Graptolites

Graptolites are an extinct group of colonial marine animals belonging to the class Graptolithina within the phylum Hemichordata. They lived from the early Cambrian to the late Devonian, with their greatest diversity in the Ordovician and Silurian. Graptolites are known mainly from their skeletonized colonies, the rhabdosomes, which consisted of numerous zooids housed in cup-shaped thecae arranged along one or more branches. The colony was built along flexible stalks and could be free-floating in the water column or anchored to the seafloor, producing a range of simple to highly branched forms.

As suspension feeders, graptolites filtered small particles from seawater. Their shells were composed of a chitin-like material and are commonly preserved as carbon films, pyrite molds, or impressions in sedimentary rocks. Because their groups evolved rapidly and had wide geographic spread, graptolites are widely used as index fossils for dating Paleozoic rocks, particularly the Ordovician through Silurian intervals.

Graptolites originated in the early Cambrian and expanded in the Ordovician and Silurian, but declined during the Devonian and became extinct by the end of that period. Their global distribution and rapid evolutionary changes make them important tools for correlating and dating Paleozoic marine rocks.