Look: Extinct porpoise had biggest underbite -- ever -- on a mammal

Scientists have discovered the biggest underbite ever found in a mammal. It belongs to an extinct porpoise that lived on the California coast 5 million years ago.

That crazy long underbite you see in the images above was not an anomaly. Scientists have found 15 specimens of this porpoise, known as Semirostrum ceruttii, all with a similarly major underbite, all from California. The fossils date from 1.5 million to 5.3 million years ago.

The protuberance is an extension of the porpoise’s jaw called a symphysis that is analogous to the human chin. It had extensive nerve canals that connected it to the brain, suggesting that the jutting jaw wasn’t just long, it was also highly sensitive.

Rachel Racicot, a doctoral candidate at Yale University and the lead author of a paper describing the porpoise in Current Biology, said this marine mammal’s jaw may have been as sensitive to touch as one of our fingers. This helped the porpoise feel for prey embedded in the sea floor. This sense of touch would be especially helpful since its eyesight was not so good. She said it likely ate anything it encountered.

The long jaw was too thin to act as a shovel to help the porpoise scoop food into its mouth. When it encountered a crustacean or squid, it probably threw its head back a little and then clamped down on the food with its upper jaw, Racicot said. There are animals living today with a pronounced underbite that feed in a similar way, including birds known as skimmers, and half-beak fish.

Although the underbite morphology seems kind of awkward, the porpoise was able to survive for millions of years, making it a fairly successful animal.

Racicot said it may have become extinct because of climate change. The porpoise lived during the Pliocene era when the Earth was warmer and there was very little ice at the poles. Sea levels were high, and there was a large area of shallow continental shelf, where this bottom feeding mammal likely thrived.

When the sea levels eventually fell in the glacier Pleistocene era, much of the shelf area was lost, resulting in less habitat for the long-jawed porpoise. Porpoises with a less distinct jaw who could feed in a variety of conditions would have been better able to adapt to the new sea conditions.

Although more research needs to be done to see if this was the cause of the porpoise’s demise, it does seem that in the end, that massive underbite may have been this poor porpoise’s undoing.

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