In a groundbreaking discovery, researchers have unearthed some of the most impeccably preserved three-dimensional trilobite fossils ever found. Dubbed “Pompeii” trilobites for their stunning preservation in volcanic ash, these 500-million-year-old fossils provide an unprecedented glimpse into the anatomy of these ancient marine arthropods. Researchers used medical-grade scans to capture details as fine as their legs and hair-like structures.

The vast success of trilobites

Trilobites are iconic marine arthropods from the Paleozoic era. Before the age of dinosaurs, trilobites were roaming the seas. They thrived for over 270 million years, leaving behind thousands and thousands of fossils. In fact, trilobites are some of the most well-studied fossils in history, with over 20,000 species described, and plenty still left to uncover.

Trilobites have a distinctive three-lobed, three-segment skeleton — which is how they got their name (tri-lobites). They had strong exoskeletons made of calcite and chitin that supported and defended them from some predators. This hard skeleton is also why they fossilized so well. Add in the fact that they were highly adaptable and occupied a variety of ecosystems, and you start to understand why there are so many trilobite fossils.

Over the years, researchers have found some stunning trilobite fossils, but these are something else.

“I’ve been studying trilobites for nearly 40 years, but I never felt like I was looking at live animals as much as I have with these ones. I’ve seen a lot of soft anatomy of trilobites, but it’s the 3D preservation here that is truly astounding,” says Greg Edgecombe, one of the study authors and a paleontologist at the Natural History Museum.

Stunningly preserved fossils

The current trilobites were uncovered in the High Atlas Mountains in Morocco. These mountains separate the Sahara Desert from the Mediterranean Sea. The trilobites were buried in a layer of thick volcanic ash. And this ash, part of volcanic ejecta called a pyroclastic flow, is what preserved them so well — much like what happened to the ancient city of Pompeii, which was buried in ash by a volcanic eruption.

“An unexpected outcome of our work is discovering that volcanic ash in shallow marine settings could be a bonanza for exceptional fossil preservation,” Edgecombe added.

When creatures become fossilized, it’s almost always the hard tissues that fossilize. Soft tissue (like plants or the softer bodies of animals) is very difficult to find because bacteria and other microorganisms typically decompose them before they can fossilize. However, the Moroccan trilobites became buried in hot ash — a dreadful fate for them, but an opportunity for excellent fossil preservation.

The ash created a mold for their bodies, preserving even intricate details such as their organs and hair-like structures.

Lead author, Abderrazak El Albani, explains how important the findings are.

“As a scientist who has worked on fossils from different ages and locations, discovering fossils in such a remarkable state of preservation within a volcanic setting was a profoundly exhilarating experience for me. I think pyroclastic deposits should become new targets for study, given their exceptional potential for trapping and preserving biological remains, including delicate soft tissues.”

“These findings are anticipated to lead to significant discoveries about the evolution of life on our planet Earth.”

Numerous fossils but still more to learn

The researchers used CT scans and computer models to visualize the trilobites’ bodies. Already, the findings are challenging some of the things we knew about them. For instance, we previously assumed that trilobites had three pairs of head appendages behind their antennae — but the Moroccan species showed four pairs. There’s also a new lobe covering the mouth that was documented for the first time in trilobites.

Co-author Harry Berks, from the University of Bristol, adds:

“The results revealed in exquisite detail a clustering of specialized leg pairs around the mouth, giving us a clearer picture of how trilobites fed. The head and body appendages were found to have an inward-facing battery of dense spines, like those of today’s horseshoe crabs.”

The finding goes to show just how much information about ancient life we’re still missing, and how much there’s still yet to discover. Who knows what other secrets are hiding in the ash?

Abderrazzak El Albani et al, Rapid volcanic ash entombment reveals the 3D anatomy of Cambrian trilobites, Science (2024). DOI: 10.1126/science.adl4540. www.science.org/doi/10.1126/science.adl4540