Paleontologists led by Dr Rachel Racicot of Yale University have described a new species of extinct porpoise that lived off the coast of what is now California during Pliocene, between 5 and 1.5 million years ago.
This is an artist’s impression of Semirostrum ceruttii. Image credit: Bobby Boessenecker.
The prehistoric marine animal, named Semirostrum ceruttii, is related to living crown porpoises.
The generic name Semirostrum refers to the animal’s unusual jaw – the rostrum is the lower half of the jaw. The species name honors Dr Richard Cerutti, a paleontologist at the San Diego Natural History Museum.
Semirostrum ceruttii had an extension of its jaw called a symphysis – the analogue of the human chin – that measured 85 cm. The typical symphysis of a crown porpoise measures only 1-2 cm.
“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,” said Dr Racicot, who is the first author of a paper published in the journal Current Biology.
“Although this morphology has been recorded in birds and fish, this is the first described mammal with this anatomy.”
Semirostrum ceruttii is known from about 15 fossil specimens, all from California, including a remarkably well-preserved specimen found at a construction site in San Diego.
Digital rendering of Semirostrum ceruttii skull and mandible. Image credit: Rachel Racicot et al.
Dr Racicot and her collaborators analyzed original fossils and CT scans of the best-preserved specimen, which they describe as almost completely preserved.
The CT scans revealed extensive nerve canals leading from the symphysis into the rear of the skull, indicating that the protuberance was highly sensitive and could relay meaningful information to the animal’s brain.
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 paleontologists said Semirostrum ceruttii depended on its very sensitive lower jaw, together with its echolocation abilities, to find prey.
Semirostrum ceruttii 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. The porpoise lineage may have become more specialized over evolutionary time,” Dr Racicot said.
“The discovery is a forerunner of many yet to come as digital 3D imaging revolutionizes the nondestructive study of 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.”
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Rachel A. Racicot et al. Unique Feeding Morphology in a New Prognathous Extinct Porpoise from the Pliocene of California. Current Biology, published online March 13, 2014; doi: 10.1016/j.cub.2014.02.031
