The unprecedented chin size of an ancient porpoise

431 The unprecedented chin size of an ancient porpoise
skimmer porpoise reconstruction by Bobby Boessenecker
This new, though already extinct, porpoise has one of the most impressive chins ever seen in mammals. Scientists believe that it used the tip of its giant mandible to probe the seabed for food in the dark. 
They call it Semirostrum ceruttii, for its bizarre snout (rostrum) and for paleontologist Richard Cerutti of the San Diego Natural History Museum who retrieved the best-preserved specimen out of rock formations back in the 1990s. Based on 15 fossils from California (including one discovered at a construction site), the species lived 1.5 to 5.3 million years ago along the coast.
The extinct porpoise has an extension of its lower jaw called mandibular symphysis -- like our chins -- that measures about 85 centimeters (over 30 inches) long. The typical symphysis of a modern porpoise measures one or two centimeters (that’s not even an inch). 
Talk about underbite, its bottom jaw protrudes 18 centimeters (7 inches) beyond the top one. "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," study researcher Rachel Racicot of Yale says in a news release.
Since this anatomy has never been seen in mammals before, “it tells us that porpoises once searched for food in a very different way than they do now,” she adds. Living porpoises look for fish and cephalopods throughout the water column: in-shore, mid-water, and bottom-dwelling. This extinct porpoise, however, appeared to be a benthic feeder, digging for food lodged in the sea floor. Its teeth showed signs of wear from sand. 
When the team analyzed the fossils and their CT scans, they discovered that the long, thin, and nearly toothless lower jaw contained extensive nerve canals leading from the symphysis into the rear of the skull. That means their big chin was highly sensitive and could relay information to the brain as it felt for its prey along the dark ocean floor. Especially useful since it had poor eyesight: Its optic canals were smaller than those in modern porpoises.
That all seems to make sense because the closest jaw structure they could find in living animals come from seabirds called skimmers and small fish called half-beaks. To hunt for food, Black Skimmers (Rynchops) fly just above the surface of the water with their symphysis slightly submerged to probe around and scoop up small fish and crustaceans. Both skimmers and half-beaks (Hemiramphus) feed at night, making their highly sensitive probes for relaying sensory information about unseen prey exceptionally important.
The work was published in Current Biology this week. 
Images: skimmer porpoise reconstruction by Bobby Boessenecker (top) & skull and jaws of skimmer porpoise by Rachel Racicot (bottom)


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