Conodonts are a group of extinct vertebrates that resemble eels. They swam the oceans of the world between the late Cambrian and late Triassic period (about 500 to 200 million years ago). Only about a dozen body fossils of conodonts have been uncovered —they are mostly known for fossils of their unusual feeding apparatus, called conodont elements. Paleontologists hesitate to call these elements "teeth" because their complex arrangement in the mouths of conodonts was unlike any known arrangements of teeth.
Conodont elements are phosphatic microfossils found in great numbers in the relevant strata, but always in isolation. Conodont elements are so common that they can easily be isolated from paleozoic rock using acetic acid. For many decades, conodonts were only known from their teeth. It wasn't until the early 1980s that fossils of the "conodont animal" were found. Despite the discovery of conodont elements clearly preserved in place with conodont animals, they are still paleontolgists who argue that conodonts are teeth of annelid worms, a theory which was popular prior to the discovery of the fossil.
Conodonts were simple animals, but very successful. Ranging in size from a centimeter to 40 cm (16 in), conodonts lacked any fins except for a small one at the tail. This is in contrast to eels, which have often have long fins over the entire length of their bodies. For a while, even after some conodont body fossils were uncovered, some thought they were worms, though subsequent finds have confirmed the existence of a primitive notocord, a flexible and primitive backbone. Hence, conodonts are usually classified in phylum chordata (the chordates, which includes all vertebrates and some close relatives). Conodonts are sometimes considered one of the earliest examples of skeletonization in chordates.
Although their teeth look ferocious, conodonts probably mostly ate algae. Their fossils contain large lateral circular organs that are interpreted by most of the paleontology community as eyes, though this interpretation is problematic for several reasons, not the least of which being that conodonts obviously had very tiny heads, not large enough to house the neurological machinery that would be necessary to make use of the incoming visual information.
Because of their abundance, conodonts elements are crucial in stratigraphy, judging the age of a rock based on its contents. Because they change color slightly as they age, conodont elements can sometimes be used to judge the age of a particular stratum at just a glance. Thus, conodont specialists are in high demand by the petroleum industry, which seeks out hydrocarbons from certain specific rock layers.