LVolcanoes located at the boundary of tectonic plates are known for their explosive and large-scale eruptions, which can generate several tens of km3 of material. 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 (Abderrazak El Albani) just published a article featured on the cover of the leading American magazine Sciencedescribing the discovery of two new species of trilobites. They are the best-preserved trilobite fossils ever discovered.
They present previously unseen anatomical details despite the millions of trilobites collected and studied over the past two centuries. These fossil arthropods found petrified in their last posture are representatives of an ecosystem 515 million years old. A marine “Pompeii”, discovered in levels of volcanic ash, at Aït Youb, in the Souss-Massa region of Morocco.
With more than 22,000 species discovered, trilobites undoubtedly represent 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 their 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. The ones we have discovered measure about 2 centimeters. Today their closest morphological “descendants” are the 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 ashes: the molds of the organisms. These preserve the smallest details of the trilobites’ outer surface, including the hairs and spines along the appendages. Their digestive tract was also preserved after being filled with ashes. Even small shells (brachiopods) attached to their exoskeleton by a peduncle have been frozen in a living 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 notably revealed in the smallest details a grouping of pairs of specialized legs around the mouth, allowing us to get a more precise idea of the way in which they fed. They also reveal, for the first time for these fossils, the presence of a labrum, a fleshy lobe serving 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 solely of volcanic materials) should become new targets for study given their exceptional potential to trap and preserve biological remains, even soft ones, without generating degradation. New windows should thus open onto the past of our planet.
To illustrate the impact of our discovery, Greg Edgecombe, curator at the National Museum of Natural History in London, specialist in arthropods 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 trilobite soft anatomy, but it’s the 3D preservation here that’s truly astounding.”
