graptolites

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graptolites The graptolites were a group of colonial marine organisms that lived from the Cambrian to the Pennsylvanian (Late Carboniferous). The colonies ranged in size from 5 mm to 1 m in length and were linear or complexly branched. They are generally preserved as carbon films, stick-like and minutely serrated along the edges like tiny saw-blades. This gives them a resemblance to writing on rock surfaces, and gave rise to their name (graptos, Greek ‘written’). Although for many years their origins were somewhat enigmatic they are now considered to be part of the phylum Hemichordata and allied therefore to the pterobranchs, a modern group of similar appearance. Their abundance in Palaeozoic black shales, together with their rapid and distinct shape changes through time, makes them very useful in Palaeozoic biostratigraphy.

Although graptolites are usually preserved as carbon films that provide little morphological detail, they are sometimes found uncrushed and unaltered, particularly in limestones, from which they can be freed by acid etching. From these specimens a considerable amount can be deduced about the growth and structure of graptolites. The members of the colony secreted a hard skeleton or rhabdosome which originated as a conical sicula housing the original zooid produced by sexual reproduction (Fig. 1a). The colony appears to have developed by the asexual budding off of new individuals, each of which formed a theca, a tubular structure forming a linear overlapping sequence. Each sequence of thecae forms a stipe, or branch, and colonies varied in the numbers of stipes that they possessed. Although no preserved soft parts are known it is assumed that the zooids could extend themselves from the thecae and that they possessed food-gathering apparatus of some kind (Fig. 1b). The material forming the rhabdosome occurs in two layers; the inner fusellar tissue consists of interlocking half-rings, while the outer cortical tissue is composed of a series of bandage-like overlapping sheets. This laminated structure must have given the rhabdosome a great deal of strength while maintaining flexibility.

The shape of the thecae can vary considerably, particularly around the aperture or main opening where ornate projections may develop. The overall shape of the colony also shows great diversity and, as an easily identifiable feature, is very useful in biostratigraphy. Colonies in which the stipes and the thecal apertures are directed down from the sicula are termed pendent, while those in which the stipes are at right angles to the sicula are termed horizontal. Reclined rhabdosomes are ones in which the stipes are inclined upwards, and those in which the stipes have joined to form a linear structure are termed scandent.

Graptolites are separated into orders that correspond to modes of life. The Dendroidea were bush-like forms that were probably attached to the sea bed like modern seaweed; they were important Cambrian forms but then dwindled to their extinction in the Late Carboniferous (Pennsylvanian). The Graptoloidea were planktonic and extremely numerous from the Ordovician to the Early Devonian; these are the forms generally thought of as ‘graptolites’. In addition there are four orders of encrusting forms that are of minor importance. Only the planktonic graptoloids are sufficiently numerous and widespread to have biostratigraphic value; major shape changes of the rhabdosome have been used to develop precise correlation schemes. In the Early Ordovician, many-branched planktonic forms developed from the attached dendroids. These early forms branched dichotomously and produced a diverse group of eight-, four-, or two-branched forms that were generally pendent. From these developed the two-stiped forms (‘tuning-fork’ graptolites) that dominated the Late Ordovician. These were pendent to reclined, and even scandent. At the base of the Silurian this fauna was replaced by the monograptids, scandent forms in which the thecae were arranged along one side of the stipe only. These were generally straight but also included forms that were curved or spiral or had lateral arms.

Graptolites are distributed in patterns similar to those seen for modern planktonic organisms. They are richest in rocks formed on the outer parts of continental shelves, where upwelling of nutrients along shelf edges would have provided abundant food resources. They also show vertical distribution; those that are most widespread were surface planktonic forms, while those that occur only in shelf-margin and shelf-slope deposits probably lived at some depth within the water column. A number of the planktonic forms display features related to floating. These include long spines projecting from the rhabdosome or loosely woven mesh-like structures surrounding it. Vane-like or bubble-like structures present particularly on scandent forms may have aided flotation if they were gas-filled. Other graptoloids with helical coiling may have been able to spin if the zooids could have generated enough activity with their tentacles, and it is possible that they might have migrated diurnally within the water column. During some parts of their faunal history the graptolites occupy faunal provinces, separate areas characterized by particular graptolite taxa. These appear to have been temperature-controlled, with graptolites preferentially inhabiting warm-water areas.

David K. Elliott

Bibliography

Clarkson, E. N. K. (1993) Invertebrate palaeontology and evolution. Chapman and Hall, London.

graptolites A group of extinct marine colonial animals that were common in the Palaeozoic era. Graptolites are generally regarded as being related to the Cnidaria. They had chitinous outer skeletons in the form of simple or branched stems, the individual polyps occupying minute cups (thecae) along these stems. Fossils of these skeletons are found in Palaeozoic rocks of all continents; they are particularly abundant in Ordovician and Silurian rock strata, for which they are used as index fossils. At the end of the Silurian many graptolites became extinct but a few groups continued into the early Carboniferous.

graptolites See GRAPTOLITHINA.