conodont

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A member of an extinct group of small primitive fishlike chordates, preserved primarily in the form of their conelike teeth.
2. n. A fossil tooth of this chordate. Conodonts are the most widespread Paleozoic microfossils and are important for biostratigraphic indexing.

Century Dictionary and Cyclopedia

1. n. A small glistening fossil organism, discovered by Pander in Silurian and Devonian rocks in Russia, and subsequently observed in other strata in different localities, and variously supposed to be a tooth of a cyclostomous fish, or a spine, hooklet, or denticle of a mollusk or an a˙nnelid: so named from its conical tooth-like appearance. These organisms are certainly not teeth of any vertebrates, and are probably the remains of worms.

Wiktionary

1. n. Any of several extinct fishlike chordates that had conelike teeth
2. n. A microfossil tooth of such an animal

GNU Webster's 1913

1. n. (Zoöl.) 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.

WordNet 3.0

1. n. small (2 inches long) 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
2. n. the tiny fossil cone-shaped tooth of a primitive vertebrate of order Conodonta