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About half a billion years ago, a volcanic eruption near a shallow sea in what’s now Morocco preserved some of the most complete specimens ever found of buglike sea creatures called trilobites, revealing anatomical details that scientists had never seen before.
Within moments, a fast-moving torrent of hot ash and volcanic gases, called pyroclastic flow, engulfed the trilobites and then cooled and hardened to solid rock. The trilobites perished on the spot — much like the people who were similarly entombed in ash at Pompeii in AD 79, during the eruption of Mount Vesuvius.
For 515 million years all evidence of those trilobites stayed hidden, buried at a site called the Tatelt Formation in the High Atlas mountain range. But an international team of researchers recently used high-resolution X-ray microtomography to peer through the layers of the trilobites’ tombs. The analysis revealed nearly pristine 3D imprints of the animals’ vaporized bodies inside 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, displaying trilobite anatomy in unprecedented detail. The hot volcanic flow that buried the trilobites preserved impressions of soft tissues that typically don’t fossilize, including gut organs, antennae, feeding structures and clusters of sensory bristles, and tiny spines on the trilobites’ appendages.
“It’s just incredible to have this in 3D without any alteration or deformation,” lead study author Dr. Abderrazak El Albani told CNN. The detailed preservation showed that trilobites were anatomically sophisticated animals, with many specialized adaptations for feeding and movement along the seafloor, he said.
Trilobites caught up in a flash of volcanic activity
Chemical analysis of oxygen levels in the sediments in and around the specimens revealed that the trilobites’ guts were stuffed with ash, likely swallowed as the animals suffocated on ash clouds in seawater, the study authors wrote.
The pressures of sediment layers often flatten delicate fossils. But after the eruption buried the trilobites, cool seawater mixed with the hot ash and quickly hardened the pyroclastic flow into a tomb of solid rock. It kept the molds of the trilobites from distorting and preserved a virtually perfect imprint of their bodies, said El Albani, a professor of geoscience at the University of Poitiers in France.
The findings also underscore the urgency of protecting fossil-rich locations in Africa such as the Tatelt Formation, El Albani added. Unlike the Tatelt, the Burgess Shale, an important Cambrian fossil site in Canada, is recognized as a UNESCO World Heritage Site. Such protections help to ensure that buried remnants of Earth’s distant past remain accessible for future study, El Albani said.
‘Extremely unusual’ discovery
Over the past 200 years, paleontologists have identified over 22,000 species of trilobites from locations 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 around 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 be more than 12 inches (30.5 centimeters) long.
Trilobites had tough exoskeletons that typically fossilize well. However, the preservation of soft tissue in the newfound 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 well-preserved enough for us to observe appendages at all,” 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 observed before,” she said. Such features are critical for understanding how new traits and new species evolve, and for tracking relationships between arthropod groups, Hopkins added.
“The more anatomical detail we have, the better inferences we can make about how fossil arthropods were related to one another.”
Trilobites had a stiff upper lip
The scientists found four trilobite specimens and identified two species new to science: Gigoutella mauretanica and Protolenus (Hupeolenus) — the second is a still-unnamed species in a known genus and subgenus. The specimens ranged from about 0.4 inches (11 millimeters) to 1 inch (26 millimeters) long.
“This is the first time we have preservation of the labrum,” a bulbous structure over the mouth that is sometimes referred to as an upper lip in insects, El Albani said. Behind the labrum, the mouth slit was also exquisitely preserved. Surrounding it were slender curved appendages, likely used for feeding, that were also previously undetected in trilobite fossils, according to the study authors.
Discovering the structures raises new questions about diversity in trilobite feeding appendages; how that may have affected what trilobites ate and where they lived; and their vulnerability to changing environmental conditions if they had highly specialized diets, Hopkins said.
The suddenness of the Cambrian volcanic eruption even preserved evidence of neighbors that shared the trilobites’ marine habitat. The research team found that one G. mauretanica trilobite had tiny shelled animals called brachiopods, measuring about 0.04 inch (1 millimeter) long, still clinging 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 for this specimen from 515 million years ago,” he said. “I hope that with other discoveries — by our team, by other teams in Morocco — we will find more or different specimens, which will give us the opportunity to see more about their life history and evolution.”
Correction: A previous version of this story incorrectly credited the GIF loop. The correct attribution is Arnaud Mazurier of the University of Poitiers.
Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American and How It Works magazine.
