Brugia Malayi - Clark Science Center - Smith College

Lab Analysis of Samples Collected from Early Triassic Moenkobi
Formation Carbonates in Nevada
Sophie Westacott
The Early Triassic is a period of slow recovery following the most massive extinction event in Earth’s history. 1 Studying the fossil
record from this time provides insight into what characterized that recovery in terms of diversity, abundance, body size, and other
factors, which in turn reveal what the environmental conditions were like. The focus of my project was the lab analysis of samples
I collected in the field this past January from the Virgin Limestone Member of the Moenkopi Formation at two localities in the
southern Nevada Muddy Mountains where silicified fossils are present.
Identifying and determining abundance of microscopic fossils while they remain trapped in rock is difficult if not impossible.
One solution to this problem, if the fossils are silicified, is to dissolve away the carbonate matrix in acid, freeing the fossils and
isolating them in the undissolved residue. This method is particularly useful for identification, as the entire 3-dimensional fossil
can be examined from every angle and under different microscopies, including the SEM. The residue can also be quickly separated
according to size using sieves. A drawback is that unsilicified or less well-preserved fossils will be underepesented.
To balance that bias, a second method was used in conjunction with the first. Thin sections, very thin rock glued to a
microscope slide, were made from each sample using rock saws, polishing wheels and other equipment. When these are viewed
under a petrographic microscope, both silicified and non-silicified fossils can be identified along with other material. Pointcounting,
which entails identifying the material directly under the cross-hairs at 200 evenly spaced points on the slide, gives
quantitative data on the relative abundance of everything in the sample, from bivalves to carbonate cement. 2
The localities these samples come from are known to have formed during the early Triassic, but more precise ages can help to
contextualize them beside other scientists’ work. Small, superficial holes were drilled into a cut slab of each rock sample, and the
powder created by the drilling was collected in test tubes (process depicted in accompanying photos). The tubes have been sent
away for testing, and the result will be a carbon isotope profile that can be matched against a profile with known ages.
I will continue this project during the academic year as an honors thesis, the ultimate goal of which is to compile data using
the methods described above to form an idea of what was happening at these localities during the early Triassic, and in doing so
contribute to our knowledge of this unique period of recovery. (Supported by the Schultz Foundation)
Advisor: Sara Pruss
References:
1
Pruss, S. B., and Bottjer, D. J., 2004, Early Triassic Trace Fossils of the Western United States and their Implications for Prolonged Environmental Stress from the
End-Permian Mass Extinction, Palaios, v. 19, p. 551-564.
2
Payne, J.L., Lehrman, D.J., Wei, J., and Knoll, A.H., 2006, The pattern and timing of biotic recovery from the end-Permian extinction on the Great Bank of
Guizhou, Ghizhou Province, China, Palaios, v. 21, p. 63-85.
2012
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