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Graptolites are examples of hemichordate animals known from the Upper Cambrian to Lower Mississippian periods. The name comes from the Greek graptos meaning “written” and lithos meaning “rock” referring to the fact that many fossil graptolites resemble hieroglyphs written on the rock surface. The name “graptolite” originates from the genus Graptolithus which was used by Linnaeus in 1735 for inorganic mineralizations and crustations which resembled actual fossils. Each graptolite colony is known as a rhabdosome consisting of a variable number of branches called stipes. Each individual animal is housed in a cone-shaped structure called a theca, many of which are attached along the axis of the stipes such that the stipes takes on the appearance of a hacksaw blade. At the beginning of Ordovician time, the colonies were pelagic and planktonic, floating freely on the surface of the sea or were attached to some type of floating devise such as a clump of seaweed. Graptolite colonies were the most abundant members of the plankton until they began to diminish in number during the Early Devonian. Graptolites are normally preserved as a black carbon film on the rock surface.


graptolite, hemichordate