How fossil collection methods can affect paleoecological datasets

The influence of collection method on paleoecological datasets: In-place versus surface-collected fossil samples in the Pennsylvanian Finis Shale, Texas, USA

Frank L. Forcino, Emily S. Stafford

Summarized by Mckenna Dyjak

What data were used?: Two different fossil collecting methods were compared using the Pennsylvanian marine invertebrate assemblages of the Finis Shale in Texas. In-place bulk-sediment methods and surface sampling methods were used to see how these different methods could influence taxonomic (groups of animals) samples. 

Methods: The bulk-sediment sampling method involves removing a mass of sediment and later washing and sieving the material to retrieve the fossil samples; surface sampling is a simpler method in which the top layer of sediment is removed and the exposed fossils are collected by hand. The samples were collected in the Finis Shale in Texas at stratigraphically equivalent (layers of rock deposited at the same time) locations to ensure continuity in the two methods. The bulk-sediment and surface pick-up samples were analyzed for differences in composition and abundance of fossil species (i.e., paleocommunities) using PERMANOVA (a type of analysis used to test if samples differ significantly from each other).

Results: The study found that the bulk-collected samples differed from the surface-collected samples. The relative abundance of the major taxonomic groups (brachiopods and mollusks), composition, and distribution varied considerably in both collecting methods. For example, there was a higher relative abundance of brachiopods in the bulk-collected samples and a higher relative abundance of gastropods in the surface-collected samples.

Figure 1. Comparison of relative abundance of fossil groups between in-place and surface samples. Note the different abundances from each of the collection methods.
(SpE = Spillway East outcrop, SpW = Spillway West outcrop, CW = Causeway Road outcrop)

Why is this study important?: Bulk-sediment sampling and surface sampling methods produce significantly different results, which would end up affecting the overall interpretation of the history of the site. The surface-collected fossils may be influenced by stratigraphic mixing (mixing of materials from different rock layers), collector bias (which can influence a fossil’s potential to be found and collected; for example, larger fossils are more likely to be collected), and destruction of fossils due to weathering. Bulk-sediment sampling will likely have a more accurate representation of the ancient community, because the fossils likely experienced the least amount of alteration during the process of the organism becoming a fossil (also known as taphonomy).

The bigger picture: The amount of things that have to go right in order for an organism to become a fossil is a lengthy list (read more about the fossilization process here). There are many biases that can contribute to the incompleteness of the fossil record such as environments that favor preservation (e.g., low oxygen), as well as poor preservation value of soft tissues, like skin. Scientists must do what they can in order to collect accurate data of the fossil record since there are already so many natural biases. Knowing which fossil collecting methods produce the most accurate results is important when advocating for the paleocommunity.

Citation: Forcino FL, Stafford ES (2020) The influence of collection method on paleoecological datasets: In-place versus surface-collected fossil samples in the Pennsylvanian Finis Shale, Texas, USA. PLoS ONE 15(2): e0228944.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.