co·no·dont /ˈkonəˌdɑnt, ˈkɑ-/
 牙形石
 Co·no·dont n.  Zool. A peculiar toothlike fossil of many forms, found especially in carboniferous rocks. Such fossils are supposed by some to be the teeth of marsipobranch fishes, but they are probably the jaws of annelids.
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 conodont
      n 1: tiny fossil cone-shaped tooth of the most primitive
           vertebrate: the conodont
      2: small (2 in) extinct eellike fish with a finned tail and a
         notochord and having cone-shaped teeth containing cellular
         bone; late Cambrian to late Triassic; possible predecessor
         of the cyclostomes