‘Bizarre’ Porpoise With Giant Underbite Is First Mammal of Its Kind in Fossil Record

Semirostrum ceruttii porpoise

An extinct species of porpoise discovered in California is revealing what paleontologists call “bizarre” features that have been seen in fish and birds but never before in mammals.

Namely, the ancient animal had a giant, pendulous lower jaw that, while nearly lacking any teeth, was shot through with sensitive nerves.

While a handy attribute that’s been found in modern fish, like half-beaks, which use their lantern jaws to probe the seafloor for food, this feature has never been observed in a mammal, living or extinct.

Semirostrum ceruttii porpoise
An artist’s reconstruction shows Semirostrum ceruttii, an ancient skimmer porpoise, that lived off the coast of California up to 5 million years ago. (Bobby Boessenecker)

“The extinct porpoise is a bizarre new animal, with the mandible extending well beyond the beak-like snout, which it may have used for probing and ‘skimming’ in the substrate,” said Rachel Racicot of Yale University, in a statement.

“Although this morphology has been recorded in birds and fish, this is the first described mammal with this anatomy.”

Racicot and her colleagues suspect that, like half-beaks, the prehistoric porpoise may have used its sensitive mandible to sift the ocean floor for edibles.

This likely proved necessary, the scientists add, because CT scans of the animal’s skull show that its optic canals — the bony channels the hold the nerves and arteries that make eyesight possible — were much narrower than in modern porpoises.

This suggests that the ancient mariner had poor vision, although it does appear to have been capable of echolocation, like its descendants.

The fossil of the newfound mammals species, named Semirostrum ceruttii, was first discovered in 1990 in a coastal outcrop dated to between 1.6 and 5 million years old. [Read about a similar recent find: “Four-Tusked Walrus, New Whale and 19 Other Fossil Mammals Discovered at California Surf Spot“]

But the discovery of the unique animal’s features were made in large part through new, non-destructive technologies that allowed researchers to probe the fossil’s inner structure.

“Many exciting new species awaiting description are lying in museum collections, but the sort of detailed descriptions that are required to do full justice to them often take a lot of time,” Racicot said.

She and her colleagues report their find in the current issue of the journal Current Biology.

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