Graptolite
The graptolites (graptolitha) are live animals in colonies. They were discovered in the geological layers of the higher Cambrien until the Dinantien (lower Carbonifère). Their appearance during the average Cambrien is possible, depend on the exact classification of a probable graptolite: the Chaunograptus .
Whereas this group was regarded as extinct since 300 million years, he was discovered in 1989 of the colonies of Cephalodiscus graptolitoides in New Caledonia close to the island of Lifou.
The graptolites are Fossile S commonly found in the whole world, they are very much used for the dating of the layers of most of the Paléozoïque. Their detailed study was initiated by Charles Lapworth which showed their interest as geological index.
The colonies of graptolites, called rhabdosome , are made up starting from an initial individual called sicule . A chitinized stolon constitutes the central axis of the colony and has a number variables of branches, the stypes . The stolon comprises a series of thèques , small side cabin, or live the individuals composing the colony.
Some graptolites live on the surface of the oceans, drifting with the liking of the currents, others are attached to algae by fine filaments or to sea-beds by kinds of roots.
Their fossils are often found in Schiste S and slate. In these layers the fossils of sea-beds are rare, this type of layer being formed out of relatively deep, defective water in oxygen and without necrophagous population of animals S.
Well preserved graptolites can be found in Calcaire but generally these rocks settled under favorable conditions has a life on the marine floor including of necrophagous as it hardly has doubts there as the majority of these graptolites dead were devoured before being able to fossilize.
The fossils are flattened in the direction of the deposit of the layers, an appearance in general in the shape of dendrites or branches. Their remainders can be confused with fossils of plants.
The classification of the graptolites is still prone to guarantee. Nowadays they are regarded as having a common ancestor with the Vertébré S.
The graptolites are subdivided in:
- dendroïdes in the shape of dendrites or branches
- graptoloidea which have less branches.
The dendroïdes are more primitive and are often enracinés at sea-beds. The graptoloidea were detached from the dendroïdes at the beginning of the Ordovicien and are prolific until the beginning of the Dévonien, starting from this system only the more primitive forms survive.
The name graptolite comes from the Greek graptos and lithographies literally writes on the stone , much of graptolites resembling hiéroglyphes written on the rock.
See too
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