This 36-Million-Year-Old Fossil Is a “Missing Link” in Whale Evolution

Discovered in Peru, the new fossil has tiny remnants of hind limbs

Whale Illo
Illustration of Mystacodon selenensis Alberto Gennari

About 50 million years ago, the earliest ancestors of whales slipped into the ocean. Generation after generation, the creatures slowly changed, losing their hind limbs and gaining flippers. A group of these early creatures, known as the basilosaurids, evolved into two broad groups of whales that are found today: the toothed whales, like modern sperm whales and orcas, and the filter-feeding baleen whales, like today's blue whales and humpbacks.

Molecular and genetic research suggests that this split took place roughly 38 to 39 million years ago, but until now no fossils of these early creatures had been found. But as Sarah McQuate reports for Nature, scientists have uncovered the oldest baleen-whale relative yet. And at 36.4 million years old, this this fossil fills in the gaps in whale evolution

As McQuate reports, the new species was excavated from Playa Media Luna in the Pisco Basin area of southern Peru and has been named Mystacodon selenensis. The creature was likely about 13-feet long, the length of a bottlenosed dolphin. But unlike modern baleen whales, which use plates made of keratin to screen krill and other small organisms out of the water, M. selenensis had teeth and likely sucked up small creatures like shrimp or squid off the bottom of the ocean floor.

That suction feeding technique links M. selenensis to older species and modern whales. “It perfectly matches what we would have expected as an intermediary step between ancestral basilosaurids and more derived mysticetes [baleen whales],” paleontologist Olivier Lambert of the Royal Belgian Institute of Natural Sciences and co-author of the paper in Current Biologysays in a press release. “This nicely demonstrates the predictive power of the theory of evolution.”

As Nicola Davis at The Guardian reports, the find also jibes with another whale fossil discovered, dubbed Alfred. That specimen dates back some 25 million years ago and was also a suction feeder, suggesting that it took a long time for modern baleen feeding to develop.

While the fossil bears out paleontologists' predictions, it did come with one big surprise: it had tiny hind limbs sticking out of its body, Davis reports. Lambert says that these tiny limbs had no real function—also known as vestigial organs. But the find upturned researchers' belief that whales completely lost their back limbs before the toothed and baleen whale ancestors split.

Paleontologists have been slow to put together the whale family tree, Lambert says, because they have been looking for fossils close to home in Europe and North America. But it turns out that much of the action in whale evolution took place in Antarctica, Peru, and India. Now that they are looking in the right places, they are finding more and more specimens.

That’s also a plus for evolutionary theory in general. “For a long time, Creationists took the evolution of whales as a favorite target to say that, 'Well, you say that whales come from a terrestrial ancestor, but you can't prove it. You can't show the intermediary steps in this evolution,” Lambert says in the press release. “And that was true, maybe thirty years ago. But now, with more teams working on the subject, we have a far more convincing scenario.”

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