笔石纲是一类已灭绝的很小的群居性半索动物，生存于寒武纪中期至石炭纪早期。志留纪时期的笔石化石甚多，被称为“笔石时代”。因为它酷似古代西方使用的羽毛笔，故得名笔石。曾长期被人误作腔肠动物，后经学者用显微镜研究后明确为半索动物。

Graptolithina is a class of hemichordate animal, the members of which are known as graptolites. Graptolites are fossil colonial animals known chiefly from the Upper Cambrian through the Lower Carboniferous (Mississippian). A possible early graptolite, Chaunograptus, is known from the Middle Cambrian.