Graptolite diversity — How much do we know?

Graptolite diversity studies usually cover only the planktic graptolites, those that are widely distributed in the fossil record and found in many marine sediments. Planktic graptolites are common from the beginning of the Ordovician (Tremadocian) to the Early Devonian (Emsian), the time interval in which they are used for biostratigraphy. Cambrian to Carboniferous dendroid graptolites were not the focus of these diversity studies. The diversity fluctuations of planktic graptolites in the Palaeozoic are considered to represent the ‘true’ record of the graptolites. However, these may in many instances only be based on or supported by the availability of data and the preservational potential of graptolites in certain sediment types. High diversity peaks may be based on the excellent preservation of graptolites in black shales (anoxic environment), when in other sediment types these faunas are too poorly preserved for proper identification. Chemical isolated graptolite faunas from limestones and shales also add to the uncertainty by producing often richer faunas than the ‘normally’ investigated material preserved on the rock surfaces. The preservation of organic material — and therefore of graptolites — is after all an exceptional and rare event in geological history. Thus, graptolites do not easily provide a reasonable view into an ancient environment and its diversity. The graptolite preservation bias appears to significantly affect the group’s diversity.