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What were the earliest ancestors of scorpions, spiders and horseshoe crabs? A PhD student from the University of Lausanne (Switzerland), with the support of a CNRS researcher, has identified a fossil that fills the gap between modern species and those from the Cambrian period (505 million years ago), solving a long paleontological mystery.

Modern scorpions, spiders, and horseshoe crabs belong to a vast lineage of arthropods that appeared on Earth nearly 540 million years ago. More precisely, they belong to a subtype that includes organisms equipped with pincers, used primarily for biting, capturing prey, or injecting venom – the chelicerae, hence their name chelicerates. But what are the ancestors of this very specific group?

This question has puzzled paleontologists since the beginning of the study of ancient fossils. It was impossible to identify with certainty any forms among early arthropods that shared enough similarities with modern species to be considered ancestral. The mystery was further complicated by the lack of available fossils for the key period between -505 and -430 million years, which would have facilitated genealogical research.

Lorenzo Lustri, then Doctor of Philosophy. student at the Faculty of Geosciences and Environment of the University of Lausanne (UNIL), provided the missing piece of the puzzle. Together with his supervisors, he studied a hundred fossils dating back to 478 million years ago from the Fezouata Shale in Morocco and identified the candidate that links modern organisms to those of the Cambrian (505 million years ago). The study was published in Nature Communications.

Fossils from the Fezouata Shale were discovered in the early 2000s and have been subjected to extensive analysis. However, the fossil illustrated in the publication, one of the most abundant at the site, has never been described before. Measuring between 5 and 10 millimeters, it is named Setapedites abundantis. This animal makes it possible for the first time to trace the entire chelicerate lineage, from the appearance of the earliest arthropods to modern spiders, scorpions and horseshoes.

“Originally we intended only to describe and name this fossil. We had absolutely no idea that it would be hiding so many secrets,” confides Lustri, the first author of the paper who defended his Ph.D. in March 2023. “It was therefore an exciting surprise to realize, after careful observation and analysis, that it also fills an important gap in the evolutionary tree of life.”

Still, the fossil has not yet revealed all its secrets. Indeed, some of its anatomical features allow a deeper understanding of the early evolution of the chelicerate group and perhaps even link to this group other fossil forms whose affinities remain highly debated.

A temporary exhibition of the biota of Fezouata, in collaboration with UNIL, will soon be held at the Palais de Rumine in Lausanne, Switzerland.

To obtain these results, the scientists examined a hundred fossils and used an X-ray scanner to reconstruct their anatomy in detail and in 3D. They were then able to make comparisons with numerous fossil chelicerates from other locations, as well as their more ancient relatives.

Finally, the significance of the Fezouata fossils is made clear using phylogenetic analyses, which mathematically reconstruct the family tree of different species based on the “coding” of all their anatomical features.