Most of the dendritic or bushy/fan-shaped organisms are classified as dendroid graptolites (order Dendroidea). They appear earlier in the fossil record during the Cambrian and were generally sessile animals. They lived attached to a hard substrate in the sea-floor, by their own weight as encrusting organisms or by an attachment disc. 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 and planktonic, drifting freely on the surface of primitive seas. They were a successful and prolific group, being the most important animal members of the plankton until they partially died out in the early part of the Devonian period. The dendroid graptolites survived until the Carboniferous period.