Applied Biostratigraphy

It is difficult to think of a fossil group that has not, at some time, been used to biostratigraphically calibrate a particular rock unit – even human fossils and their various accoutrements and accessories have been used. Nevertheless it has become clear to biostratigraphers that some fossil groups are of more practical use then others in global age-dating and biozonation. It is a rare student of geology that is not at least vaguely familiar with at least the mention of using conodonts, graptolites and goniatites in the Paleozoic, ammonites in the Mesozoic and various planktonic microfossils in the Cenozoic.

Although these form a highly disparate group of organisms to use as global correlative tools, they all – more or less – have one thing in common, which is their mode of life. All of the above fossil groups live in the marine realm and, more specifically, they all live above the actual sea floor / sediment substrate. These fossil groups fall into one of two main categories:

Planktonic – organisms which float in the water column and have little or no control over their movements, for example…

• Planktonic foraminifera
• Calcareous nannoplankton
• Dinoflagellate cysts
• Radiolaria
• Graptolites

Nektonic – organisms which float or swim in the water column and have direct, conscious control over their movements, for example…

• Ammonites
• Goniatites
• Trilobites
• Conodonts

This is not to say that organisms which dwell on or within the sea floor / sediment substrate (the so-called “benthonic” realm) do not have any biostratigraphic utility – they do – but their application may be more restricted in geographical range compared with their floating and swimming brethren.

It should also not be taken to imply that non-aquatic organisms are also without biostratigraphic utility. Terrestrial organisms such as plant spores and pollen are equally as important in biostratigraphic correlation in both terrestrial and marine sediments as they can be transported by wind over vast distances.

Why are planktonic / nektonic fossils preferred for global correlation?

An individual fossil species would be “ideal” for biostratigraphy if it had the following characteristics…

• the instantaneous first appearance – worldwide – of the new species
• a short, though measureable, period of existence
• near-global distribution
• universal preservation
• easily identifiable with little or no variation in its appearance
• instantaneous extinction, again worldwide

Such an organism would have a distribution in space and time (called the “biochronal envelope”) like this…