Graptolithina

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Graptolithina (graptolites; phylum Hemichordata) A class of extinct, colonial, marine organisms which secreted a chitinous exoskeleton with characteristic growth bands and lines. The complete skeleton (colony) is referred to as a rhabdo-some. They first appeared in the Middle Cambrian and are last known from the Lower Carboniferous. The class comprises two principal orders: true (graptoloid) graptolites (Graptoloidea), which are important zonal fossils in Ordovician and Silurian rocks; and the dendroid graptolites (Dendroidea).

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