Graptolites

Graptolites (literally translated "writing on rock") are an extinct group of Paleozoic colonial organisms, most often found as thin carbonized films in various shales or limestones from the Cambrian to the Carboniferous. It is believed that they were either planktonic (floating) or benthonic (bottom dwelling) in habit. They are classified in the hemichordates due to their affinity to a modern group of hemichordates, the pterobranchs, and it is believed that they were filter-feeders as are their modern counterparts. An entire graptolite colony is termed a rhabdosome, composed of a variable number of stipes diverging from the initial individual of the colony, the sicula. The small "sawtooth" like structures along the stipes are termed thecae (singular theca)--it is the thecae that housed the individual members of the graptolite colony. In some genera of graptolites, the individual stipes are strung together by a whip-like structure called the nema.