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In page Chordate:

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All of the earliest chordate fossils have been found in the Early Cambrian Chengjiang fauna, and include three species that are regarded as fish,[citation needed] and hence vertebrates. Because the fossil record of early chordates is poor, only molecular phylogenetics offers a reasonable prospect of dating their emergence. However, the use of molecular phylogenetics for dating evolutionary transitions is controversial. It has proven difficult to produce a detailed classification within the living chordates. Attempts to produce evolutionary "family trees" shows that many of the traditional classes are paraphyletic.[citation needed]