Geology is all too poorly served on the Internet. The company has taken the opportunity to provide some general information about the Silurian geological system.
Graptolites were an extremely significant group of animals flourishing in the Palaeozoic until declining in the Devonian and Carboniferous. A colony of zooids living in the 'bumps' (theca) along the spiral filtered tiny food particles in the sea, in a similar way to modern day corals but they are quite unrelated. Graptolites in the Lower Palaeozoic are extremely useful in correlating rocks of the same age across the World as the individual species were relatively short lived but found over a wide area. Spirograptus turriculatus is a marker fossil within the Telychian Stage of the Llandovery Series.