Gastropoda, Hyalitha, Brachipoda and Related Microbes of Cambrian Reefs in South-facing Exposures of Pilgrim Formation in Central Montana

Abstract

Throughout evolution some organisms have left traces of their existence by the process of fossilization. By studying an organism’s fossils, its structure and identity can be linked with a particular age or era. Each fossil provides insight into our evolutionary history. I conducted research in five sites in Grizzly Gulch, south of Helena , Montana. All sites contained reefs in the Pilgrim Formation of Late Cambrian age, which is approximately 520 million years old. I analyzed the reefs, extracted a total of 51 specimens and prepared them in the lab for identification. I identified 10 skeletal structures, including organisms of the phyla Hyalitha, Brachiopoda and Mollusca. I also identified other structures on the reefs that helped me to understand specific environmental conditions of this area. These structures included trace fossils which helped identify organisms that existed but were soft bodied. Evidence of their presence was preserved through burrows. In addition, graintypes such as oolites and intraclasts indicated that this area was one shallow water; stromatolites and channels suggested the presence of strong tides. My study concluded from these five reef sites that several different invertebrates dominated the floors of a shallow sea 520 million years ago in the area now called Grizzly Gulch, in central Montana.