Graptolite, any member of an extinct group of small, aquatic colonial animals that first became apparent during the Cambrian Period (542 million to 488 million years ago) and that persisted into the Early Carboniferous Period (359 million to 318 million years ago). Graptolites were floating animals . They possessed a chitinous (fingernail-like) outer covering and lacked mineralized hard parts. The graptolite animal was bilaterally symmetrical and tentacled. It has been suggested that graptolites are related to the hemichordates, a primitive group of invertebrates.