Silurian graptolite biofacies on Arctic Canada: taxonomy, biostratigraphy, biogeography, paleoenvironments and evolution

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The Silurian (~445 to 415 million years) of the Canadian Arctic Islands, as elsewhere in the then-existing world, were occupied by many unusual organisms, including one dominant group of very abundant marine plankton animals, the graptolites. Graptolites evolved very rapidly and through the Silurian existing species disappeared and new species appeared very frequently, making them excellent fossils to subdivide the Silurian geologic column into many fine time slices, a procedure known as biostratigraphy. The graptolites of the Arctic Islands while continuous through the entire Silurian rock sequence, as is normal elsewhere, are virtually unique in the world in their mode of preservation: their original 3D skeletal material is encased within limestone concretions, and are easily extracted by dissolving the rock in dilute acid, yielding beautifully preserved specimens. As such, the Arctic graptolite sequence is considered to be one of the best in the world. For that reason, I have for years studied, and continue to study, these beautifully preserved graptolites. The procedure for newly recovered material continues as follows: firstly examination of their morphology for new information on their morphology and for species recognition, secondly the continual process of the development and refinement of their biostratigraphy to enable comparison and correlation with the fine time slices of rock elsewhere in the world, and finally from this background to advance the knowledge of their evolutionary history. Graptolites witnessed a number of mass extinctions during the Silurian. One event, well marked in the Arctic, witnessed the abrupt reduction of 50 pre-extinction species to only two post-extinction species, and coincided with a strong drop in sea level, and also with a strong increase Carbon 13. Future studies will examine this in more detail, and extend studies into earlier and later Silurian extinction events, with the goal of better understanding the nature of plankton extinction, and of extinctions in general. Finally, my overriding goal while continuing to learn more about graptolites and their habitats, is to extend that knowledge to help understand the geological and biological history, including climate, of the Arctic Canada during the Silurian Period.