About graptolites

What are graptolites? What use are they? Where can I find them?
The name 'Graptolite' comes from the greek words 'graphein' - to write, and 'lithos' - stone, and thus, in a metaphorical sense, graptolites are 'writing in the rocks'!

Much of the following discussion is greatly expanded in our publications 'Graptolites in colour' and 'Fossils Illustrated - Graptolites' (please see our publications page).

What are graptolites? Graptolites are the fossil remains of small colonial animals that dominated the marine plankton between 500 to 400 million years ago. Their skeleton is made of collagen, a complex protein something like your finger nails. In life, the graptolite colony was connected together by living tissue, but after death only the resistant collagen skeleton is preserved, and this is what we find as fossils. Graptolites may be related to some living groups such as Rhabdopleura, though this comparison is not without controversy. The basic visible features of the graptolite skeleton (the rhabdosome) are the sicula (the first part of the colony), stipe(s) and thecae. The thecae are the living quarters for the graptolite zooids, the members of the colony. Graptolite skeletons (and theca shapes) come in a multitude of morphologies, some examples of which are demonstrated on our Gallery page.


The specimen of the 'fish-hook'-shaped Silurian graptolite Pseudostreptograptus, shown to the left (scale bar=250 microns), demonstrates all the typical morphological features. At the proximal end (the oldest part of the rhabdosome) is the 'cone-shaped' sicula. This lies on the left side of the rhabdosome at the tip of the 'fish-hook' and is shown in detail at the top left of the image to the right.



There is a series of thecae (on the specimen of Pseudostreptograptus on the inner side of the 'fish-hook'): you can count 10 in all, the first one appearing near the proximal end of the rhabdosome, opposite the sicula. As the colony grows more are added. Each of these thecae is connected by the main stipe (which in this case is strongly curved into a 'fish-hook' shape). In life, the thecae (plural) housed the zooids (the living colony members) of the graptolite colony. The thecae of this graptolite are complex. They are incurled and have paired spines (see close-up of 2 thecae in the left hand image). Other graptolites, such as the pristiograptids, had much simpler thecal morphology.



Although we don't know for sure what the graptolite zooids looked like, they may have been something like the image to the left. This is a polyp from the hydroid Antennularia (not a graptolite) prepared by Prof. Oliver Bulman. The scale bar represents 100 microns.


Why study graptolites? Graptolites lived in the Earth's oceans from 520 to 320 million years ago, when they became extinct. For much of that time they dominated the upper layers of the ocean as the earth's first large zooplankton. They lived by the countless billion and their skeletons are preserved today in vast numbers in varied strata in every continent. Because of their diversity they are a powerful correlative tool: units of time of much less than a million years are identifiable, and within individual rock sequences evolutionary changes can be studied. Their refined biostratigraphy makes them important fossils for the oil industry also.





Graptolites such as this specimen of Expansograptus similis (see right) can tell us very accurately where we are in relative time. Scale bar= 1mm.

In the case of the planktonic graptolites (graptoloids), which arose in the earliest Ordovician, they reached their greatest diversity soon after (about 480 million years ago) with almost 200 species known from this time. Thereafter the graptoloids declined in overall diversity, culminating in their final extinction during the mid-Devonian. The benthonic (bottom dwelling) dendroid graptolites arose during the mid-Cambrian (around 520 million years ago). They outlived the graptoloids surviving on until their final extinction during the mid-Carboniferous period (320 million years ago).

Where can I find graptolites? Fossils Illustrated - Graptolites, edited by Douglas Palmer & Barrie Rickards gives information on where to find graptolites (see our Publications page). Many British Geological Survey Memoirs (particularly those detailing areas of North Wales, The English Lake District or southern Scotland) also provide detailed information on where to find graptolites and usually provide good illustrations of the fossils. If you want further information about these contact Mark Williams or the BGS enquiries service. Other good sources of information include: Geology in southwest Scotland by Stone et al.; British Cambrian to Ordovician Stratigraphy by A.W.A. Rushton, A.W. Owen, R.M. Owens & J.K. Prigmore; and Silurian Field Excursions. A Geotraverse across Wales and the Welsh Borderland by D.J. Siveter, A. Thomas & R.M. Owens (1989). National Museum of Wales, Cardiff.

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