Graptolite

Graptolites are an extinct group of colonial marine animals belonging to the class Graptolithina, within the phylum Hemichordata. They first appear in the fossil record during the early Cambrian and persisted until the Early Devonian, with peak diversity in the Ordovician. Graptolites are primarily known from their fossilized skeletons, which form colonies called rhabdosomes. A rhabdosome consists of a central axis bearing numerous individual zooids housed in protective thecae, often arranged in branches or sheets that can resemble a feather, a fan, or a tree-like structure.

Ecology and morphology: The colony could be attached to the seafloor by a holdfast in dendroid forms or, in many planktonic types, drifted in the water column. The individual zooids were filter-feeders; growth occurred by budding, producing a characteristic pattern of thecal tubes that is diagnostic for species. Rhabdosome shapes and the arrangement of thecae vary among taxa, providing valuable paleontological and stratigraphic information.

Preservation and significance: Graptolite fossils are most commonly preserved as carbonaceous films or impressions and, less frequently, as calcite or pyrite replacements of the tube walls. Because graptolites evolved rapidly and had wide geographic distribution, they are among the most important Paleozoic index fossils for correlating and dating sedimentary sequences, especially in Ordovician and Silurian strata. Well-known genera and species, such as Monograptus, define widely used biostratigraphic zones.

Extinction: Graptolites declined through the Devonian and disappeared by its end, leaving a lasting legacy in stratigraphy and paleoenvironments through their fossil record.