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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 found of the beetle-like sea creatures called trilobites, revealing anatomical details scientists had never seen before.
Within moments, the trilobites were engulfed by a fast-moving stream of hot ash and volcanic gases, called a pyroclastic flow, and then cooled and hardened into solid rock. The trilobites perished on the spot – much like the humans who were similarly buried in the ash at Pompeii in AD 79, during the eruption of Mount Vesuvius.
For 515 million years, all evidence of these trilobites remained 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 trilobite graves. The analysis revealed nearly intact 3D prints of vaporized animal bodies inside chunks of volcanic rock, researchers reported June 27 in the journal Science.
From scans of these prehistoric forms, scientists have reconstructed 3D digital models that show the anatomy of the trilobite in unprecedented detail. The hot volcanic flow that buried the trilobites preserved imprints of soft tissues that usually did not fossilize, including intestinal organs, antennae, feeding structures, and clusters of sensory bristles and tiny spines on trilobite appendages.
“It’s just incredible to have it in 3D without any changes or distortions,” study lead author Dr. Abderrazak El Albani. Detailed preservation showed that trilobites are anatomically sophisticated animals with many specialized adaptations for feeding and moving about on the sea floor, he said.
Chemical analysis of oxygen levels in the sediments in and around the samples revealed that the trilobites’ guts were stuffed with ash, likely swallowed when the animals suffocated on ash clouds in seawater, the study authors wrote.
The pressures of sediment layers often flatten fine fossils. But after the eruption buried the trilobites, cool seawater mixed with hot ash and quickly hardened the pyroclastic flow into a tomb of solid rock. It prevented the trilobites from deforming their forms and preserved a virtually 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 deposit in Canada, is recognized as a UNESCO World Heritage Site. Such protections help ensure that buried remnants of Earth’s distant past remain accessible for future study, El Albani said.
Over the past 200 years, paleontologists have identified over 22,000 species of trilobites from places around the world that were once covered by oceans. Trilobites were arthropods, like modern insects, spiders, centipedes, and crustaceans, and evolved into a wide variety of shapes and sizes before becoming extinct about 252 million years ago. Most trilobite species are no longer than 1 inch (2.5 centimeters), but some, such as Hungoides bohemicus, have grown to lengths of more than 12 inches (30.5 centimeters).
Trilobites had solid exoskeletons that usually fossilize well. However, soft tissue preservation in newly found 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 preserved enough for us to even observe appendages,” said Hopkins, who studies trilobites but was not involved in the new research. “The level of detail preserved in the Tateltu specimens is extremely unusual, so much so that there are some features that have not been seen before,” she said. Such traits are crucial for understanding how new traits and new species evolve, and for tracking relationships between arthropod groups, Hopkins added.
“The more anatomical details we have, the better conclusions we can make about how the fossil arthropods were related to each other.”
The researchers found four specimens of the trilobite and identified two new scientific species: Gigoutella mauretanica and Protolenus (Hupeolenus) – the latter being an as yet unnamed species in a known genus and subgenus. Specimens ranged from about 0.4 inches (11 millimeters) to 1 inch (26 millimeters) in length.
“This is the first time we’ve preserved the labrum,” the bulbous structure above the mouth that is sometimes referred to as the upper lip in insects, El Albani said. Behind the labrum, the oral slit was also excellently preserved. Around it were slender, curved appendages, probably used for feeding, which the study authors said were also previously undetected in trilobite fossils.
The discovery of the structures raises new questions about the diversity in trilobite food appendages; how this might have affected what trilobites ate and where they lived; and their vulnerability to changing environmental conditions if they had a highly specialized diet, Hopkins said.
The suddenness of the Cambrian volcanic eruption even preserved evidence of neighbors who shared the marine environment of the trilobites. The research team found that one trilobite, G. mauretanica, had tiny shell-like animals called brachiopods, measuring about 0.04 inches (1 millimeter), still attached to its face. This example of commensalism — different species of animals living together — is also extremely 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 further discoveries – by our team, by other teams in Morocco – we will find more or other specimens that will give us the opportunity to see 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.