Graptolithina are extinct, colonial, marine organisms that existed from the Middle Cambrian to the Lower Carboniferous. The name graptolite is derived from the Greek word graptos, meaning 'written' and lithos, meaning 'stone,' since the majority of fossils resemble Egyptian hieroglyphics. Straight, curved, single-branching, bifid, several-branching and spiral forms are found. These fossils are commonly referred to as 'saw-blade' fossils because of their striking resemblance to the sharp-toothed tools used by carpenters. Graptolites are common fossils with worldwide distributions making them important index fossils for Palaeozoic biostratigraphic studies.


Graptolite morphology and anatomy has been reconstructed by looking at rarely preserved three-dimensional forms (unlike the common, two-dimensional carbonised films) extracted from rock using chemicals. This technique reveals fine detail and structures. The 'skeletons' of graptolites are composed of periderm, a thin, sheet-like material once thought to be a type of chitin. This tissue is composed of several, organic fabric-types. The primary 'blade' of a graptolite colony is called the sicula. Along the length of the sicula are individual cup-shaped thecae (singular = theca). These are thought to house the individual animals, or zooids, although there is no direct evidence for this. Each zooid is simple, possessing some type of food gathering mechanism, possibly a lophophore. These structures have possibly been found in pyratized fossils and it has been suggested that the zooids are connected to the rest of the colony, known as the rhabdosome, by a common canal that runs the length of the sicula. In branching forms, the branches are referred to as stipes.



 Anatomical illustrations.

Reproduced with permission from ENK Clarkson



Fossil graptolites are colonial, marine invertebrates with two main modes of life; benthic and planktic. Benthic forms were attached to the substrate by encrusting themselves to hard surfaces via a holdfast and were common in moderately deep water settings. In contrast, planktic forms drifted within the water column and evolved numerous systems, which prevented them from sinking, such as gaseous or fatty bubbles within the skeletal tissues, or by having the tissues isotonic with surrounding ocean water. Graptolite zooids filter-feed, and share their food with the entire colony by ingesting it and passing the nutrients through a common canal.

 Rare three dimensionally preserved graptolite extracted using chemical preparation and photographed using scanning electron microscopy. Width of specimen is 1mm across.



Reproduced with permission of ENK Clarkson


Author: LShychoski
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