Discovery of a 515 million year old marine “Pompeii” in Morocco

Volcanoes located at the boundaries of tectonic plates are known for their explosive and large-scale eruptions, which can generate several tens of km³ of matter. These eruptions can trap present life almost instantly, thus preserving under their ashes the evidence of entire civilizations, as in Pompeii, buried under the ashes of Vesuvius.

An international team of researchers, which I coordinated, has just published an article presented on the cover of the leading American journal Science, describing the discovery of two new species of trilobites. These are the best preserved trilobite fossils ever discovered.

They present unprecedented anatomical details despite the millions of trilobites collected and studied over the last two centuries. These fossil arthropods found petrified in their last posture are the representatives of a 515 million year old ecosystem (Ma), a marine “Pompeii”, discovered in layers of volcanic ash, in Aït Youb, in the Souss-Massa region of Morocco. This work is crowned by the cover of Science magazine.

With over 22,000 species discovered, trilobites are probably the best-known fossil invertebrates. While their calcite exoskeleton gives them a high potential for fossilization (which explains their large number), their non-mineralized appendages and internal organs are only known through a limited number of specimens. Trilobites have been extinct since the end of the Paleozoic (539 to 252 million years ago). They are arthropods whose size varies from one to a few centimeters. They lived exclusively in the marine environment. Those we have discovered measure about 2 centimeters. Today, their closest morphological “descendants” are horseshoe crabs. They are also marine arthropods, but they are distant cousins.

Trilobite molds

In Aït Youb, during a volcanic eruption, living organisms were buried by fiery clouds. The biological tissues were then consumed by the intense heat, leaving only cavities in the solidified ash: the molds of the organisms. The latter preserve the smallest details of the trilobites’ outer surface, including the hairs and spines along the appendages. Their digestive tract was also preserved after filling with ash. Even small shells (brachiopods) attached to their exoskeleton by a peduncle were frozen in life position.

Using an imaging technique, X-ray microtomography, we were able to study the fossils in 3D without extracting them from their matrix. This technique is based on the property of X-rays to pass through matter and be absorbed depending on the nature and density of the constituents they encounter. By digitally filling their mold, the missing bodies were reconstructed with a striking level of detail.

This work, carried out by Arnaud Mazurier, Research Engineer at the University of Poitiers, sheds new light on the anatomical organization of trilobites. The results have notably revealed in minute detail a grouping of pairs of specialized legs around the mouth, allowing us to get a more precise idea of ​​how they fed. They also reveal, for the first time for these fossils, the presence of a labrum, a fleshy lobe that acts as an upper lip in current arthropods.

Optimal conservation thanks to volcanic ash

This discovery demonstrates the essential role of volcanic ash deposits for the preservation of fossils and the critical importance of exploring underwater volcanic environments.

It also demonstrates that X-ray microtomography is a powerful tool for observing fossilized objects in very hard rocks in 3D, without the risk of altering them. Thus, pyroclastic deposits (rocks composed mainly or only of volcanic materials) should become new study targets given their exceptional potential to trap and preserve biological remains, even soft ones, without generating degradation. New windows should thus open onto our planet’s past.

To illustrate the impact of our discovery, Greg Edgecombe, curator at the National Museum of Natural History in London, arthropod specialist and co-author of the study said:

“I’ve been studying trilobites for almost 40 years, but I’ve never felt like I was looking at living animals like I did with these. I’ve seen a lot of soft anatomy of trilobites, but it’s the 3D preservation here that’s truly stunning.”