A graduate student at the University of Lausanne in Switzerland, supported by a CNRS researcher, has discovered a fossil that bridges the evolutionary gap between modern species such as scorpions, spiders and horseshoes and their ancient cousins of about 505 million people from the Cambrian period. Years ago. This discovery solves an old mystery of paleontology.
Modern scorpions, spiders, and horseshoe crabs belong to a large arthropod lineage that appeared on Earth almost 540 million years ago. More precisely, they belong to a subphylum—chelicerae—that includes organisms equipped with pincers used primarily to bite, capture prey, or inject venom, hence the name chelicerae. So what are the ancestors of this very special group?
This question has puzzled paleontologists since the study of ancient fossils began. Among the earliest arthropods, it has not been possible to identify with certainty any species that bears enough ancestral resemblance to modern species. The mystery is further complicated by the lack of fossils from the key period between -505 and -430 million years ago; This would make genealogical research easier.
Lorenzo Lustri, a master’s student at the Faculty of Earth and Environmental Sciences at the University of Lausanne (UNIL), provided the missing piece of the puzzle. With his advisors, he studied a hundred fossils found in the Fezouata Shale in Morocco, dated to 478 million years ago, and identified a candidate linking modern organisms to the Cambrian period (505 million years ago). The study was published on: Nature Communication .
Breakthrough in evolutionary biology
Fossils from the Fezuat Shale were discovered in the early 2000s and analyzed extensively. However, the fossil seen in the publication, which is one of the most commonly seen fossils in the bed, had not been described before. Its size between 5 and 10 millimeters takes its name from Setapedites abundance . This animal makes it possible for the first time to trace the entire lineage of chelicerates, from the emergence of the earliest arthropods to modern spiders, scorpions and horseshoe crabs.
“Initially, we only aimed to describe and name this fossil. “We had no idea that there would be so many secrets in it,” admits Lorenzo Lustri, the first author of the paper, who defended his doctoral thesis in March 2023. “It filled an important gap in the evolutionary tree of life.”
However, the fossil has not yet revealed all its secrets. In fact, some anatomical features allow a more in-depth understanding of the early evolution of the chelicerate group and perhaps even link other fossil forms to this group, whose affinities are controversial.
A temporary exhibition of the Fezouata biota in collaboration with UNIL will soon be held at the Palais de Rumine in Lausanne, Switzerland.
To obtain these results, scientists examined nearly a hundred fossils and reproduced their anatomy in detail and in 3D with the help of an X-ray scanner. They were then able to make comparisons with numerous fossil chelicerates and their older relatives from elsewhere. Finally, the significance of the Fezouata fossils was clarified thanks to phylogenetic analysis, which mathematically reconstructed the family tree of different species based on the “coding” of all their anatomical features.
Source: Port Altele
