; 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 rhabdosome. 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).
Subjects: Zoology and Animal Sciences — Earth Sciences and Geography.