An extinct porpoise sets the record for the largest underbite that scientists have ever found on a mammal. Rachel Racicot, a doctoral candidate at Yale University and the lead author of a paper describing the porpoise in Current Biology, used its unusual jaw to sift for prey through expanses of now-vanished underwater embankments off the present-day California coast.
The species is Semirostrum cerutti, and it lived a few million years shy of the rise of humans. Credit for discovery of the species goes to Richard Cerutti, a collector from the San Diego Natural History Museum who first identified fossils of this ancient marine mammal in old rock formations on the California coast in 1990. Years later, Racicot and colleagues examined the fossils, which came from 15 different specimens all dating from 1.5 to 5.3 million years ago, to produce the Current Biology study.
Based on her studies, Racicot discerned that the jaw had extra nerve endings that would have made it highly sensitive to touch, and therefore helpful for feeling for prey embedded in the sea floor. Its eyesight was most likely sub-par.
The porpoise was probably not a picky eater and would have considered anything that it found fair game. Crustaceans and squid would have been typical meals, though. When the porpoise found one of them, it would probably throw its head back a bit and then clamp down on the unlucky critter with its upper jaw, Racicot explained. She noted that several present-day birds, including skimmers and half-beak fish, have similar underbites and feed in comparable ways.
This jaw may have done the species in, however, as the climate changed. Its heyday was the Pliocene era, a time when the Earth was warmer and the poles had very little ice cover. Sea levels were higher and submerged a sizable share of the planet’s present-day land mass. Within that submerged land area was a shallow continental shelf where the porpoise most likely did most of its feeding.
Eventually, the planet cooled and the sea levels retreated. The porpoises lost much of this shelf. They had to suddenly make do with less habitat area, and the area that they had featured other kinds of terrain that wasn’t as easy to sift through with an oversized jaw. Porpoises with less-distinct jaws that could feed in a range of conditions would have fared better. These smaller-jawed porpoises lived on to evolve into the porpoises of today, while the Semirostrum cerutti made its exit.