Unveiling the Mystery: The True Source of Brilliance in “Golden” Fossils

Researchers from The University of Texas at Austin and their collaborators have made a remarkable discovery regarding the shimmering fossils found in Germany’s Posidonia shale. Contrary to popular belief that their radiance originated from fool’s gold (pyrite), the scientists have debunked this notion. Instead, they have found that a combination of minerals is responsible for the golden gleam, offering insights into the ancient environment where these fossils were formed.

The significance of this revelation lies in unraveling the formation process of these exceptionally preserved sea life specimens from the Early Jurassic era. Moreover, it sheds light on the role of oxygen in the environment during fossilization. Rowan Martindale, an associate professor at the UT Jackson School of Geosciences and co-author of the study, explains, “When you visit the quarries, you can spot golden ammonites among the black shale slabs. Surprisingly, we struggled to find pyrite in the fossils. Even the ones that appeared golden were preserved as phosphate minerals with yellow calcite. This fundamentally alters our understanding of this renowned fossil deposit.”

Published in Earth Science Reviews, the research was led by Drew Muscente, a former assistant professor at Cornell College and former postdoctoral researcher at the Jackson School. The Posidonia Shale fossils, including rare specimens with soft tissues such as ichthyosaur embryos, squids with ink-sacs, and lobsters, date back 183 million years. To investigate the conditions that facilitated such remarkable preservation, the researchers subjected dozens of samples to scanning electron microscopes for an analysis of their chemical composition.

Jim Schiffbauer, an associate professor at the University of Missouri Department of Geological Sciences and co-author of the study, expressed his enthusiasm, stating, “I was eagerly waiting to examine them under my microscope and help unravel their preservation story,” as he handled some of the larger samples.

The researchers discovered that phosphate minerals constituted the primary composition of the fossils in all cases, despite the presence of microscopic clusters of pyrite crystals, known as framboids, in the surrounding black shale rock. Sinjini Sinha, a doctoral student at the Jackson School and co-author of the study, recounted her meticulous search for framboids on the fossils, saying, “I spent days looking for framboids on the fossils. While there were perhaps only three or four framboids on the fossils, I counted 800 framboids on the matrix.”

The fact that pyrite and phosphate minerals were found in different locations on the specimens holds vital implications for understanding the fossilization environment. Pyrite forms in oxygen-deprived (anoxic) environments, whereas phosphate minerals require oxygen. The research suggests that although anoxic conditions on the seafloor set the stage for fossilization by preventing decay and predators, a burst of oxygen was necessary to facilitate the chemical reactions essential for fossilization.

These findings align with earlier research conducted by the team on the geochemical conditions of sites known for their exquisitely preserved fossils, known as konservat-lagerstätten. However, the results contradict long-standing theories regarding the conditions required for exceptional fossil preservation in the Posidonia.

Sinha explains, “For a long time, it was believed that anoxia was the cause of exceptional preservation, but it doesn’t directly contribute. Anoxia creates an environment conducive to faster fossilization, which leads to preservation, but it’s the oxygenation that enhances preservation.”

Interestingly, the oxygenation, along with the presence of phosphate and other minerals, also contributes to the stunning shine exhibited by these fossils.

Funding for the research was provided by Cornell College and the National Science Foundation. The Posidonia fossil specimens used in the study are now part of the collections at the Non-Vertebrate Paleontology Laboratory of the Jackson School.

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More about Golden fossils

• Earth Science Reviews – The published research article on the study of golden fossils and their formation in the Posidonia shale.
• The University of Texas at Austin Jackson School of Geosciences – The official website of the university’s geosciences department, where additional information about the research and related studies can be found.
• Non-Vertebrate Paleontology Laboratory – The website showcasing the collections and work conducted at the Non-Vertebrate Paleontology Laboratory at the Jackson School, which houses the Posidonia fossil specimens used in the study.