Biostratigraphy for Invertebrate Fossils
Biostratigraphy is a tool in palaeontology where fossils are used to date rocks in a relative sense. Throughout Earth’s history, in times of bursting diversity, species in the fossil record can be used to tell the relative age of the surrounding rocks.
For example, let’s say you were in the field and discovered ‘Species Y’. You know ‘Species Y’ appeared in the rock record after ‘Species X’ but before ‘Species Z’, so you can determine a relative age for the rocks you are looking at. In many cases, the best fossils for biostratigraphy are those that are found in high abundances over a large geographic area that preserve very well and live for a short time interval before dying out.
Therefore, marine invertebrates are common fossils used for biostratigraphy and often include ammonites, bivalves, and foraminifera.
Dr. Ahmed Abdelhady and colleagues recently published an article in Palaeontologia Electronica (PE) where they tried to rank invertebrate fossils on their effectiveness for biostratigraphy.
Although biostratigraphy has several useful implications, there are a few common problems associated with the technique. “One common issue is the validity and reliability of the biozone, the smallest age unit in biostratigraphy” says Dr. Abdelhady.
A biozone is an interval of geologic time represented by specific fossils in the rock record. These can be hard to read in the rocks and know exactly what the upper and lower limits of time are. “Ideally they will be reproducible across a considerable geographic area or global scale” says Dr. Abdelhady.
In order to determine which of the most commonly used fossils (ammonites, bivalves, benthic and planktic forams) are most effective for biostratigraphy Dr. Abdelhady and colleagues analyzed them with the Unitary Association Method.
“This is a deterministic method which generates a number of temporal biozones based on the occurrences of the fossil assemblages. Although different taxa may have different lower and upper boundaries in time, most of their ranges overlap” says Dr. Abdelhady.
Through analyzing the results, the team was able to rank the fossils from least to most effective and generate an Index of Stratigraphic Potential (ISP) of the fossil groups. They found that mode of life was an important factor in determining which fossils were best for biostratigraphy.
“Mode of life (benthic vs. nectic or mobile vs. stationary) will impact the ability to disperse across a large geographic area, which is an important factor for an ideal biostratigraphic fossil. Nektic (free-swimming) species can disperse for longer distances and thus have a wider geographic range. However, this results in having to compete with more species for survival, where less competent species will go extinct. In contrast, a smaller geographic range (such as for stationary benthic species) there may be less chance for competition where local species are more likely to live longer” explains Dr. Abdelhady.
Although bivalves and benthic forams persisted for longer periods of time, their geographic ranges were limited and therefore are only useful within those small regions. Ammonites and planktic forams are more valuable because of their high diversity and species turnover across greater geographic ranges, which can be correlated across the globe. Based on their results, planktic forams seem to be the best fossils for biostratigraphy.
“Planktic forams are ideal biostratigraphic fossils because they occur within different marine environments, are easily identifiable being backed by thorough systematics work, and their small size makes them discoverable in any core or sample cutting” says Dr. Abdelhady.
Few studies have looked at the biostratigraphic resolution of different fossil groups and there has never been an index ranking the stratigraphic potential of fossils which makes this article a valuable base for the field of biostratigraphy.
When asked what the next steps for this project were Dr. Abdelhady stated “we want to apply this index to other fossils across different timescales as well as compare species ranges across high and low latitudes for both the Paleozoic and Mesozoic.” Hopefully this will help further evaluate the index as a promising new tool for biostratigraphy.
To read more about this research you can find the original article here.