Why do these fossils have a golden shine? New study reveals origins of preservation

A team of researchers breaks down the mystery behind the golden hue atop Posidonia Shale fossils.
Sejal Sharma
Ammonite fossil from the Ohmden quarry, Posidonia shale lagerstatte
Ammonite fossil from the Ohmden quarry, Posidonia shale lagerstatte

Sinjini Sinha 

It was earlier believed that the shimmering golden hue atop Posidonia Shale fossils came from pyrite, deceptively similar to gold, also commonly known as fool’s gold. But a new study is now claiming that it is a result of something else entirely.

“When you go to the quarries, golden ammonites peek out from black shale slabs,” said Rowan Martindale, an associate professor at the University of Texas at Austin and a co-author of the study, in a statement. “But surprisingly, we struggled to find pyrite in the fossils. Even the fossils that looked golden are preserved as phosphate minerals with yellow calcite. This dramatically changes our view of this famous fossil deposit.”

The mystery of the golden hue

The fossils of the Posidonia Shale, found in southwest Germany, date back to 183 million years ago. These fossils are exquisitely preserved and are rare soft-bodied specimens like ichthyosaur embryos, squids, and lobsters. To learn more about their fossilization conditions, the researchers studied the samples under scanning electron microscopes to learn how the fossils may have come about.

“I couldn’t wait to get them in my microscope and help tell their preservational story,” said co-author Jim Schiffbauer, who handled some of the larger samples, in the statement.

The findings of the microscopic analysis of these shiny specimens revealed that although there were traces of pyrite crystals, called framboids, in the surroundings of the Posidonia Shale fossils, they were mostly made up of phosphate minerals.

The press release says this reveals key details about the manner in which these specimens were fossilized because pyrite and phosphate were found in different places on the specimens. Pyrite needs non-oxygen environments to grow, whereas phosphate minerals need oxygen. The research suggests that although the seafloor was anoxic (no oxygen), which helped with the preservation of the fossils, there is evidence to suggest that there was some influx of oxygen which catalyzed the chemical processes needed for fossilization.

And it is due to this oxygenation that the fossils have a golden shine.

“It’s been thought for a long time that the anoxia causes the exceptional preservation, but it doesn’t directly help,” said co-author and doctoral student Sinjini Sinha. “It helps with making the environment conducive to faster fossilization, which leads to the preservation, but it’s oxygenation that’s enhancing preservation.”

The study was published in the journal ScienceDirect.

Study abstract:

Konservat-Lagerstätten—deposits with exceptionally preserved fossils of articulated multi-element skeletons and soft tissues—offer the most complete snapshots of ancient organisms and communities in the geological record. One classic example, the Posidonia Shale in southwestern Germany, contains a diverse array of fossils preserved during the ∼183 Ma Toarcian Oceanic Anoxic Event. Seminal work on this deposit led to the hypothesis that many Konservat-Lagerstätten were preserved in stagnant basins, where anoxic conditions limited soft tissue degradation. To date, however, no studies have thoroughly investigated the geomicrobiological processes that drove fossil mineralization in the Posidonia Shale. As a result, the role of anoxia in its exceptional preservation remains uncertain. Here, we address these issues by reviewing the geology of the Posidonia Shale; describing the mineralization of its fossils; and synthesizing novel and existing data to develop a new model for the paleoenvironment and taphonomy of the Lagerstätte. Although shells and carbonate skeletal elements were preserved as pyritized and carbonaceous fossils, non-biomineralized tissues were primarily preserved via phosphatization (transformation of remains into calcium phosphate minerals). Unambiguous examples of phosphatization include ammonite shells, crustacean carapaces, ichthyosaur remains, coprolites, and coleoid gladii, mantle tissues, and ink sacs. Phosphatized crustaceans and coleoids contain cracks filled with pyrite, sphalerite, and aluminosilicate minerals. Such cracks were likely generated during burial compaction, which fractured phosphatized tissues, exposed their organic matter to focused microbial sulfate reduction, and thereby led to formation of, and infilling by, sulfide and clay minerals. These observations indicate that phosphatization happened early in diagenesis, prior to burial compaction and microbial sulfate reduction, beneath (sub)oxic bottom water, and corroborate the hypothesis that the animals were preserved during ephemeral pulses of oxygenation in the basin and/or within environments located along boundaries of anoxic water bodies. Overall, our findings support the view that anoxic bottom water does not directly promote exceptional preservation; in fact, it may impede it. Konservat-Lagerstätten, particularly “stagnation deposits”, tend to form in (sub)oxic depositional environments with steep redox gradients and/or high sedimentation rates. Under these conditions, organisms are rapidly buried below the redox boundary, where their mineralization is promoted by focused geomicrobiological processes, and degradation is limited by the supply of oxidants in the microenvironments around them.

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