“Golden” fossils reveal origins of exceptional preservation

Not everything that glitters is gold, in the case of fossils even fool’s gold.

A recent study by scientists from the University of Texas at Austin and collaborators found that many of the fossils from the German Posidonia Shale do not get their luster from pyrite, commonly known as fool’s gold, which has long been thought to be the source of shine is. Instead, the golden hue comes from a mix of minerals, which indicates the conditions under which the fossils formed.

The discovery is important for understanding how the fossils – which are among the world’s best preserved specimens of Early Jurassic marine life – were formed in the first place and the role of environmental oxygen in their formation.

“If you go to the quarries, golden ammonites peek out from black slabs of slate,” said study co-author Rowan Martindale, associate professor at the UT Jackson School of Geosciences. “But surprisingly, we had trouble finding pyrite in the fossils. Even the fossils that appeared golden are preserved as phosphate minerals with yellow calcite. This dramatically changes our view of this famous fossil deposit.”

The study was published in Geoscientific reviews. Drew Muscente, a former assistant professor at Cornell College and a former Jackson School postdoctoral fellow, led the study.

The fossils of the Posidonia Shale date back to 183 million years ago and include rare soft-bodied specimens such as ichthyosaur embryos, ink-sac squid and lobsters. To learn more about the fossilization conditions that led to such exquisite preservation, the researchers placed dozens of specimens under scanning electron microscopes to examine their chemical composition.

“I couldn’t wait to get them in my microscope and help tell their conservation story,” said co-author Jim Schiffbauer, an associate professor in the University of Missouri’s Department of Geological Sciences, who edited some of the larger samples.

The researchers found that in each case the fossils consisted mainly of phosphate minerals, although the surrounding black shale rock was dotted with microscopic clusters of pyrite crystals called framboids.

“I spent days looking for the framboids on the fossil,” said co-author Sinjini Sinha, a graduate student at the Jackson School. “In some specimens I counted 800 framboids on the matrix, while on the fossils it was maybe three or four.”

The fact that pyrite and phosphate are found at different locations on the specimens is important because it reveals important details about the fossilizing environment. Pyrite forms in anoxic (without oxygen) environments, but phosphate minerals require oxygen. The research suggests that although an anoxic seabed sets the stage for fossilization — keeping decay and predators at bay — it needed a pulse of oxygen to power the chemical reactions needed for fossilization.

These results complement previous research by the team on the geochemistry of sites known for their caches of exceptionally well-preserved fossils, known as conservation deposits. However, the results of these studies contradict long-standing theories about the conditions required for exceptional preservation of the fossils in the Posidonia.

“It has long been thought that anoxia is causing the extraordinary preservation, but it doesn’t help directly,” Sinha said. “It helps make the environment conducive to faster fossilization, which leads to conservation, but it’s oxygenation that improves conservation.”

It turned out that the oxygen supply — and the phosphate and accompanying minerals — also enhanced the fossil’s luster.