Graptolite
From Freepedia
Graptolites (Graptolithina) are colonial animals known chiefly from the Upper Cambrian through the Mississippian (Lower Carboniferous). They are possibly hemichordates. A possible graptolite, Chaunograptus, is known from the Middle Cambrian.
They are closely allied to the extant hemichordates Cephalodiscus and Rhabdopleura. Cephalodiscus numbers about about 18 species. It was first discovered in 1882; before this, graptolites had been believed extinct.
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.
Image:Graptolite tetragraptus.jpg
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 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 were not firmly established until the 1970s, when advances in electron microscopy demonstrated their relation to the Pterobranch. Linneaus, originally regarded as being 'pictures resembling fossils rather than true fossils', though later workers supposed them to be 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 hemichordates (phylum Hemichordata), a primitive group which probably shares a common ancestry with the vertebrates and echinoderms.
Many graptolites appear to have been pelagic 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.
Grapolite 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 favorable 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.
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 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.
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.
External link
- Graptolite Net - Graptolite.net
- Classification of the Cephalodiscoidea (Graptolithoidea) - Graptolite.net - Cephalodiscus
- BIG G - The British & Irish Graptolite Group - British and Irish Graptolite Group (BIG-G)
