A Pompeii trilobite preserves beautiful fossils in volcanic ash

Hundreds of millions of years ago, trilobites were found everywhere on Earth. Clad in sturdy exoskeletons, the animals left behind countless fossils that can be studied by paleontologists today. Despite all those preserved shells, centuries of research have left scientists unable to understand certain aspects of trilobite anatomy, especially the soft internal structures of the ancient arthropods.

But a group of trilobite fossils buried in volcanic ash in Morocco may provide the best picture yet of the segmented seafarers, a paper published Thursday in the journal Scienceresearchers describe a group of trilobites that were fossilized in a manner similar to the Romans of Pompeii, who were frozen to death by the eruption of Mount Vesuvius.

Abderrazak El Albani, a geologist at the University of Poitiers in France, led the dig that led to the discovery of the new fossils in the High Atlas Mountains in 2015. During the Cambrian period, 510 million years ago, the area was a shallow marine environment surrounded by erupting volcanoes. One of those eruptions left behind a creamy-colored layer of fine-grained volcanic ash in which the trilobites fossilized.

When the researchers broke open the volcanic rock, they found incredibly detailed impressions of the trilobites etched into the stone. “Volcanic ash is so fine-grained, like talcum powder, that it can form the tiniest anatomical features on the surface of these animals,” said John Paterson, a paleontologist at the University of New England in Australia and one of the co-authors of the new study.

Dr. El Albani and his team propose that a short and sudden burst of volcanic activity buried the trilobites as ash-like debris flooded the marine environment. In fact, a smothered trilobite’s digestive tract is full of sediment that it may have ingested before death. When the ash turned to stone, it created three-dimensional molds of the buried trilobites.

This froze the trilobites in time, like the doomed inhabitants of Pompeii, who were buried in ash as they fled the eruption of Vesuvius. Some trilobites are curled up in a ball, while others look as if they are about to run around. One specimen is even covered in tiny bivalves, which have hitched a ride on the animal’s shell using fleshy stalks.

“These brachiopods are still in their life position, which shows how quickly the burial occurred,” said Dr. El Albani.

To get a closer look at the fossilized anatomies, the scientists used micro-CT scans and X-ray imaging to create 3D images of the specimens. This allowed them to see delicate structures like antennae, digestive tracts, and even the hair-like bristles on the trilobites’ walking legs.

The team also discovered previously unknown anatomical features. These included several small appendages that helped scoop food into the trilobite’s slit-like mouth, and a flap of soft tissue called a labrum, which attached to the trilobite’s hard mouthpart and is now a common feature in living arthropods.

“The labrum is a kind of fleshy lip associated with the mouth that is part of the oral chamber where food is processed,” Dr. Paterson said. “The labrum has long been thought to exist in trilobites, but it has never been seen in the fossil record.”

According to Thomas Hegna, a paleontologist at the State University of New York at Fredonia who was not part of the study, the appendages observed in the new specimens were most likely not shared by all trilobites in the same form. For example, some bulging-eyed species in the genus Carolinites “would have had to drag their eyes through the mud with legs,” which were as short as those in the Moroccan specimens, he said.

But the intricate structures preserved in these “breathtaking” specimens will help place trilobites in the arthropod family tree, he says.

“This gets into the details of the anatomy, but such debates are relevant if we want to figure out which group of living arthropods is most closely related to extinct trilobites,” he said.

For Dr. El Albani, who is Moroccan, the incredible trilobite specimens also represent more than a taxonomic tool. He hopes they will inspire greater protection of Morocco’s paleontological heritage, which has been exploited by commercial fossil traders to the point that some call it a “trilobite economy.”

“We want to protect the place where the discovery was made to make it available to science,” he said.