Graptolite

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

The name graptolite comes from the Greek graptos, meaning "written", and lithos, meaning "rock", as many graptolite fossils resemble hieroglyphs written on the rock. Linnaeus originally regarded them as 'pictures resembling fossils rather than true fossils', though later workers supposed them to be related to the hydrozoans.

Taxonomy
The name originates from the genus Graptolithus, which was used by Linneus in 1735 for inorganic mineralizations and crustations which resembled actual fossils. In 1768, in the 12th volume of Systema Naturae, he included G. sagittarius and G. 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). Later workers used the name to refer to a specific group of organisms. Graptolithus was officially abandoned in 1954 by the ICZN, partly because of its original purpose as a grouping for inorganic mimicries of fossils. (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). Comparisons are drawn with the modern hemichordates Cephalodiscus and Rhabdopleura. Cephalodiscus numbers about 18 species, and was first discovered in 1882.

Graptolites as zone fossils
Graptolites are common fossils and have a worldwide distribution. They are important index fossils for dating Palaeozoic rocks as they evolved rapidly with time and formed many different species. British geologists can divide the rocks of the Ordovician and Silurian periods into graptolite biozones; these are generally less than one million years in duration.

Graptolites can also be used to estimate water depth and temperature.

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). Each subsequent individual (zooid) was housed within a tubular or cup-like structure (called a theca). In some colonies, there are two sizes of theca, and it has been suggested that this difference was due to sexual dimorphism. 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). They appear earlier in the fossil record (in the Cambrian period), and were generally benthic animals (attached to the sea-floor by a root-like base). Graptolites with relatively few branches were derived from the dendroid graptolites at the beginning of the Ordovician period. This latter type (order Graptoloidea) were pelagic, drifting freely on the surface of ancient seas or attached to floating seaweed by means of a slender thread. They were a successful and prolific group, being the most important animal members of the plankton until they died out in the early part of the Devonian period. The dendroid graptolites survived until the Carboniferous 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. The dead planktonic graptolites, having sunk to the sea-floor, would eventually become entombed in the sediment and are thus well preserved.

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. They vary in shape, but 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 by the casual observer.

Graptolites are normally preserved as a black carbonized film on the rock's surface or as light grey clay films in tectonically distorted rocks. They may be sometimes difficult to see, but by slanting the specimen to the light they reveal themselves as a shiny marking. Pyritized graptolite fossils are also found.

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