Very useful as guide fossils but geologists not quite sure what conodonts were or if they were actually part of another larger organism. Most accepted idea is that they belong to a chordate, perhaps some sort of jawless fish. All chordates have a central hollow nerve chord or notochord, pharyngeal slits in the embryo and an internal skeleton. The first chordate, Pikaia, appeared in the Cambrian and resembled an eel. There are many classes of vertebrates.
Paleozoic fossils contain a record of fish evolution.
Agnatha first appear in the Ordovician. Fins are not well developed and they are not good swimmers. During the Paleozoic some possessed bony armor. Appear in the Silurian, extinct in the Permian, Placodermi were not efficient swimmers because of rudimentary fin arrangement. Appear in the Silurian, extinct in the Permian.