Research shows anatomy of ancient trilobite preserved in 3D by volcanic eruption

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About half a billion years ago, a volcanic eruption near a shallow sea in what is now Morocco produced some of the most complete specimens ever found of trilobites, insect-like marine creatures, revealing anatomical details that scientists had never seen before.

Within moments, a fast-moving stream of hot ash and volcanic gases, called a pyroclastic flow, engulfed the trilobites, then cooled and hardened into solid rock. The trilobites died on the spot—as did the people similarly buried in ash at Pompeii in 79 A.D., during the eruption of Vesuvius.

For 515 million years, all evidence of those trilobites lay hidden, buried in a place called the Tatelt Formation in the High Atlas Mountains. But an international team of researchers recently used high-resolution X-ray microtomography to peer through the layers of the trilobite graves. The analysis revealed nearly pristine 3D imprints of the animals’ vaporized bodies in chunks of volcanic rock, the scientists reported June 27 in the journal Science.

From scans of these prehistoric molds, the scientists reconstructed 3D digital models that revealed the trilobite anatomy in unprecedented detail. The hot volcanic flow that buried the trilobites preserved impressions of soft tissues that normally do not fossilize, including gut organs, antennae, feeding structures and clusters of sensory bristles, and small spines on the trilobites’ appendages.

“It’s just incredible to have this in 3D without any change or distortion,” lead researcher Dr. Abderrazak El Albani told CNN. The detailed preservation showed that trilobites were anatomically advanced animals, with many specialized adaptations for feeding and moving across the seafloor, he said.

Chemical analysis of oxygen levels in the sediments in and around the specimens showed that the trilobites’ guts were filled with ash, likely swallowed as the animals suffocated in ash clouds in the seawater, the study authors wrote.

The trilobite Protolenus is shown in side view. The digestive system is shown in blue, the hypostome, or mouth structure, in green (far left), and the labrum, a bulbous structure above the mouth that is sometimes called an upper lip in insects, in red.

The pressure of sediment layers often flattens fragile fossils. But after the eruption buried the trilobites, cold seawater mixed with the hot ash and quickly hardened the pyroclastic flow into a tomb of solid rock. It prevented the trilobite molds from deforming and preserved a near-perfect imprint of their bodies, said El Albani, a professor of geosciences at the University of Poitiers in France.

The findings also underscore the urgency of protecting fossil-rich sites in Africa, such as the Tatelt Formation, El Albani added. Unlike the Tatelt, the Burgess Shale, a major Cambrian fossil site in Canada, is recognized as a UNESCO World Heritage Site. Such protection helps ensure that buried remains from Earth’s distant past remain accessible for future research, El Albani said.

Over the past 200 years, paleontologists have identified more than 22,000 species of trilobites from places around the world that were once covered by oceans. Trilobites were arthropods, like modern insects, spiders, millipedes, and crustaceans, and they evolved into a wide range of shapes and sizes before going extinct about 252 million years ago. Most trilobite species are no more than 1 inch (2.5 centimeters) long, but some, such as Hungoides bohemicus, grew to more than 12 inches (30.5 centimeters) long.

Microtomographic reconstruction shows the newly discovered trilobite species Gigoutella mauretanica, found in the Tatelt Formation in the High Atlas Mountains.

Trilobites had tough exoskeletons that typically fossilize well. However, the preservation of soft tissue in the newly discovered trilobites is exceptionally rare, said Dr. Melanie Hopkins, curator-in-charge of invertebrate paleontology at the American Museum of Natural History in New York City.

“Only a small fraction of trilobite species are preserved well enough to observe appendages,” said Hopkins, who studies trilobites but was not involved in the new research. “The level of detail preserved in the Tatelt specimens is extremely unusual, so much so that there are some features that have not been seen before,” she said. Such features are crucial for understanding how new traits and new species evolve, and for tracking relationships among arthropod groups, Hopkins added.

“The more anatomical detail we have, the better we can draw conclusions about how fossil arthropods were related to each other.”

The scientists found four trilobite specimens and identified two species that are new to science: Gigoutella mauretanica and Protolenus (Hupeolenus) — the second is an as-yet unnamed species in a known genus and subgenus. The specimens ranged from about 0.4 inches (11 millimeters) to 1 inch (26 millimeters) in length.

“This is the first time we’ve seen the labrum preserved,” a bulbous structure above the mouth that’s sometimes called an upper lip in insects, El Albani said. Behind the labrum, the oral slit was also beautifully preserved. Surrounding it were slender, curved appendages, likely used for feeding, that also had not been detected in trilobite fossils before, the study authors said.

The discovery of the structures raises new questions about the diversity of trilobite feeding appendages. It also raises questions about how that might have affected what trilobites ate and where they lived. It also raises questions about their vulnerability to changing environmental conditions if they had a highly specialized diet, Hopkins said.

The sudden eruption of the Cambrian volcanic eruption even preserved evidence of neighbors sharing the trilobites’ marine habitat. The research team found that one G. mauretanica trilobite had small shelled animals called brachiopods that were about 0.04 inches (1 millimeter) long and still attached to its face. This example of commensalism — different types of animals living together — is also exceedingly rare in the trilobite fossil record, El Albani said.

“It’s a unique window into the life history of this specimen from 515 million years ago,” he said. “I hope that with other discoveries — by our team, by other teams in Morocco — we’ll find more or different specimens, giving us the opportunity to see more about their life history and evolution.”

Mindy Weisberger is a science journalist and media producer whose work has appeared in Live Science, Scientific American, and How It Works magazine.

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