Volcanic eruptions preserved ancient trilobite anatomy in 3D, study finds

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Nearly half a billion years ago, a volcanic eruption near a shallow sea off present-day Morocco preserved the most complete specimens ever of bug-like marine creatures called trilobites, revealing anatomical details that scientists had never seen before.

Within moments, a torrential stream of hot ash and volcanic gases, called a pyroclastic flow, engulfed the trilobites and then cooled and turned into solid rock. The trilobites died on the spot – just as people buried in ash in Pompeii died during the eruption of Mount Vesuvius in 79 A.D.

For 515 million years all evidence of those trilobites remained hidden in a place called the Tatelt Formation in the High Atlas mountain range. But an international team of researchers recently used high-resolution X-ray microtomography to look at the layers of the trilobites’ graves. The analysis revealed nearly pristine 3D imprints of the animals’ vaporized bodies inside fragments of volcanic rock, the scientists reported June 27 in the journal Nature. Science,

From scans of these prehistoric molds, scientists reconstructed 3D digital models that displayed the trilobite’s anatomy in unprecedented detail. The hot volcanic flows that buried the trilobites preserved traces of soft tissues that do not usually fossilize, including visceral organs, antennae, feeding structures and clusters of sensory bristles, and tiny spines on the trilobites’ appendages.

“To see this in 3D without any changes or distortion is incredible,” the lead study author said. Dr. Abderrazak Al Albani he told CNN. He said the detailed preservation shows that trilobites were anatomically sophisticated animals, with many special adaptations for feeding and movement on the sea floor.

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

Arnaud Mazurier/Institute of Environmental and Materials Chemistry of Poitiers/University of Poitiers

The trilobite Protolenus is shown in side view. The digestive tract appears in blue, the hypostome, or mouth structure, in green (far left) and the labrum, a bulbous structure above the mouth sometimes referred to as the 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 hot ash and pyroclastic flows to quickly turn the trilobites into a tomb of solid rock. This protected the trilobites’ molds from being distorted and preserved a nearly perfect imprint of their bodies, said L. Albani, a professor of geology at the University of Poitiers in France.

El Albani said these findings also underscore the urgent need to preserve fossil-rich places such as the Tatelt Formation in Africa. Unlike Tatelt, the Burgess Shale, an important Cambrian fossil site in Canada, is recognized as a UNESCO World Heritage Site. Such protection helps 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 becoming extinct about 252 million years ago. Most trilobite species grew 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.

Arnaud Mazurier/Institute of Environmental and Materials Chemistry of Poitiers/University of Poitiers

Microtomographic reconstruction reveals a new trilobite species, Gigoutella mauritanica, found in the Tatelt Formation in the High Atlas Mountains.

Trilobites had strong exoskeletons that typically fossilize well. However, the preservation of soft tissue in newly found trilobites is exceptionally rare, he said Dr. Melanie HopkinsCurator 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 that we can see the appendages,” said Hopkins, who studies trilobites but was not involved in the new research. “The level of detail preserved in the Tatel specimens is extremely unusual, so much so that there are some features that have not been seen before,” she added. Such features are important for understanding how new traits and new species evolve, and for tracking relationships among arthropod groups, Hopkins said.

“The more anatomical details we have, the better we can guess how fossil arthropods were related to one another.”

The scientists found four trilobite specimens and identified two species new to science: Gigautella mauretanica and Protolenus (Hupolenus) — the second is a still-unnamed species in a known genus and subgenus. The specimens were 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 above the mouth sometimes referred to as the upper lip in insects, El Albani said. Behind the labrum, the mouth hole was also excellently preserved. Surrounding it were thin curved appendages, possibly used for feeding, which were also not seen in trilobite fossils before, according to the study authors.

Hopkins said the discovery of the structures raises new questions about the diversity of trilobite feeding appendages; how this may have affected trilobites’ diets and habitats; and, if their diets were highly specialized, how this may have affected their sensitivity to changing environmental conditions.

The suddenness of the Cambrian volcanic eruption also preserved evidence of the neighbors that shared the trilobites’ marine habitat. The research team found that one G. mauretanica trilobite had tiny shelled animals called brachiopods, about 0.04 inches (1 millimeter) in length, still stuck to its face. This example of commensalism — different kinds of animals living together — is also extremely rare in the trilobite fossil record, L’Albani said.

“This is a unique glimpse 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 will find more or different specimens, which 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 magazine.

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