Applied Biostratigraphy

Interpreting paleoenvironments from fossils could be regarded as more of an art than a science. Or perhaps more correctly as a detective story. The evidence and clues will be there but it is how they are put together to form the conclusion that can be the tricky part. We know that certain fossils live(d) in certain environments. Surely if the life habitat maps of each species/genus/group in a particular sample are overlain it will show us the most likely environment of deposition?

Such an assumption might be true if we could be sure that all the fossils present in any one sample should in fact be there. The surprising thing is that many fossils in a sample are not “supposed” to be there at all. Some may be there through direct contamination because our sampling methods may be inadequate or samples may be contaminated by mechanical processes (see the section on Operational Biostratigraphy and “caving”). Others may be in a sample because of the geological process known as “reworking”, where older fossils have become incorporated in to younger sediments because they have been eroded out of older strata. Another consideration might be that the organism that results in our fossil may have died in one place, but has become a fossil somewhere far removed from its natural habitat (see below).

It is the role of the biostratigrapher to take such matters under consideration. On the upside, even fossils that are not “supposed” to be in a particular sample can also provide us with useful information. Let’s take the simplest option first and assume that all our fossils in a sample are there “honestly”. How do we as geologists use fossils to interpret paleoenvironments?

• By direct comparison with living forms (however, points to remember…)
  • - the modern ocean is currently at a “highstand” setting with the continental shelves more-or-less fully submerged. Sea levels have been (much) higher and lower in the past and change constantly under geologic timescales
  • - modern global oceanic circulation patterns date only from the Pliocene after the Isthmus of Panama finally closed, separating the Atlantic from the Pacific Ocean and allowing the Gulf Stream to form. Another significant event occurred in the earlier Cenozoic with the opening of the Drake Passage between South America and Antarctica which first allowed complete global circulation around the Southern Ocean.
  • - there is always a loss of information through the fossilisation process & diagenesis
  • - some extinct fossil groups(e.g. graptolites, trilobites, conodonts etc.) have no modern analogues with which to compare
• Morpho-functional analysis – working out what “bits” of an organism has what function and how that function was “useful” to the living organism
• Association of fossils with sedimentary facies and their associated trace fossils for which the environment is already known
• Comparison with ancient facies models and their fossils
• Statistical methods