Fossil of the month: Saivodus sp.
What did Saivodus look like?
Shark skeletons are composed of cartilage. Teeth and in some cases, fin spines, are the only bony material in sharks. Cartilage does not fossilize well. For that reason, most of what we know about ancient sharks comes from fossil teeth and spines. Several Paleozoic shark fossils, however, have been found with rare soft parts of the original shark body (brain cases, fins, etc.) preserved. From these soft-part fossils, scientists can reconstruct what the bodies of some extinct sharks looked like. The new Mammoth Cave fossil is providing more information to help scientists more accurately reconstruct what Saivodus sharks looked like.
During the Paleozoic era, there were many different kinds of sharks. Some of those sharks had fin spines, others did not. Fin spines are long bony spines that are positioned in front of dorsal (back) fins of some sharks. Paleozoic soft-part shark fossils which are associated with cladodont teeth, are also usually associated with Ctenacanthus or Ctenacanthus−like, fin spines (Ginter and Maisey, 2007). The Mammoth Cave fossil shows Saivodus had fin spines, so it may have had a shape similar to Ctenacanthus. Ctenacanthus is a fossil shark which has different soft parts preserved in different fossils, so there is a relatively good understanding of its shape and appearance. It had a head which was more rounded and torpedo-shaped than most modern sharks. It had two dorsal (back) fins, and a split caudal (tail) fin. Its pectoral (front lateral) fins were proportionally shorter than most modern sharks. Different ctenacanthid sharks appear to have had similar shapes although the size and shape of their fins varied (Schaeffer and Williams, 1977; Zangerl, 1981).
Saivodus appears to have been a ctenacanthid shark, so may have had a shape similar to another Paleozoic fossil shark, Ctenacanthus.
Text, photographs, and illustrations by Stephen Greb