A PhD student at the University of Lausanne in Switzerland, with the support of researchers at the French National Center for Scientific Research (CNRS), has discovered a fossil that narrows the evolutionary gap between contemporary species such as scorpions, spiders and horseshoe crabs and ancient species from the Cambrian period about 505 million years ago. This discovery unravels a long-standing mystery in paleontology.
Scientists have discovered a key fossil that links modern chelicerads such as scorpions and spiders to their ancient Cambrian ancestors, clarifying a major gap in the evolutionary history of these species.
Modern scorpions, spiders, and horseshoe crabs belong to the massive phylum Arthropods, which appeared on Earth nearly 540 million years ago. More precisely, they belong to a subphylum that includes creatures equipped with chelera, which are mainly used to bite, catch prey, or inject venom, hence the name chelicera. But what is the ancestor of this very particular taxon?
This question has puzzled paleontologists since the beginning of studying paleontological fossils. We are unable to determine whether there were any morphologies in early arthropods that were sufficiently similar to modern species to be considered their ancestors. The mystery was compounded by the lack of fossils during a critical period between -505 million and -430 million years ago, which could have facilitated genealogical investigations.
Lorenzo Lustri, then a PhD student at the Faculty of Earth Sciences and Environment at the University of Lausanne (UNIL), provided the missing piece of the puzzle. Together with his mentor, he studied hundreds of fossils dating back to 478 million years ago in the Fezu Ata Shale in Morocco and identified candidates to link modern organisms with those of the Cambrian (505 million years ago) period. The study was published in Nature Communications.
Lorenzo Lustri inspects the collection of the University of Lausanne. Image courtesy of UNIL
The fossils in the Fezu Atta Shale were discovered in the early 2000s and have undergone extensive analysis. However, the fossils presented in this publication are among the richest in the deposit and have never been described before. It is between 5 and 10 mm in size and is named "Setapedites abundantis". For the first time, this animal made it possible to trace the entire lineage of cheleropods, from the earliest appearance of arthropods to modern spiders, scorpions, and horseshoe crabs.
A fossil fills in the gap between scorpions, spiders, and horseshoe crabs and Cambrian (505 million years ago) species. Source: UNIL
"Originally, we only intended to describe and name the fossil. We had no idea that it would hold so many secrets," admits Lorenzo Lustri, first author of the paper, who will defend his PhD in March 2023. "So, after careful observation and analysis, we were pleasantly surprised to find that it also fills an important gap in the evolutionary tree of life."
However, this fossil has not yet revealed all its secrets. In fact, some of its anatomical features could give us a deeper understanding of the early evolution of chelicerapods, and may even link other fossil forms that are still highly related to this class.
In collaboration with the UNESCO Institute for Lifelong Learning (UNIL), a temporary exhibition of the Fezzuata biota will soon be held at the Palais de Rumine in Lausanne, Switzerland.
The Fizuata landform where fossils were found. Image courtesy of UNIL
To achieve these results, scientists studied hundreds of fossils and used X-ray scanners to perform detailed 3D reconstructions of their anatomy. Subsequently, they compared these fossils to a large number of chelicerapod fossils and their more ancient close relatives from other sites. Finally, with the help of phylogenetic analysis, the importance of the Fizuata fossil became apparent. Phylogenetic analysis is the mathematical reconstruction of the family tree of different species based on the "coding" of all their anatomical features.
编译来源：ScitechDaily