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PRECAMBRIAN PERIOD

555-545 million years ago

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Yunnanozoon

Yunnanozoon

By the end of the Precambrian, the hemichordate lineage had separated from that of the chordates (as evidenced by the fossils of ancestral echinoderms in the Ediacaran fauna). Yunnanozoon is a fossil hemichordate from the Early Cambrian which was very similar to the fossil chordates of the Early Cambrian.

Yunnanozoon lividum (3 cm) is known from the Early Cambrian of China. Although it was first classified as a chordate, some feel that a notochord is not present and it should be classified as a hemichordate based on its body structure (proboscis, collar, and a trunk composed of gill and digestive regions), pharynx, branchial arches, and gonads. It may have had segmented muscles homologous to chordates (Chen, 1995; Shu, 1996).