Hemichordata
Most of the tree-like forms are classified as dendroid graptolites (order Dendroidea). They appear earlier in the fossil record (in the Cambrian), and were benthic animals attached to the sea-floor by a root-like base. Graptolites with relatively few branches were derived from the dendroid graptolites at the beginning of the Ordovician period. This latter type (order Graptoloidea) were pelagic, drifting freely on the surface of ancient seas or attached to floating seaweed by means of a slender thread. They were a successful and prolific group, being the most important animal members of the plankton until they died out in the early part of the Devonian period. The dendroid graptolites survived until the Carboniferous period.