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Trilobites preserved in rapid volcanic ash reveal ‘unprecedented’ details

Trilobites preserved in rapid volcanic ash reveal ‘unprecedented’ details

The recently found specimens, which were killed and preserved in volcanic ash more than 500 million years ago, reveal intricate details never before seen in a trilobite.

It was the Pompeii of the Cambrian. About 515 million years ago, a volcanic eruption covered the shallow waters of what are now the High Atlas Mountains in Morocco. The ash encased the invertebrates living in the soft mud of the ocean floor, and quickly reacted with seawater chemicals to solidify into rock, preserving a pristine impression of the creatures.

Using imaging techniques to create these impressions, scientists have published in the journal Science the most complicated 3D anatomy of trilobites, the multilegged arthropods common in the Paleozoic seas. Despite centuries of research, this discovery offers never-before-seen insights into these ancient creatures.

Led by Professor Abderrazak El Albani, an international team of scientists has made remarkable discoveries about Cambrian trilobites. This groundbreaking research, including experts Harry Berks and Philip Donoghue from the University of Bristol’s School of Earth Sciences, sheds light on the feeding habits of these prehistoric organisms, including the unique arrangement of specialised pairs of legs around their mouths.

“The head and body appendages had an inward-facing battery of dense spines like those of horseshoe crabs, which manipulated and tore apart prey or ate carcasses as they were moved toward the mouth,” Harry Berks explained. “The mouth, a narrow slit behind a fleshy lobe called a labrum, familiar to living arthropods, has never been seen so clearly in a trilobite.”

Microtomographic reconstruction of the head and anterior trunk (“body”) limbs of the trilobite Protolenus (Hupeolenus) in ventral view.Trilobites preserved in rapid volcanic ash reveal ‘unprecedented’ details
Microtomographic reconstruction of the head and anterior trunk (“body”) limbs of the trilobite Protolenus (Hupeolenus) in ventral view. Credit: Arnaud MAZURIER, IC2MP, Univ. Poitiers

The buccal appendages have spoon-like curved bases and are so small that they go unnoticed in poorly preserved fossils. Trilobites were previously thought to have three pairs of head appendages behind their long antennae, but both Moroccan species indicate that they actually had four pairs.

The Moroccan trilobites date back to the Cambrian period, some 515 million years ago, and were discovered in rocks formed from volcanic ash, which was deposited on the shallow seafloor where the trilobites lived.

The trilobites, as well as the tiny ‘lamp shells’ (brachiopods) that were attached to them by a delicate stalk during life, were killed by the hot, suffocating ash and were quickly fossilized as the surrounding ash turned to stone.

The exterior of the trilobites, their legs and the lampshades attached to them were preserved as imprints in the volcanic rock. The digestive system of the trilobites was also preserved after being filled with ash.

Microtomographic reconstruction of the trilobite Gigoutella mauretanica in ventral view.Microtomographic reconstruction of the trilobite Gigoutella mauretanica in ventral view.
Microtomographic reconstruction of the trilobite Gigoutella mauretanica in ventral view. Credit: Arnaud MAZURIER, IC2MP, Univ. Poitiers

To observe the appearance of these rock impressions shortly after the trilobites perished, the team used high-resolution X-ray microtomography (XRμCT). X-rays were used to distinguish between the density of the rock where a trilobite was preserved and the empty space (air) where the body was before it disintegrated. Co-author Harry Berks used computer modeling of X-ray slices through the fossils to analyze the trilobites’ full body anatomy in 3D, separate from the surrounding rocks.

Harry said “The computer work is difficult, but it has certainly been worth it. These trilobites look so alive, it’s almost as if they could crawl out of the rock.”

The ‘Pompeii’ trilobites are notable for not being flattened or deformed like many other fossils, and all of their legs are positioned exactly as they would have been in life, with even tiny spines and sensory bristles along the leg joints preserved.

This research provides new insights into the structure and biology of the long-extinct trilobites while highlighting the significant potential for finding exceptionally preserved fossils in volcanic ash deposited in shallow marine environments.

Co-author Philip Donoghue said: “No one expects to find fossils in volcanic rocks, but our study shows that volcanic ash deposits are definitely worth looking at. Who knows what secrets are yet to be discovered in these underexposed rocks?”

Trilobites belong to a completely extinct group of arthropods, a category of arthropod animals that includes more than a million species of insects, crabs, spiders, and millipedes that still exist today. Trilobites are abundant and diverse life forms in fossil deposits from the Paleozoic Era, surviving from 521 million years ago to 250 million years ago. More than 20,000 species of trilobites have been identified by paleontologists, with body lengths ranging from less than two millimeters to more than 90 centimeters.

Most trilobite species are known only by their hard exoskeleton, which resembles a lobster shell. In about 30 species, a pair of antennae and/or a pair of two-branched legs are preserved beneath the head shield and each body segment.

Journal reference:

  1. Rapid burial by volcanic ash reveals the 3D anatomy of Cambrian trilobites. Science, 2024; DOI: 10.1126/science.adl4540.