Long-standing paleontological mystery solved – revolutionary fossil discovery in Morocco reveals origin of spiders and scorpions

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Setapedites is an abundant fossil

Scientists have discovered an important fossil that links modern chelicerates such as scorpions and spiders to their ancient Cambrian ancestors, clarifying a significant gap in the evolutionary history of these species. One of the Setapedites abundant fossils that have been used to trace the origins of spiders, scorpions and horseshoe crabs. Credit: UNIL

A PhD student at the University of Lausanne in Switzerland, with the support of a CNRS researcher, has discovered a fossil that bridges the evolutionary gap between modern-day kind such as scorpions, spiders and horseshoe crabs, and their ancient counterparts from the Cambrian period, about 505 million years ago. This finding solves a long-standing mystery in paleontology.

Modern scorpions, spiders and horseshoe crabs belong to the extensive lineage of arthropods that appeared on Earth almost 540 million years ago. To be precise, they belong to a subphylum that includes organisms equipped with pincers used mainly for biting, grabbing prey or injecting venom – the chelicerae, hence their name chelicerates. But what are the ancestors of this very specific group?

Reconstruction of Setapedites is abundant

Reconstruction of Setapedites abundant. Credit: Elissa Sorojsrisom

This question has puzzled paleontologists since the beginning of the study of ancient fossils. It was impossible to identify with certainty among early arthropods any form that was sufficiently similar to modern species to be considered ancestors. The mystery was compounded by the lack of fossils available for the key period between -505 and -430 million years ago, which would have facilitated genealogical research.

Lorenzo Lustri, then a PhD candidate 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 478 million years from the Fezouata Shale of Morocco and identified the candidate that links modern organisms to those of the Cambrian (505 million years ago). The research was published in Nature communication.

Lorenzo Lustri

Lorenzo Lustri looks at the collections of the University of Lausanne. Credit: UNIL

A breakthrough in evolutionary biology

Fossils from the Fezouata Shale were discovered in the early 2000s and have undergone extensive analysis. However, the fossil illustrated in the paper, one of the most common in the deposit, had never been described before. It is between 5 and 10 millimeters in size and has been given a name Setapedites abundant. This animal makes it possible for the first time to trace the entire lineage of chelicerates, from the appearance of the earliest arthropods to modern spiders, scorpions and horseshoe crabs.

Scorpions, spiders and horseshoe crabs Fossil images

A fossil fills the gap between scorpions, spiders and horseshoe crabs, and species from the Cambrian (505 million years ago). Credit: UNIL

“Initially we just wanted to describe and name this fossil. We had absolutely no idea that it would contain so many secrets,” says Lorenzo Lustri, the first author of the article, who defended his PhD in March 2023. “It was therefore an exciting surprise to realize, after careful observations and analysis, that it also filled an important gap in the evolutionary tree of life.”

Yet the fossil has not yet revealed all its secrets. Some of its anatomical features allow a deeper understanding of the early evolution of the chelicerate group, perhaps even linking other fossil forms to this group, whose affinities are still highly debated.

A temporary exhibition on the Fezouata biota will soon take place at the Palais de Rumine in Lausanne, Switzerland, in collaboration with UNIL.

Fezouata landscape

Fezouata landscape, where the fossils were found. Credit: UNIL

Method

To obtain these results, the scientists studied 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 sites, as well as with their older relatives. Finally, the importance of the Fezouata fossil became clear with the help of phylogenetic analyses, which mathematically reconstruct the family tree of several species based on the ‘coding’ of all their anatomical features.

Reference: “Lower Ordovician synziphosurine reveals early euchelicerate diversity and evolution” by Lorenzo Lustri, Pierre Gueriau and Allison C. Daley, May 7, 2024, Nature communication.
DOI: 10.1038/s41467-024-48013-w

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