A paper published on March 13 in the journal Current Biology described the new face of extreme mammal adaptations. In it, authors Rachel Racicot of Yale University, Thomas Deméré of the San Diego Natural History Museum, Brian Beatty of NYIT College of Osteopathic Medicine, and Robert Boessenecker of the University of California attempted to explain why an unassuming species of extinct porpoise grew the biggest chin ever recorded in mammals.

Closely related to dolphins, the porpoise is the smallest member of the whale and dolphin order Cetacea, which means they breathe air, produce milk to feed their young, and hunt their prey from the ocean. The unique facial features of Semirostrum ceruttii, however, tell scientists that this porpoise had an interesting hunting technique.

Semirostrum lived in the area that’s now San Diego during the Pliocene Epoch, between 1.5 and 5.3 million years ago, when California was covered by the ocean. The closest living relative to S. ceruttii is the crown porpoise, which boasts a chin one or two centimeters in length. The chin of S. ceruttii was a whopping 85 centimeters long. For years, the reason why has been a mystery.

The skeletons offered clues to S. ceruttii’s lifestyle. Strong shoulder blade and breastbone along with unfused neck vertebrae tell scientists that S. ceruttii probably hunted near the bottom of the ocean, searching for the crabs, small squid, and fish that hid in the fine mud. Like porpoises and dolphins, S. ceruttii’s sharp, conical teeth were designed for hunting and catching prey, not for chewing. Wear on teeth suggests that it spent its time digging through the sediment to flush out its food, much like a seabird called the Black Skimmer, which uses its pronounced under bite to hunt for fish by skimming its lower beak just under the water.

Racicot and her team analyzed the 15 specimens of S. ceruttii found in California, and submitted the best specimen, an extremely well-preserved skull, to a computerized tomography (CT) scan. This allowed the scientists to get a detailed look inside the bone without destroying the rare fossil. What they found was an extensive network of tunnels in the bone that once housed nerves running from the chin to the brain. Also found in the Black Skimmer, these structures indicate that S. ceruttii’s chin was extremely sensitive to touch. The nerve canals that run from the eye sockets to the brain are relatively small, suggesting that S. ceruttii’s eyesight was not as good as that of a modern porpoise.

Semirostrum ceruttii probably used its sensitive super chin to skim the sediment at the bottom of the ocean and feel the prey it couldn’t see, allowing it to snap up a quick meal. Although familiar to scientists who study birds and fish, this is the first such adaptation to be found in a mammal.

Source: Racicot, R. A., Deméré, T. A., Beatty, B. L., & Boessenecker, R. W. (2014). Unique feeding morphology in a new prognathous extinct porpoise from the Pliocene of California. Current Biology, 24(7): 774-779.

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Published On: July 24, 2014

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