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| 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.
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| 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.
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| 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.
