Hundreds of millions of years ago, trilobites were found all over the Earth. Covered by tough exoskeletons, these animals left behind countless fossils that are studied by paleontologists today. Despite the preservation of many shells, scientists have been unable to work out certain aspects of trilobite anatomy after centuries of study, particularly the soft internal structure of these ancient arthropods.
But a group of trilobite fossils buried in volcanic ash in Morocco may offer the best glimpse yet of the segmented sailors. In a paper published Thursday in the journal Science, researchers describe a group of trilobites that were petrified in a similar way to the Romans of Pompeii who froze to death in the eruption of Mount Vesuvius.
The new fossils were discovered in the High Atlas Mountains in 2015, led by geologist Abderrazak El Albani of the University of Poitiers in France. During the Cambrian period 510 million years ago, this area was a shallow sea surrounded by erupting volcanoes. One of those eruptions left behind a layer of fine, cream-colored ash, within which the trilobite fossil was preserved.
When the researchers cracked the volcanic rocks, they found highly detailed impressions of trilobites engraved into the stone. “Volcanic ash is a very fine particle, like talcum powder, so it can imprint the tiniest anatomical features onto the surface of these animals,” said John Patterson, 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 speculate that the trilobites were buried when a sudden, short-lived volcanic explosion flooded the marine environment with ash debris. The asphyxiated trilobites' digestive tracts were filled with sediment that they likely ingested before dying. As the ash turned to stone, it created a three-dimensional mold of the buried trilobite.
This has frozen the trilobites in time, like the doomed inhabitants of Pompeii who were buried in ash while fleeing the eruption of Vesuvius. Some of the trilobites are curled up in a ball, while others look as if they might scurry about at any moment. One specimen is covered in minuscule bivalve creatures that have used their fleshy stalks to latch onto animal shells.
“These brachiopods are still in the posture they were in when alive, showing how quickly they were buried,” Dr El Albani said.
To get a closer look at the fossilized anatomy, the scientists used micro-CT scanning and X-ray imaging to create 3D images of the specimens, which allowed them to see delicate structures such as the antennae, digestive tract, and even the hair-like setae on the trilobite's walking legs.
The team also discovered previously unknown anatomical features, including several tiny appendages that helped scoop food into the trilobite's slit-like mouth, and a flap of soft tissue called a lip that attached to the trilobite's hard mouthparts and is now a common feature in modern arthropods.
“The lips are like fleshy lips attached to the mouth and form part of the oral cavity where food is processed,” Dr Patterson said. “Lips have long been hypothesized to exist in trilobites but have never been observed in fossils.”
The appendages observed in the new specimen likely aren't shared by all trilobites in the same way, says paleontologist Thomas Hegna of the State University of New York at Fredonia, who was not involved in the study. For example, the worm-eyed species of Carolinite “would have had to rub their eyes in the mud with their legs,” but their legs are just as short as those in the Moroccan specimen, Hegna says.
But the complex structures preserved in these “breathtaking” specimens could help place trilobites in the arthropod family tree, he says.
“This gets into the anatomical details, but these discussions are important if we want to understand the group of modern arthropods that are most closely related to extinct trilobites,” he said.
For Moroccan Dr El Albani, this remarkable trilobite specimen is more than a taxonomic tool: he hopes it will lead to greater protection of Morocco's palaeontological heritage, which has been exploited by commercial fossil traders in what has been dubbed the “trilobite economy.”
“We want to protect the sites where discoveries were made so they can be used for scientific research,” he said.