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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 hulking sea creatures called trilobites, revealing anatomical details scientists had never seen before.
Within moments, a fast-moving torrent of hot ash and volcanic gases, known as a pyroclastic flow, swallowed the three lobes and then cooled and hardened into solid rock. The trilobites perished on the spot — much like those who were in the same way in ashes at Pompeii in AD 79, during the eruption of Mount Vesuvius.
For 515 million years all the evidence of those trilobites remained hidden, presented in what is known as the Tatelt Formation in the High Atlas mountain range. But an international team of researchers recently used high-resolution X-ray micrography to peer through a series of trilobite tombs. The analysis showed almost pristine 3D prints of the vaporized bodies of the animals inside a chunk of volcanic rock, the scientists reported June 27 in the journal Science.
From scans of these prehistoric models, the scientists recreated 3D digital models, showing the anatomy of a trilobite in unprecedented detail. The hot volcanic flow that engulfed the trilobites preserved soft tissues that do not normally fossilize, including gut organs, antennae, feeding structures and clusters of sensory bristles, and small pins on the appendages of the trilobites.
“It’s incredible to have this in 3D without any change 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 sea floor, he said.
Trilobites were caught in a flash of volcanic activity
Chemical analysis of oxygen levels in the sediments in and around the specimens showed that the guts of the trilobites were filled with ash, suggesting that the animals likely swallowed ash clouds in the seawater, the study authors wrote.
The pressure of the sediment layers often flattens delicate fossils. But after the eruption buried the trilobites, the cool seawater mixed with the hot ash and quickly hardened the pyroclastic flow into a solid rock tomb. It kept the models of the trilobites from distorting and preserved an almost perfect trace of their bodies, said El Albani, a professor of geology 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, the Burgess Shale, an important Cambrian fossil site in Canada, is recognized as a UNESCO World Heritage Site. Such protections help ensure that buried remains of Earth’s distant past remain accessible for future study, El Albani said.
‘Extremely unusual’ discovery
In the last 200 years, paleontologists have identified more than 22,000 species of trilobites from parts of the world that were once covered by oceans. Trilobites were arthropods, like modern insects, spiders, millipedes and crustaceans, and evolved into a wide variety of shapes and sizes before becoming 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 usually had exoskeletons that fossilized well. However, the preservation of soft tissue in the newly discovered trilobites is extremely 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 number of trilobite species are well preserved for us to look at 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 several features that have not been observed before,” she said. Such features are critical to understanding how new traits and species evolve, and to tracing relationships between groups of arthropods, Hopkins said.
“The more anatomical data we have, the better we can infer the relationships between fossil arthropods.”
Trilobites had a stiff upper lip
The scientists found four trilobite specimens and identified two previously unknown species: Gigoutella mauretanica and Protolenus (Hupeolenus) – the latter is an unnamed species in a recognized 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 have preserved the labrum,” said El Albani, a bulbous structure over the mouth sometimes called the upper lip in insects. Behind the labrum, the oral cleft was also exquisitely preserved. It was surrounded by slender curved appendages, likely used for feeding, which had not previously been observed in trilobite fossils, according to the study authors.
The discovery of the structures raises new questions about the diversity of trilobite feeding appendages; how that might have affected what trilobites ate and where they lived; and their vulnerability to changing environmental conditions if they had highly specialized diets, Hopkins said.
A sudden Cambrian volcanic eruption preserved evidence of neighbors sharing the marine habitat of the trilobites. The research team found that tiny shell-like animals called brachiopods, one G. mauretanica trilobite, about 0.04 inches (1 millimeter) long, were still clinging to its face. This example of mutualism — different types of animals living together — is also extremely rare in the fossil record of trilobites, El Albani said.
“It’s a unique window in the history of life 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 specimens or different specimens, which will give us the opportunity to see more about the history and evolution of their lives.”
Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American and How It Works magazine.
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