Conodonts (Greek kōnos, "cone", + odont, "tooth") are extinct agnathan (jawless) vertebrates resembling eels, classified in the class Conodonta. For many years, they were known only from their tooth-like oral elements found in isolation and now called conodont elements. Knowledge about soft tissues remains limited. The animals are also called Conodontophora (conodont bearers) to avoid ambiguity. Conodonts are considered key index fossils, fossils used to define and identify geological periods.