graptolite - Taxonomy, Graptolites as zone fossils, Morphology, Preservation

An extinct marine animal (a hemichordate), mostly found in surface plankton, living in colonies; known from the Cambrian to the Carboniferous periods; individual polyps of colony lived in chitinous tubes arranged in single or double rows along the main axes. (Class: Graptolithina.)

Graptolites (Graptolithina) are fossil colonial animals known chiefly from the Upper Cambrian through the Mississippian (Lower Carboniferous).

The name graptolite comes from the Greek graptos, meaning "written", and lithos, meaning "rock", as many graptolite fossils resemble hieroglyphics written on the rock.

Taxonomy

The name originates from the genus Graptolithus, which was used by Linneus in 1735 for inorganic mineralizations and crustations which resembled actual fossils. scalaris, respectively a possible plant fossil and a possible graptolite. His 1751 Skånska Resa, he included a figure of a "fossil or graptolite of a strange kind" currently thought to be a type of Climacograptus (a genus of biserial graptolites). (Bulman, 1970: V 6)

Since the 1970s, as a result of advances in electron microscopy, graptolites have generally been thought to be most closely allied to the pterobranchs, a rare group of modern marine animals belonging to the phylum Hemichordata (hemichordates).

Graptolites as zone fossils

Graptolites are common fossils and have a worldwide distribution. British geologists can divide the rocks of the Ordovician and Silurian periods into graptolite biozones;

Morphology

Each graptolite colony is known as a rhabdosome and has a variable number of branches (called stipes) originating from an initial individual (called a sicula). The number of branches and the arrangement of the thecae are important features in the identification of graptolite fossils.

Most of the dendritic or many-branched types are classified as dendroid graptolites (order Dendroidea). Graptolites with relatively few branches were derived from the dendroid graptolites at the beginning of the Ordovician period.

Preservation

Graptolite fossils are often found in shales and slates where sea-bed fossils are rare, this type of rock having formed from sediment deposited in relatively deep water that had poor bottom circulation, was deficient in oxygen, and had no scavengers.

Graptolites are also found in limestones and cherts, but generally these rocks were deposited in conditions which were unfavorable for bottom-dwelling life, including scavengers, and undoubtedly most graptolite remains were generally eaten by other animals.

Graptolite fossils are often found flattened along the bedding plane of the rocks in which they occur, though may be found in three dimensions when they are infilled by iron pyrite.

Graptolites are normally preserved as a black carbonized film on the rock's surface or as light grey clay films in tectonically distorted rocks. Pyritized graptolite fossils are also found.

A well known locality for graptolite fossils in the United Kingdom is Abereiddy Bay, Dyfed, Wales where they occur in rocks from the Ordovician period.