The Graptolithina became extinct during the Lower Carboniferous and there are no surviving forms found today. Some have suggested that graptolites are within the phylum Hemichordata, and most closely related to pterobranchs. Pterobranchs are a class of small, benthic, colonial, deep-marine organisms that produce a skeleton composed of cuticle, thought to be similar to graptolite tissues. They feed using a lophophorate system, the same proposed for graptolites.
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