Hundreds of millions of years ago, trilobites could be found all over the Earth. Covered in hard exoskeletons, the animals left behind countless fossils for paleontologists to study today. Despite all those preserved shells, scientists have been unable to understand some aspects of trilobite anatomy after centuries of study, particularly 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 segmented marines. In a paper published Thursday in the journal Science, researchers describe a cluster of trilobites that became solidified 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 excavation that resulted in the discovery of 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 from erupting volcanoes. One of those eruptions left a cream-colored layer of fine-grained volcanic ash in which trilobites fossilized.
When the researchers opened up the volcanic rock, they found incredibly detailed trilobite impressions carved into the stone. “Volcanic ash is so fine, like talcum powder, 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 new co-authors. study.
Dr. El Albani and his team posit that a brief and sudden burst of volcanic activity buried the trilobites when ash debris flooded the marine environment. The digestive tract of a drowned trilobite is even filled with sediment it may have ingested before death. As 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 of the trilobites are curled up in a ball, while others look like they’re about to slide. One specimen is even covered in tiny bivalves, which they mounted on the animal’s shell using fleshy stalks.
“These brachiopods are still in their vital position, which shows how quickly the burial took place,” 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 3-D images of the specimens. This allowed them to look at delicate structures such as antennae, the digestive tract and even the bristle-like hairs on the legs of the walking trilobites.
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 the labrum that attached to the hard part of the trilobite’s mouth and is now a feature common 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 hypothesized 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 observed in the new specimens are most likely not shared by all trilobites in the same form. For example, some eyed species of insects in the genus Carolinites “would have had to drag their eyes through the mud with their feet,” he said.
But the intricate structures preserved in these “breathtaking” specimens will help place trilobites within the arthropod family tree, he says.
“It gets into the details of anatomy, but such debates are important when we want to understand which group of living arthropods is most closely related to the extinct trilobites,” he said.
For 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 palaeontological 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 to science,” he said.