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Conodont
Conodonts are extinct worm-like forms with distinctive multi-bladed teeth made of apatite (calcium phosphate). The tiny teeth are quite common in Paleozoic rocks, but body fossils were not found until the late 1980s. Conodonts and their presumed relatives are known from the Precambrian to the Late Triassic. The earliest forms are identified as Protoconodonts, followed by Paraconodonts, followed by Euconodonts, followed finally by the true Conodonts. Some authors regard the Euconodonts and Conodonts as being a continuous lineage.
Following the discovery of several body fossils in Scotland and South Africa, most paleontologists think conodonts — which turn out to have fins, cheveron shaped muscles, and eyes — are in the phylum chordata. There are a lot of opinions about where the conodonts belong amongst the chordates/vertebrates. Some paleontologists (Cochrane) place the Protoconodonts (therefore possibly also the conodonts) in a phylum along with the chaetognath worms.
A cladistic analysis by Donoghue et. al (1998). suggests that conodonts and Euconodonts are vertebrates. The paraconodonts (known only from teeth) are thought to be related, but the relationship is unclear. Per Donoghue, Paraconodonts are not related to the rest.
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