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Fossil reveals the first ancestors of scorpions and spiders

Researchers at the University of Lausanne have solved a long-standing paleontological mystery. The team has recently discovered a fossil that bridges the gap between modern scorpions, spiders, horseshoe crabs, and their Cambrian ancestors. 

Mystery of the arthropod lineage 

Scorpions, spiders, and horseshoe crabs belong to the arthropod lineage, which appeared nearly 540 million years ago. They are part of a subphylum called chelicerates, characterized by pincers known as chelicerae, used for biting, grasping prey, or injecting venom. Identifying the ancestors of this specific group has long puzzled paleontologists.

Early arthropod fossils did not show enough similarities with modern species to be considered ancestors, and a lack of fossils from the key period between 505 and 430 million years ago compounded the mystery.

Fossils from Morocco 

Lorenzo Lustri, a PhD student at the University of Lausanne’s Faculty of Geosciences and Environment, provided the missing piece of the puzzle. 

Lustri and his supervisors studied a hundred fossils from the Fezouata Shale of Morocco, dating back 478 million years, and identified a fossil that links modern organisms to those of the Cambrian period (505 million years ago). 

Discovered in the early 2000s, fossils from the Fezouata Shale have undergone extensive analysis. However, the fossil illustrated in the recent study, Setapedites abundantis, had never been described before. 

Measuring between 5 and 10 millimeters, this fossil allows scientists to trace the lineage of chelicerates from early arthropods to modern species like spiders, scorpions, and horseshoe crabs.

Analyzing the anatomy of fossil chelicerates

To achieve these results, the scientists used an X-ray scanner to reconstruct the anatomy of the fossils in detail and in 3D. They compared these findings with numerous fossil chelicerates from other sites and their more ancient relatives. 

The significance of the Fezouata fossil was clarified through phylogenetic analyses, which mathematically reconstruct the family tree of different species based on the “coding” of all their anatomical traits.

Surprising discovery 

“Initially, we only intended to describe and name this fossil. We had absolutely no idea that it would hold so many secrets,” said Lustri, who defended his PhD in March 2023. “It was therefore an exhilarating surprise to realize, after careful observations and analysis, that it also filled an important gap in the evolutionary tree of life.”

The fossil has yet to reveal all its secrets. Some anatomical features provide insights into the early evolution of chelicerates and may link this group to other debated fossil forms.

A temporary exhibition on the Fezouata biota will soon be held at the Palais de Rumine in Lausanne, Switzerland.

Fossils of scorpions and spiders 

Fossils of scorpions and spiders provide intriguing insights into the ancient history of these arachnids, tracing their existence back hundreds of millions of years. 


Scorpions are among the oldest known terrestrial animals, with their earliest fossils dating back to the Silurian period, over 430 million years ago. These ancient scorpions were similar in form to modern ones, though some were aquatic or semi-aquatic, showing adaptations like gills.


Spider fossils, while not as ancient as those of scorpions, are still quite old, with the oldest known spider fossils dating back to the Carboniferous period, around 300 million years ago. These early spiders already displayed key features such as spinnerets, which are used to produce silk. 

The preservation of spiders in the fossil record is often linked to amber deposits, where fine details of their anatomy, including their silk-producing structures, can be exquisitely preserved.

Evolutionary stability 

Both scorpions and spiders have shown remarkable evolutionary stability. Their basic body plans and behaviors have changed little over millions of years, indicating a successful adaptation to their ecological niches. 

The study of spider and scorpion fossils not only helps in understanding their evolution and distribution over time but also provides insights into the ancient environments they inhabited.

The study is published in the journal Nature Communications.

Image Credit: UNIL


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