Porpoise With Distinct Underbite Revealed After Analysis Of Museum Specimen

By Shweta Iyer on March 13, 2014 1:42 PM EDT

porpoise
According to researchers, the extinct porpoise, which mainly inhabited the coast of California, had a significant underbite, which means its lower jaw extended well beyond its upper jaw (Photo: Bobby Boessenecker)

One of the many species that inhabited the coast of America and are now extinct was a species of porpoise called Semirostrum ceruttii. Porpoises are small marine mammals closely related to whales and dolphins. According to researchers, the extinct porpoise, which mainly inhabited the coast of California, had a significant underbite, which means its lower jaw extended well beyond its upper jaw, according to a press release Thursday. This discovery was made by researchers at the Yale University, whose report is published in the Cell Press journal Current Biology.

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CT scans of the mammal's skull revealed a large innervated beak, which means the beak had a distribution of nerves around it. This must have made its lower jaw very responsive to stimuli and allowed it recognize prey along the ocean bed. Rachel Racicot, one of the researchers said, "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. Although this morphology has been recorded in birds and fish, this is the first described mammal with this anatomy."

The fossilized remains of this mammal were unearthed in 1990 from a 1.6 to 5 million-year-old rock formation along the coast of California, by Richard Cerutti, a collector from the San Diego Natural History Museum.  CT scans of the specimen's skull were analyzed by the Yale University researchers and they found a long, thin, and nearly toothless jaw with a fused mandible.

The researchers also discovered sensory structures in the lower jaw of the specimen. A similar sensory structure can also be found in seabirds called black skimmer and small fish called half-beaks, both of which have elongated lower jaws, which help them to skim the surface of the water and the sea bed respectively, in search of prey at night.

Another interesting find is that the porpoise may have had poor eyesight since its optic canals were smaller than those in current porpoise species. These findings have led researchers to conclude that the extinct porpoise may have used its sensitive lower jaw and echolocation abilities to forage for prey. Morphological studies suggest that the Semirostrum ceruttii is related to modern porpoises like whales and dolphins and is similar to today's freshwater dolphin.

"Today we don't find anything resembling river dolphins in the same kinds of habitats that Semirostrum likely occupied," Racicot said, suggesting a possible evolution of the extinct porpoise into a more sophisticated species.The study of this specimen has paved the way for many more researches that can be conducted on museum specimens.    

"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.

Source: Racicot et al. "Unique feeding morphology in a new prognathous extinct porpoise from the Pliocene of California." Current Biology. 2014.

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