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About half a billion years ago, a volcanic eruption near a shallow sea in what is now Morocco preserved some of the most complete specimens ever discovered of insect-like sea creatures called trilobites, revealing anatomical details scientists had never seen before.
Within moments, a torrent of hot ash and volcanic gases, called a pyroclastic flow, engulfed the trilobites, then cooled and solidified. The trilobites perished on the spot, just as the people who were buried in the ash in Pompeii in 79 A.D. when Mount Vesuvius erupted.
For 515 million years, all traces of these trilobites lay hidden, buried in a site called the Tatelt Formation in the High Atlas Mountains. But an international team of researchers recently used high-resolution X-ray microtomography to peer into the layers of the trilobite graves. The analysis revealed nearly intact 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 casts, scientists reconstructed 3D digital models, illustrating the trilobites’ anatomy in unprecedented detail. The hot volcanic flow that buried the trilobites preserved imprints of soft tissues that don’t usually fossilize, including intestinal organs, antennae, feeding structures and groups of sensory bristles, as well as tiny spines on the trilobites’ appendages.
“It’s just incredible to have this in 3D without any alteration or deformation,” Dr. Abderrazak El Albani, lead author of the study, told CNN. The detailed preservation showed that trilobites were anatomically sophisticated animals, with many specialized adaptations for feeding and moving around on the seafloor, he said.
Chemical analysis of oxygen levels in sediments in and around the specimens revealed that the trilobites’ intestines were filled with ash, likely swallowed when the animals suffocated in ash clouds in seawater, the study authors wrote.
Sedimentary layers tend to flatten fragile fossils. But after the eruption, the trilobites were buried under the pressure of cold seawater mixed with hot ash, which quickly solidified the pyroclastic flow, preventing the trilobite molds from deforming and preserving a near-perfect imprint of their bodies, says El Albani, a professor of geosciences at the University of Poitiers in France.
The findings also highlight the urgency of protecting fossil-rich sites in Africa, such as the Tatelt Formation, El Albani added. Unlike the Tatelt Formation, the Burgess Shale, a major Cambrian fossil site in Canada, is recognized as a UNESCO World Heritage Site. Such protective measures help ensure that buried remains of Earth’s distant past remain accessible for future study, 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 variety of shapes and sizes before going extinct about 252 million years ago. Most trilobite species are no more than an inch long, but some, like Hungoides bohemicus, have grown to over 12 inches long.
Trilobites had tough exoskeletons that generally fossilize well. However, soft tissue preservation in newly discovered trilobites is exceptionally rare, said Dr. Melanie Hopkins, curator of invertebrate paleontology at the American Museum of Natural History in New York.
“Only a small fraction of trilobite species are well enough preserved for us to see 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 features that have never been observed before,” she said. Such features are critical for understanding how new traits and species evolve, and for tracking relationships among arthropod groups, Hopkins added.
“The more anatomical detail we have, the better our conclusions about the relationships between fossil arthropods.”
Scientists found four specimens of trilobites and identified two species unknown to science: Gigoutella mauretanica and Protolenus (Hupeolenus) — the second is a still unknown species belonging to a known genus and subgenus. The specimens measured between 11 and 26 millimeters long.
“This is the first time we have a preservation of the labrum,” a bulbous structure above the mouth that is sometimes called the upper lip in insects, El Albani said. Behind the labrum, the oral slit was also beautifully preserved. Surrounding it were thin, curved appendages, likely used for feeding, that also had not been detected before in trilobite fossils, the study authors said.
The discovery of these structures raises new questions about the diversity of trilobites’ feeding appendages; how that may have affected what trilobites ate and where they lived; and how vulnerable they were to changing environmental conditions if they had highly specialized diets, Hopkins said.
The suddenness 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 tiny shelled animals called brachiopods, about 1 millimeter long, still attached to its face. This example of commensalism (different types of animals living together) is also extremely rare in the trilobite fossil record, El Albani said.
“This is a unique window into the life history of this 515-million-year-old specimen,” he said. “I hope that with further discoveries – by our team, by other teams in Morocco – we will find more different specimens, which will give us the opportunity to learn more about their life history and evolution.”
Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American, and How It Works magazine.