Graptolites are commonly found in Lower Palaeozoic black shales and infrequently in argillaceous rocks of similar age on a global scale. The evolution of graptolite form has been described from flattened, carbonised fossils found on the bedding planes of slates and shales. The earliest graptolites (Middle Cambrian) are small and stick-like, and throughout the Palaeozoic, dendritic, or branching forms are the most common. Upper Cambrian forms are dendroidal and display increasing diversity throughout this period. They become less common throughout the Ordovician and maintain this low diversity until becoming extinct in the Lower Carboniferous. Graptolite diversity was the greatest during the Ordovician and Devonian with increased complexity of forms. Spiralling forms developed during the Silurian period.
SEM photo of a rare, three-dimensional spiral-form graptolite from the Silurian. Scale for this specimen is unavailable.
Reproduced with permission from ENK Clarkson
Biostratigraphy is a type of stratigraphy (the study of layers of rock deposited through time and space) developed utilising fossils in dating and correlating sequences of rock. Graptolites have been extremely useful in describing biostratigraphic units, especially rocks of Lower Palaeozoic age. Sequences of graptolite faunas have been used to subdivide Ordovician and Silurian rocks. Graptolites are valuable for this use because 1) they were planktonic and therefore widely distributed, 2) most types were able to tolerate a wide range of temperatures and so were not confined to latitudinal gradients, 3) most were not confined to deep or shallow waters, and therefore are found preserved in both facies-types and 4) the time-range of many graptolites is short.
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