Posidonia shale fossils’ golden gleam comes from a mix of minerals, not fool’s gold, study finds

A recent study by scientists from The University of Texas at Austin and their collaborators has challenged the long-held belief that the golden glimmer in fossils from Germany’s Posidonia shale comes from pyrite, also known as fool’s gold. The researchers found that the shine actually comes from a mixture of minerals, which offers insight into the conditions under which the fossils formed, and the role of environmental oxygen in their formation.

The fossils from the Early Jurassic period are some of the best-preserved specimens of sea life in the world. According to study co-author Rowan Martindale, a UT Jackson School of Geosciences associate professor, “golden ammonites peek out from black shale slabs” in the quarries where they are found. However, the team struggled to find pyrite in the fossils, even those that appeared golden. Instead, the fossils were preserved as phosphate minerals with yellow calcite.

This new understanding of the famous fossil deposit has significant implications for our comprehension of how the fossils came to be and how environmental factors played a role in their formation. The study, led by former Cornell College assistant professor and Jackson School postdoctoral researcher Drew Muscente, was published in Earth Science Reviews.

Golden ammonite fossils at Ohmden quarry. Credit: Rowan Martindale/ The University of Texas at Austin Jackson School of Geosciences.

The Posidonia Shale fossils are rare and remarkable, dating back to 183 million years ago and featuring soft-bodied specimens such as ichthyosaur embryos, squids with ink-sacs, and lobsters. To investigate the fossilization process responsible for their exceptional preservation, scientists analyzed the chemical composition of dozens of samples using scanning electron microscopes.

Co-author Jim Schiffbauer, an associate professor at the University of Missouri Department of Geological Sciences, was excited to examine the larger samples under the microscope. The team found that although the surrounding black shale rock had microscopic clusters of pyrite crystals called framboids, the fossils were primarily composed of phosphate minerals.

Interestingly, pyrite and phosphate were found in different locations on the specimens. This is significant because it provides clues about the fossilization environment. Pyrite forms in anoxic environments without oxygen, whereas phosphate minerals require oxygen. The research indicates that while an anoxic seafloor is necessary for fossilization to occur, it took a pulse of oxygen to drive the chemical reactions that enabled fossilization.

These findings build upon earlier research on the geochemical conditions of fossil-rich sites known as konservat-lagerst├Ątten. The results contradict long-held beliefs about the conditions necessary for exceptional fossil preservation in the Posidonia, which had been attributed to anoxic environments.

As co-author Sinjini Sinha, a doctoral student at the Jackson School, explained, anoxic environments do not directly aid preservation but rather create conditions conducive to faster fossilization. In contrast, oxygenation enhances preservation and the fossil’s shine through phosphate and accompanying minerals.

Source: University of Texas at Austin

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