542-488 million years ago
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).