A group of extinct marine colonial animals that were common in the Palaeozoic era. Graptolites are generally regarded as being related to colonial hemichordates known as pterobranchs (see Hemichordata). They had chitinous outer skeletons in the form of simple or branched stems, the individual polyps occupying minute cups (thecae) along these stems. Fossils of these skeletons are found in Palaeozoic rocks of all continents; they are particularly abundant in Ordovician and Silurian rock strata, for which they are used as index fossils. At the end of the Silurian many graptolites became extinct but a few groups continued into the early Carboniferous.
Subjects: Biological Sciences — Earth Sciences and Geography.