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This ancient whale likely sucked prey into its mouth like a giant vacuum cleaner

Meet Mystacodon selenensis

An illustration of Mystacodon selenensis diving down to catch eagle rays along the seafloor off the coast of present-day Peru.
Illustration by Alberto Gennari

Scientists have found a new species of whale ancestor — a toothy cetacean that sucked prey into its mouth and likely hunted at the bottom of the seafloor. The 36-million-year-old fossil gives us new information on how whales evolved, showing, for example, that the marine animals likely lost their hind limbs more recently than was previously thought.

The fossil, which is described in a study published today in Current Biology, was first discovered in 2010 in Peru by paleontologist Mario Urbina of the Museo de Historia Natural. It’s identified as the oldest known member of a group of whales called mysticetes, which includes modern baleen whales, according to the study. Today, these whales — like the blue whale or the humpback whale — don’t have teeth, but keratin fibers called baleen that filter out the ocean water to catch fish or plankton. The fossil unearthed in Peru was found to have teeth, and so the scientists called it Mystacodon selenensis, which basically means “toothed mysticete.”

The specimen helps us understand the evolution of whales, according to study co-author and paleontologist Olivier Lambert of the Royal Belgian Institute of Natural Sciences. It can be considered “an intermediary step” between younger mysticetes and an ancient group of extinct cetaceans called the basilosaurids, he says. These archaic sea monsters, which lived about 41 million to 23 million years ago, had teeth and jaws suited for biting and attacking prey. They also had small hind limbs.

Lambert and his colleagues analyzed the Mystacodon’s skeleton and compared it to that of basilosaurids. “We noted interesting differences,” Lambert says. They found that the Mystacodon’s snout was wider and the skull shape was different from that of basilosaurids. This suggested that this ancient whale was a “suction feeder” — an animal that catches prey by sucking water into its mouth. The researchers also noticed that the teeth of the Mystacodon were more abraded and the eye sockets were located higher on the skull. These are signs that the animal hunted at the bottom of the ocean. The sand of the seafloor can wear down the teeth, and “higher eyes will allow the animal to look forward above its snout,” Lamber says. “This would be useful for locating prey along the seafloor, with the snout along the bottom.”

Members of the excavation team digging around the skeleton of Mystacodon selenensis at the Media Luna locality in the Pisco Basin, Peru.
Photo by Giovanni Bianucci

The Mystacodon fossil provided one more surprise: its pelvis bone showed that the ancient whale must have had hind limbs sticking out of its body. Scientists had previously thought that ancient whales lost their hind limbs in the basilosaurid phase of their evolution. But the discovery suggests instead that modern baleen whales and modern toothed whales lost the hind limbs independently.

Next, Lambert and his colleagues want to better analyze the fossil to confirm the findings. For example, if the Mystacodon really fed at the bottom of the ocean, its bones should be denser and more compact. Using CT scans could help find that out. “This is a very preliminary description,” Lamber says. “We want to go much more into detail.” And we can’t wait to learn more about this wild toothed suction feeder of the past.