Graptolithina

Graptolithina, commonly known as graptolites, are an extinct group of colonial hemichordates known from the Paleozoic fossil record. They produced slender, branching skeletons called rhabdosomes, built by numerous zooids housed in tubular thecae. The colony architecture varies, with forms ranging from linear chains to complex dendroid shapes, and the arrangement of thecae underpins graptolite taxonomy.

Most graptolites were planktonic, floating or drifting in the water column, though some attached to substrates or debris. They were filter feeders, capturing fine particles from seawater. Graptolites are especially valuable for biostratigraphy because many species have rapid, worldwide or near-global range limits, enabling precise dating of Paleozoic rocks.

The fossil record shows their first appearance in the Cambrian, with major radiations during the Ordovician and Silurian. They declined in the Devonian and became extinct by the Early Devonian. Fossils are commonly preserved in fine-grained rocks such as shales or limestones, where their delicate rhabdosomes may appear as carbonaceous films or impressions.

Taxonomically, Graptolithina is placed among the Hemichordata, with modern pterobranchs as their closest living relatives. Graptolite research has been foundational to Paleozoic paleontology and stratigraphy, and their fossils continue to inform correlations and dating of Paleozoic rocks worldwide.