#semirostrum

Semirostrum!

semirostrum is a p cool dolphin(?)

Art by Bobby Boessenecker.

A restoration of the long-jawed porpoise Semirostrum.

by Brian Switek

Two years ago I was exploring the San Diego Natural History Museum when a fossil skull stopped me in my tracks. The Pliocene bones were clearly those of a toothed whale, but the specimen wasn’t quite like anything I had ever seen before. The porpoise’s lower jaw jutted far ahead of the upper – a thick, bony “chin” that would make Bruce Campbell jealous. What was that prodigious jaw for, and how did it evolve?

I wanted to know more, but the exhibit was short on details. The extinct whale hadn’t been officially named, and why a porpoise would have such a peculiar profile was anyone’s guess. That’s why I was thrilled to see that Yale University doctoral candidate Rachel Racicot and coauthors have now published answers to some of the questions that transfixed my mind when I first saw the skull.

The skull of Semirostrum ceruttii at the San Diego Natural History Museum. Photo by Brian Switek.

The porpoise’s name, Racicot and colleagues decided, is to be Semirostrum ceruttii, the genus name following from the shortened appearance of the snout compared to the  lower jaw and the species epithet honoring the man who found the skull, Richard A. Cerutti. There’s more of the cetacean – including vertebrae, ribs, pectoral girdle, and arm – but it’s the skull that makes the Pliocene porpoise an enigma.

Measured by Racicot and coauthors, the toothless, fused part of the porpoise’s lower jaw takes up at least 40% of the length of the entire lower jaw. The look is superficially similar to some species of fish and a bird called the black skimmer that does exactly as its name suggests as it tries to snag small meals from the surface of the water. The question is how the whale used this unusual protuberance as it swam the seas of southern California between 5.3 and 1.3 million years ago.

[See linked video here]

What a prehistoric creature was capable of doing and what that animal actually did is a persistent paleo puzzle. Fortunately, the skull of Semirostrum preserves a few delicate clues as to how this unusual whale made a living.

Along the stiff lower jaw of Semirostrum are numerous small openings. These are the end-points of canals that housed soft tissues and ran through the bone in life, indicating that the lower jaw was a very sensitive appendage. This was a specialized sensory structure.

But specialized for what? Racicot and colleagues propose that the porpoise’s jaw may have evolved to probe for prey hiding in the murky ocean bottom. And the cetacean’s teeth offer another reason to imagine Semirostrum tracking fish hiding beneath the surface. The porpoise’s teeth are worn down and polished, suggesting that a great deal of abrasive sediment was brushing by as the whale snatched small prey.

Not that the jaw would have been all about feeding. Such a sensitive jaw, Racicot and coauthors note, could have also been a bony antenna that helped Semirostrum communicate with each other in turbid or shadowy seas. If you could put yourself in the porpoise’s Pliocene waters, perhaps you would hear them click and buzz as they pointed each other to the best spots to swipe fish from the sand.

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