“Golden” fossils show where the excellent preservation first occurred.

In the case of fossils, even fool’s gold is not gold at all.

Many of the fossils from Germany’s Posidonia shale don’t get their shine from pyrite, also known as fool’s gold, as was once thought to be the source in a recent study by researchers at the University of Texas at Austin and their collaborators. All things being equal, the brilliant tint is from a blend of minerals that clues at the circumstances under which the fossils were framed.

Understanding how the fossils, which are among the world’s best-preserved sea life from the Early Jurassic, formed in the first place and the role that oxygen in the environment played in their formation are important outcomes of the discovery.

Rowan Martindale, a co-author of the research and an associate professor at the UT Jackson School of Geosciences, stated, “When you go to the quarries, golden ammonites peek out from black shale slabs.” However, to our surprise, we had a hard time finding pyrite in the fossils. Phosphate minerals with yellow calcite preserve even the golden-colored fossils. This dramatically alters how we perceive this well-known fossil deposit.”

“Golden ammonites may be seen protruding from slabs of black shale when you visit the quarries. Surprisingly, though, we had trouble locating pyrite in the fossils. Even the golden-looking fossils are still present as phosphate minerals with yellow calcite. This fundamentally alters how we perceive this well-known fossil deposit.”

 Rowan Martindale, an associate professor at the UT Jackson School of Geosciences.

Earth Science Reviews was the journal that published the study. The study was led by Drew Muscente, a former Jackson School postdoctoral researcher and Cornell College assistant professor.

Ammonite fossil From the Ohmden quarry, Posidonia shale lagerstatte. Credit: Sinjini Sinha/ The University of Texas at Austin Jackson School of Geosciences.

The Posidonia Shale contains rare soft-bodied fossils from 183 million years ago, including ichthyosaur embryos, squids with ink sacs, and lobsters. The researchers examined the chemical composition of dozens of samples under scanning electron microscopes to learn more about the conditions of fossilization that resulted in such exquisite preservation.

“I was unable to hold back to get them in my magnifying lens and help recount their preservational story,” said co-creator Jim Schiffbauer, an academic administrator at the College of Missouri Branch of Geographical Sciences, who dealt with a portion of the bigger examples.

Despite the fact that the surrounding black shale rock was dotted with microscopic clusters of pyrite crystals known as framboids, the researchers discovered that the fossils were primarily composed of phosphate minerals in every instance.

“I went through days searching for the framboids on the fossil,” said co-creator Sinjini Sinha, a doctoral understudy at the Jackson School. “I counted 800 framboids on the matrix of some of the specimens, but only three or four on the fossils.”

It is important that phosphate and pyrite are found in different places on the specimens because this reveals important information about the environment in which fossils were formed. Pyrite structures in anoxic (without oxygen) conditions; however, phosphate minerals need oxygen. According to the findings, despite the fact that an anoxic seafloor prevents decay and predators from forming fossils and sets the stage for fossilization, the chemical reactions that are necessary for fossilization require a pulse of oxygen.

Geosciences students from The University of Texas at Austin with ichthyosaur specimens from the Posidonia shale. Credit: Rowan Martindale

These results add to the team’s previous work on the geochemical conditions of konservat-lagerstätten, locations known for their caches of exceptionally preserved fossils. Notwithstanding, the aftereffects of these examinations go against well-established hypotheses about the circumstances required for extraordinary fossil conservation in the Posidonia.

Sinha stated, “It’s been thought for a long time that the anoxia causes the extraordinary preservation, but it doesn’t directly help.” It contributes to the environment’s facilitation of faster fossilization, which results in preservation, but oxygenation enhances preservation.

It turns out that the fossil’s shine was also improved by the oxygenation, phosphate, and minerals that came with it.

More information: A.D. Muscente et al, What role does anoxia play in exceptional fossil preservation? Lessons from the taphonomy of the Posidonia Shale (Germany), Earth-Science Reviews (2023). DOI: 10.1016/j.earscirev.2023.104323