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).

Graptolithina (graptolites; phylum Hemichordata, subphylum Stomochordata) An extinct class of stick-like, colonial, marine organisms that existed from the Middle Cambrian to the Lower Carboniferous. Their fossils are used to establish a stratigraphical time-scale for the Lower Palaeozoic. There were 2 principal orders, Dendroidea and Graptoloidea, and a number of minor, short-lived orders.