Hundreds of millions of years ago, trilobites could be found all over the Earth. Encased in tough exoskeletons, the animals left behind countless fossils that paleontologists study today. Despite all these preserved shells, scientists have been unable to understand some aspects of trilobite anatomy after centuries of study, 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 look yet at segmented mariners. In a paper published Thursday in the journal Science, scientists describe a group of trilobites that petrified in a similar way to the Romans of Pompeii, who were frozen when Mount Vesuvius erupted.
Abderrazak El Albani, a geologist at the University of Poitiers in France, led the excavations 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 spewing volcanoes. One of these eruptions left a cream-colored layer of fine-grained volcanic ash in which were fossilized trilobites.
When scientists broke open the volcanic rock, they found incredibly detailed trilobite prints etched into the stone. “Volcanic ash is so fine-grained, like talc, that it can form the smallest 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 hypothesize that a short and sudden burst of volcanic activity buried the trilobites when ash debris flooded the marine environment. The digestive tract of one suffocated trilobite is even full of sediment it may have ingested before death. When the ash turned to stone, it created the three-dimensional forms of the buried trilobites.
This froze the trilobites in time like the doomed inhabitants of Pompeii who were buried in the ash as they fled the eruption of Vesuvius. Some trilobites are curled up into a ball, while others look like they’re about to wander off. One specimen is even covered in tiny bivalves that have hitched themselves to the animal’s carapace using fleshy stalks.
“These brachiopods are still in their life position, which shows how quickly the burial took place,” said Dr. El Albani.
To get a closer look at the fossilized anatomy, the researchers used micro-CT scans and X-ray imaging to create 3D images of the specimens. This allowed them to see fine structures such as antennae, digestive tracts, and even hairy 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 flap of soft tissue called a labrum that attached to the trilobite’s hard mouth and is now a common feature among living arthropods.
“The labrum is a kind of fleshy lip attached to the mouth that forms part of the oral cavity where food is processed,” said Dr. Paterson. “The labrum has long been assumed to exist in trilobites, but has never been 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 seen in the new specimens most likely were not shared by all trilobites in the same form. For example, some bug-eyed species of 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 complex structures preserved in these “breathtaking” specimens will help place trilobites in the arthropod family tree, he says.
“That gets into the details of anatomy, but such debates are relevant when we want to figure out which group of living arthropods is mostly closely related to the extinct trilobites,” he said.
For Dr. For El Albani, who is Moroccan, the incredible trilobite specimens are also more than just 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 extent that some call it the “trilobite economy.”
“We want to protect the place where the discovery was made to make it available to science,” he said.