A Trilobite Pompeii Preserves Exquisite Fossils in Volcanic Ash

Hundreds of millions of years ago, trilobites could be found all over the Earth. Cloaked in tough exoskeletons, the animals left behind countless fossils to be studied by paleontologists today. Despite all those preserved shells, scientists have been unable to understand certain aspects of trilobite anatomy after centuries of study, especially the soft internal structures of the ancient arthropods.

But a group of trilobite fossils entombed in volcanic ash in Morocco may provide the best glimpse yet of the segmented seafarers. In a paper published Thursday in the journal Science, researchers describe a batch of trilobites that were petrified in a manner similar to the Romans of Pompeii who were frozen in death by the eruption of Mount Vesuvius.

Abderrazak El Albani, a geologist at the University of Poitiers in France, led the dig that resulted in 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 spewing volcanoes. One of those eruptions left a cream-colored layer of fine-grained volcanic ash in which the trilobites were fossilized.

When the researchers cracked 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 mold 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 coauthors of the new study.

Dr. El Albani and his team posit that a short and sudden burst of volcanic activity buried the trilobites when ashy debris flooded the marine environment. One smothered trilobite’s digestive tract is even packed with sediment that it may have ingested before death. As the ash turned to stone, it created three-dimensional molds of the entombed trilobites.

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

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

To get a closer look at the fossilized anatomies, the scientists used micro-C.T. scans and X-ray imaging to create 3-D images of the specimens. This allowed them to view delicate structures like antennae, digestive tracts and even the hairlike bristles on the trilobites’ walking legs.

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

“The labrum is a kind of fleshy lip associated with the mouth that forms part of the oral chamber where food is processed,” Dr. Paterson said. “The labrum has long been hypothesized to exist in trilobites, but never observed in fossils.”

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 bug-eyed species in the genus Carolinites “would have had to drag their eyes through the mud with legs,” that were as short as those in the Moroccan specimens, he said.

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

“This gets into the minutiae of anatomy, but such debates are relevant when we want to figure out what group of living arthropods is mostly closely related to extinct trilobites,” he said.

To Dr. El Albani, who is Moroccan, the incredible trilobite specimens also represent something more than a taxonomic tool. He hopes they will inspire greater protection for 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 in order to make it available for science,” he said.