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CAMBRIAN PERIOD

542-488 million years ago

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CHORDATE WITH GILL BARS

THE SKELETAL SYSTEM

The most primitive part of the vertebrate skull, the splanchnocranium, originated as gill supports composed of a "cartilage-like" tissue in ancestral cephalochordates.

The pharynx of the modern lancelet Amphioxus stretches for half the length of the body and contains 100-200 gill slits (depending on the species). Each gill bar contains a skeletal rod for support composed of collagen and glycosaminoglycans. (Ruppert, from Harrison, 1997, p. 434). There is disagreement over whether the material which composes the gill bars of hemichordates (acorn worms and enteropneusts) and lancelets should be classified as cartilage or simply as a tissue similar to cartilage. The mucopolysaccharides of the Amphioxus skin and notochord, although different from craniates (including lamprey and hagfish), are more similar to vertebrates than to those of invertebrates (Anno, 1975).
The splanchnocranium, or at least its precursor, exists in cephalochordates around gills forming supports for gill arches. Although the splanchnocranium in vertebrates is a product of neural crest cells, true neural crest cells do not exist in cephalochordates. The splanchnocranium supports the roof of the pharynx in jawless fish (Kardong, 2002).