Learning that a shiny rock is fool’s gold rather than the valuable mineral is generally cause for disappointment. But for scientists who discovered a new 450-million-year-old arthropod preserved in iron pyrite, the substance could be considered priceless.

Fossils of ancient arthropods — a group of animals including spiders, centipedes, and insects — are especially rare, because they are made up primarily of soft tissue, which usually starts decaying days after death. Due to some rare geochemical good fortune, the newly discovered species Lomankus edgecombei did not decay. The creatures — distant relatives of contemporary spiders, scorpions, and horseshoe crabs — were preserved in remarkably good shape, according to a report in Current Biology.

“We get an almost complete look at their anatomy,” says Luke Parry, an Earth sciences professor at Oxford University and an author of the paper. “Fossils like this tell us a great deal about what life on Earth looked like hundreds of millions of years ago, which is pretty astonishing, especially when you consider that these animals would have rotted to nothing in a matter of days under normal conditions.”

Preserving the Ancient Arthropod

Preservation in pyrite is “extremely rare,” says Parry, adding that he was “astonished” at how well Lomankus was preserved. He can point to only a handful of similar examples. They include the Beecher’s Trilobite Bed (in upstate New York, where the new fossil is from), the Devonian Hunsrück Slate in Germany, and a fossil site from the Cambrian period called the Chengjiang Biota.

Finding ancient fossils that pyrite has preserved is rare because the process requires specific conditions: organic material, iron, and a lack of oxygen. Pyrite forms through the action of sulfate, which reduces the bacteria that break down organic material in the absence of oxygen and produce hydrogen sulfide. When iron is also present, the hydrogen sulfide reacts with iron to form pyrite, which is iron sulfide.

The team that found L. edgecombei essentially hit a geological jackpot.

“Sediments that contain the fossils are low in organic material but high in iron and so the carcasses of the animals were like small islands where the conditions for pyrite to form are just right,” says Parry. “They were in a goldilocks zone of exceptional preservation.”

Read More: Scientists Find Fossilized Brains From 500-Million-Year-Old Arthropods

An Adaptation for a Dark Environment

This particular arthropod is especially interesting, because it shows one adaptation to its appendages, which Parry calls a “biological Swiss army knife.”

Contemporary arthropods use the legs at the front of their bodies for many functions. Some deploy them like insect antennae to sense their surroundings. Others adapted them into pinchers, to grasp prey. The adaptability of these appendages may be one reason why there are more species of arthropods on Earth than any other group of animals.

In Lomankus, the claws are much smaller, with three long and flexible whip-like flagella at their end. This suggests that the creature probably used these appendages to probe their environment. That adaptation, combined with the lack of eyes, indicates that Lomankus probably lived in a deep, dark environment.

Parry says scientists are not done prospecting for fossils at the site.

“The trilobites from this site are extremely famous but other fossils with soft parts are much rarer from this site,” Parry says. “Finding Lomankus hints at many more new fossils that are yet to come from this site.”

They may yet hit “Fool’s Gold” again.

Read More: 6 Unusual Traits of Animal Evolution

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Before joining Discover Magazine, Paul spent over 20 years as a science journalist, specializing in U.S. life science policy and global scientific career issues. He began his career in newspapers, but switched to scientific magazines. His work has appeared in publications including Science News, Science, Nature, and Scientific American.