Rapid volcanic ash preserved trilobites reveal ‘unprecedented’ detail

Stunning fossils reveal 3D anatomy of arthropods that lived a half-billion years ago.

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The recently found specimens, which were killed and preserved in volcanic ash over 500 million years ago, reveal intricate details never previously observed in any trilobite.

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

Using imaging techniques on these impressions, scientists have published in the journal Science the most intricate 3D anatomy of trilobites, the many-legged arthropods that were prevalent in the Paleozoic seas. Despite centuries of study, this discovery presents never-before-seen insights into these ancient creatures.

Led by Professor Abderrazak El Albani, an international team of scientists has uncovered remarkable findings about the trilobites from the Cambrian period. This breakthrough research, featuring experts such as 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 specialized leg pairs around their mouths.

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

Microtomographic reconstruction of the head and anterior trunk (“body”) limbs of the trilobite Protolenus (Hupeolenus) in ventral view.
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 mouth-edge appendages have spoon-like curved bases and are so small that they remain undetected in poorly preserved fossils. It was previously believed that trilobites possessed three pairs of head appendages behind their long antennae, but both Moroccan species indicate that they actually had four pairs.

Dating back to the Cambrian Period around 515 million years ago, the Moroccan trilobites are discovered in rock formed from volcanic ash, which was deposited on the shallow seafloor where the trilobites resided.

The trilobites, as well as small ‘lamp shells’ (brachiopods) that were attached to them in life through a delicate stalk, were killed by the hot, suffocating ash and were rapidly fossilized when the ash surrounding them turned into rock.

The trilobites’ outer surface, their legs, and the lamp shells attached to them were preserved as impressions in the volcanic rock, while the trilobites’ digestive tract 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. Credit: Arnaud MAZURIER, IC2MP, Univ. Poitiers

In order to observe the appearance of these rock impressions shortly after the trilobites perished, the team utilized high-resolution X-ray micro-tomography (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 employed computer modeling of X-ray slices through the fossils to analyze the trilobites’ entire body anatomy in 3D, freed from the surrounding rock.

Harry said “The computer work is pain-staking but it’s definitely been worth it. These trilobites look so alive, it’s almost as though they could crawl out of the rock.”

The ‘Pompei’ trilobites stand out because they are not flattened or distorted like many other fossils, and all their legs are positioned exactly as they were in life, with even small spines and sensory bristles along the leg joints being preserved.

This research provides new insights into the structure and biology of the long-extinct trilobites while also 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 a look. Who knows what secrets remain to be discovered in these understudied rocks?”

Trilobites belong to a completely extinct group of arthropods, which is a category of jointed-legged animals that includes over a million species of insects, crabs, spiders, and centipedes that exist today. Trilobites are abundant and diverse lifeforms in fossil deposits of the Palaeozoic 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 over 90 centimeters.

The majority of trilobite species are known only from their hard exoskeleton, similar to a lobster’s shell, while approximately 30 species have preserved a pair of antennae and/or pairs of two-branched legs under the head shield and each segment of the body.

Journal reference:

  1. Abderrazzak El Albani et al. Rapid volcanic ash entombment reveals the 3D anatomy of Cambrian trilobites. Science, 2024; DOI: 10.1126/science.adl4540.
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