Millions of years ago, the coast of California was home to a species of porpoise distinguished from its living relatives by a lower jaw that extended well beyond the upper, according to researchers who report their findings in the Cell Press journal Current Biology on March 13. In other words, the long-lost porpoise had a rather distinct and unusual underbite.
Careful analysis of the fossilized Semirostrum ceruttii skull also shows that the porpoise's pronounced beak included innervated jaws, which the animal likely used to feel for prey along the ocean floor.
"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," says Rachel Racicot of Yale University. "Although this morphology has been recorded in birds and fish, this is the first described mammal with this anatomy."
Although Richard Cerutti, the collector from the San Diego Natural History Museum who first found the specimen in 1.6 to 5 million-year-old rock formations along the California coast in 1990, immediately recognized it as special, Racicot and her colleagues didn't really know what they had until they began to pore over medical CT scans taken years later. Those images revealed a long, thin, and nearly toothless lower jaw with a fused mandible and, of course, that underbite.
The scans uncovered other hidden details, including sensory structures in the lower jaw similar to those found in seabirds called black skimmers and small fish called half-beaks, both of which use their lower jaws to help them feel around for food at night or under low-light conditions. Further study showed that the porpoise's optic canals were smaller than those in modern porpoise species as well, suggesting that the extinct animal also had poor eyesight. The evidence led the researchers to suspect that Semirostrum depended on its very sensitive lower jaw, together with its echolocation abilities, to find prey.
The porpoise—which comparative morphology analysis suggests is a sister species to modern-day porpoises—was in some ways similar to today's freshwater river dolphins. "Today we don't find anything resembling river dolphins in the same kinds of habitats that Semirostrum likely occupied," Racicot says, adding that the porpoise lineage may have become more specialized over evolutionary time.
The discovery is a forerunner of many yet to come as digital 3D imaging revolutionizes the nondestructive study of museum specimens, the researchers say.
"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 says.
Current Biology, Racicot et al.: "Unique feeding morphology in a new prognathous extinct porpoise from the Pliocene of California."
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