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PRECAMBRIAN PERIOD

555-545 million years ago

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hemichordate with gill bars

The "cartilage-like" tissue in the head and pharynx of hemichordates may represent the ancestral condition of the vertebrate skeleton (the splanchnocranium of the skull, more specifically).

Acorn worms possess a cartilage-like skeletal rod ventral to the stomachord in the head region (Benito, form Harrison 1997, p. 16). Enteropneusts possess gill bars and a proboscis "skeleton" which has been described as being similar to cartilage (Benito, form Harrison 1997, p. 34).