Silicification, Bias through Time
I worked with Susan Butts, Ph.D., the Senior Collections Manager of Invertebrate Paleontology at the Peabody Museum, and Richard Krause, Ph.D., a Post-Doctoral Associate of the Geology and Geophysics Department of Yale University, on a research project that revolved around the analysis of prevalence of silicification across the Paleozoic Era. Silicification is the process by which the original shell material of a fossil dissolves in the environmental acid and is replaced by the precipitation of silica into the fossil cavity. In a similar manner to all forms of taphonomy, silicification is affected by different factors of the environment in which it is taking place, such as climate and global ocean chemistry. Also, the formation of silica fossils is mediated by the composition and structure of the original shell material and depositional factors, such as permeability, porosity, lithology, and stratigraphy. The occurrence of silicified fossils is much more prevalent in the Paleozoic, disappearing in the fossil record as you move through the younger time periods. This may have something to do with either the abundance of carbonate shelly fossils during the Paleozoic or the abundance of available silica mineral during the era. However, the exact reason(s) for the large quantity of silicified fossils during the Paleozoic, but not in time periods afterwards, is not agreed upon.
The stratigraphic collections of the Peabody Museum represent the invertebrate species of the Paleozoic, organized by age. These specimens have been acquired over a period of at least 150 years, including irreplaceable collections from lost localities and modern inaccessibility. In testing the fossils for silicification, acid was used to determine whether the fossils still contained the original calcium carbonate material or had been silicified. Also, silica is harder than metal, so the fossils could be tested by scratching them with metal. Finally, they are also more resistant to weathering and often stand out in relief. When the testing of all of the specimens was completed, I was able to record my data across multiple individual time periods, producing a display of the changes in prevalence of silicification across the Paleozoic.
Additionally, I performed a literature search across articles published in the Journal of Paleontology. This literature search had the same function as that of the physical testing, but was extremely valuable to increase the number of data points and to widen our field of study. Each paper represented an additional specimen and the literature search as a whole produced very similar data to the physical specimens.
By the end of the summer, patterns began to form in the data. Our results showed that silicification was most prevalent in the Middle to Late Ordovician, the Late Silurian, and the Middle Permian. Possible explanations for these ranges include global climate and mass extinctions. In fact, modern hypotheses of prehistoric global climate seem to correlate very well with our data. Additionally, there is a drastic decrease in silicification near the end of the Permian period, correlating with the largest mass extinction of Earth’s history, and other additional possible correlations. However, the true reasons for these changes are still unknown.