Graptolita

Graptolites (Graptolita) are colonial animals known chiefly from the Upper Cambrian through the Mississippian (Lower Carboniferous). Possibly hemichordates. A possible graptolite, Chaunograptus, is known from the Middle Cambrian.

One surviving genus Cephalodiscus is known, with about 18 species, first discovered in 1882; prior to this, graptolites had been believed extinct.

Graptolites are common fossils and have a worldwide distribution. They are important zone (or index) fossils for dating Palaeozoic rocks as they evolved rapidly with time and formed many different species. British geologists recognize some 36 graptolite zones in the rocks of the Cambrian, Ordovician and Silurian periods.

Each graptolite colony is known as a rhabdosome and had a variable number of branches (called stipes) originating from an initial individual (called a sicula). Each subsequent sicula was housed within a tubular or cup-like structure (called thecae). The number of branches and the arrangement of the thecae are important features in the identification of graptolite fossils.

The biological affinities of the graptolites have always been debatable. Originally regarded as being related to the hydrozoans, graptolites are now considered to be related to the pterobranchs, a rare group of modern marine animals. The graptolites are classed as hemicordates (phylum hemicordata), a primitive group which probably shares a common ancestry with the vertebrates.

Many graptolites appear to have been planktonic, drifting freely on the surface of ancient seas, or attached to floating seaweed by means of a slender thread. Some forms of graptolite lived attached to the sea-floor by a root-like base.

Their 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. The dead planktonic graptolites, having sunk to the sea-floor, would eventually become entombed in the sediment and are thus well preserved.

Some well preserved graptolites are found in limestones and cherts, but generally these rocks were deposited in conditions which were favorable for bottom-dwelling life, including scavengers, and undoubtedly most graptolite remains were generally eaten by other animals.

Graptolite fossils are found flattened along the bedding plane of the rocks in which they occur. They vary in shape, but are are most commonly dendritic or branching (such as Dictoyonema), saw-blade like, or "tuning fork" shaped (such as Didymograptus murchisoni). Their remains may be mistaken for fossil plants.

The dendritic or branching type are known as dendroid graptolites (dendroidea). They are more primitive and were generally rooted to the sea-floor. The graptolites which have few branches were derived from the dendroid graptolites at the beginning of the Ordovician period. This latter type (the graptoloidea) were planktonic and were successful and prolific until the early part of the Devonian period when they died out, to be survived by the more primitive dendroid graptolites.

Graptolites are normally preserved as a black or white carbonized film on the rocks surface, and sometimes may be difficult to see, but by slanting the specimen to the light they reveal themselves as a shiny marking. Pyritized graptolite fossils are also found.

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

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.