How ‘Worms’ End Up in Fool’s Gold Fossils
Tubelike fossils of an animal known as Conotubus hemiannulatus.
Credit: Schiffbauer et al.

Fool's gold helps explain why many fossils of soft-bodied animals that lived more than 540 million years ago still survive, a new study finds.

Understanding the relationship between decay and fossilization will inform future study and help researchers interpret fossils in a new way.

“The vast majority of the fossil record is composed of bones and shells,” says James Schiffbauer, assistant professor of geological sciences at the University of Missouri.

“Fossils of soft-bodied animals like worms and jellyfish, however, provide our only views onto the early evolution of animal life. Most hypotheses as to the preservation of these soft tissues focus on passive processes, where normal decay is halted or impeded in some way, such as by sealing off the sediments where the animal is buried,” he says.

“Our team is instead detailing a scenario where the actual decay helped ‘feed’ the process turning the organisms into fossils—in this case, the decay of the organisms played an active role in creating fossils.”

Fool’s Gold and Wormy Creatures

Schiffbauer studied a type of fossil animal from the Ediacaran Period called Conotubus, which lived more than 540 million years ago. He notes that these fossils are either replicated by, or associated with, pyrite—commonly called fool’s gold.



The tiny fossils are tube-shaped and believed to have been composed of substances similar at least in hardness to human fingernails. These fossilized tubes are all that remain of the soft-bodied animals that inhabited them and most likely resembled worms or sea anemone-like animals.

“Most of the animals that had once lived on the Earth—with estimates eclipsing 10 billion species—were never preserved in the fossil record, but in our study we have a spectacular view of a tinier fraction of soft-bodied animals,” says Shuhai Xiao, professor of geobiology at Virginia Tech and a coauthor of the study.

“We asked the important questions of how, and under what special conditions, these soft-tissued organisms can escape the fate of complete degradation and be preserved in the rock record.”

Schiffbauer and his team performed a sophisticated suite of chemical analyses of these fossils to determine what caused the pyrite to form. They found that the fool’s gold on the organisms’ outer tube formed when bacteria first began consuming the animal’s soft tissues, with the decay actually promoting the formation of pyrite.

“Normally, the earth is good at cleaning up after itself,” Schiffbauer says. “In this case, the bacteria that helped break down these organisms also are responsible for preserving them as fossils. As the decay occurred, pyrite began replacing and filling in space within the animal’s exoskeleton, preserving them.

“Additionally, we found that this process happened in the space of a few years, perhaps even as low as 12 to 800. Ultimately, these new findings will help scientists to gain a better grasp of why these fossils are preserved, and what features represent the fossilization process versus original biology, so we can better reconstruct the evolutionary tree of life.”

The explanation helps to solve the mystery of why about 80 percent of the fossils in the Gaojiashan formation are preserved in three dimensions, with fool's gold around them, while others are preserved in two dimensions in a second process called carbonaceous compression. It seems that, as long as sediments didn't continue to bury the fossils too quickly, the pyrite process could continue. If the fossils buried faster, the compression process took over, creating pancake-flat fossils instead of fossils in three dimensions. 

The above story is based on materials provided by University of Missouri.
 
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