The Pompeian trilobite preserves fascinating fossils in volcanic ash

Hundreds of millions of years ago, trilobites could be found all over the Earth. Covered in hard exoskeletons, these animals left behind countless fossils for today’s paleontologists to study. 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 ancient arthropods.

But a group of trilobite fossils buried in volcanic ash in Morocco may provide the best glimpse yet of the fragmented mariners. In a research paper published Thursday in the journal SciencesResearchers describe a group of trilobites that fossilized in a manner similar to the Roman Pompeii that were frozen dead by the eruption of Mount Vesuvius.

Abdel Razzaq Al-Albani, a geologist at the University of Poitiers in France, led the excavations 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 volcanoes. One of these eruptions left a cream-colored layer of fine-grained volcanic ash in which the trilobites fossilized.

When the researchers opened up the volcanic rocks, they found incredibly detailed impressions of trilobites etched into the stone. “The volcanic ash is so finely grained, like talcum powder, that it can create the smallest anatomical features on the surface of these animals,” said John Patterson, a paleontologist at the University of New England in Australia and one of the authors of the new study.

Dr. Albani and his team hypothesize that a short, sudden burst of volcanic activity buried the trilobites as ash-filled debris flooded the marine environment. Even the digestive tracts of the strangled trilobites are filled with sediment that they may have swallowed before death. As the ash turned to stone, it created three-dimensional casts of the buried trilobites.

This caused the trilobite to be frozen in time like the doomed inhabitants of Pompeii, who were buried in ash as they fled Vesuvius’ outburst. Some trilobites curl up into a ball, while others look as if they are about to set sail. One specimen was even covered with tiny bivalves, which mounted on the animal’s shell using fleshy stalks.

“These armadillos are still in their life position, which shows how quickly the burial happened,” Dr. Al-Albani said.

To get a closer look at the fossilized anatomy, the scientists used micro-CT scans and X-ray imaging to create 3D images of the specimens. This allowed them to see tiny 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 numerous small appendages that helped push food into the trilobite’s slit-like mouth, and a soft tissue flap called the upper lip that was attached to the trilobite’s hard mouthpart and is now a common feature among living arthropods.

“The labial lip is a type of fleshy lip attached to the mouth that forms part of the oral chamber where food is processed. The labial lip has long been assumed to be present in trilobites, but has never been observed in fossils,” Dr Patterson said.

The appendages seen in the new specimens were probably not common to all trilobites in the same shape, according to Thomas Hegna, a paleontologist at the State University of New York at Fredonia who was not involved in the study. For example, some of the bulging-eyed Carolinianites “would have had to drag their eyes through the mud with legs,” he said, that were as short as those in the Moroccan specimens.

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

“This gets into the details of anatomy, but such discussions are relevant when we want to know which group of living arthropods are most closely related to the extinct trilobites,” he said.

For Dr. Al-Albani, a Moroccan, the stunning trilobite specimens also represent more than just a taxonomic tool. He hopes these discoveries will inspire greater protection of Morocco’s paleontological heritage, which has been exploited by commercial fossil dealers to the point that some call it “Trilobite economy“.”

“We want to protect the place where the discovery was made so that it is available to science,” he added.