale that reached the Pacific Ocean.” Peregocetus is likely the oldest four-legged whale ever found in the Americas and “the most complete outside India and Pakistan,” Olivier Lambert, lead study author and a vertebrate paleontologist at the Royal Belgian Institute of Natural Sciences, says in a Natural History Museum statement.

Because the fossil was unusually well preserved, scientists were able to observe a number of features adapted for life both on land and in the water. Peregocetus had a long snout and sharp teeth, which suggests it was snacking on bony fish. The excavation team did not uncover the whale’s last vertebrae, so they do not know if it had a fluke—the paddle-like appendage that modern whales use for propulsion. But Peregocetus was likely a powerful swimmer.

“[T]he anatomy of the first vertebrae of the tail resembles that of amphibious mammals such as otters and beavers,” says Lambert. “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.”

Peregocetus’ fore and hind legs are very similar to those of its ancestors from India and Pakistan, and its fingers and toes were crowned with hooves that would have helped it move about outside of the water. But Peregocetus probably wasn’t particularly adept at walking “and certainly not at running” on land, Lambert tells Miller of the L.A. Times. The animal may have only surfaced onto terrestrial environments for specific activities, like breeding and giving birth.

The location of the fossil on the coast of the southeastern Pacific suggests that early whales arrived in the Americas by crossing the south Atlantic between Africa and South America, the researchers say. At this point in prehistory, the distance between the continents was half of what it is today, and the whales may have been helped along by surface currents. But they still would have needed to spend days, or even weeks, at sea—another indication that they were becoming increasingly adapted to marine environments.

Peregocetus thus represents an important evolutionary link between the earliest whales and the fully aquatic ones we know today. According to the Royal Belgian Institute of Natural Sciences, some of Peregocetus’ four-legged relatives eventually spread to the east coast of North America and evolved into the basilosaurids, a group of aquatic whales that emerged around 41 million years ago and retained very small hind limbs. The basilosaurids gave rise to the two groups of modern whales: baleen whales, like the humpback and blue whale, and toothed whales, like the dolphin and sperm whale.

“They went from small hoofed mammals to the blue whale we have today,” Travis Park, a postdoctoral fellow at the Natural History Museum who studies cetacean evolution and was not involved in the recent study, tells the Guardian’s Hannah Devlin. “It’s so interesting to see how they conquered the oceans.”

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