A unique feeding specialization in a prehistoric porpoise

Life reconstruction of a pair of Semirostrum ceruttii

      Porpoises (Family Phocoenidae) are among the smallest cetaceans (whales). They live in coastal regions, feeding on fish and squids on the sea floor. The six living species of porpoises are distributed in all the oceans of the world. Porpoises  are a quite recent addition to the cetacean world, having diverged from the dolphins probably during the middle of the Miocene period. Fossil porpoises are all very similar to their modern counterparts and do not show much specialization, with the exception of the newly discovered Semirostrum ceruttii from the Pliocene San Diego and Purisima formations of California. 

The Black Skimmer (Rynchops niger) (photo by Dan Pancano)

       Semirostrum is unique among the cetaceans for its nearly toothless lower jaw that is some 40% longer than its upper jaw. The many mandibular canals found in this protruding lower jaw indicate that this was a richly innerved and vascularized area and was most probably used for prey detection. The slightest touch would provoke the immediate closure of the mouth. A modern equivalent of such apparatus is given by the beak of the skimmer (Rhynchops), a tern-like seabird that hunts by flying close to the water surface, skimming the waves with its lower jaw. Semirostrum would have done the same at the bottom of the seafloor in search of small preys living close to the sand surface. This high degree of feeding specialization (called skim-feeding)  is rather unprecedented among mammals. Semirostrum lived alongside other benthic foraging mammals such as the near toothless walrus Valenictus and the baleen whale Herpetocetus, indicating an unique shallow water ecosystem. Semirostrum is known from an almost complete skull and several referred specimens that include postcranial elements.

Semirostrum scale diagram

Ref: Rachel A. Racicot, Thomas A. Deméré, Brian L. Beatty & Robert W. Boessenecker. 2014. Unique Feeding Morphology in a New Prognathous Extinct Porpoise from the Pliocene of California. Current Biology. Published online.