In the early 2000s, a local fossil collector named Mohamed ‘Ou Said’ Ben Moula discovered a treasure trove of fossils at Fezouata Shale in Morocco. Among these fossils, a team of researchers from the University of Lausanne (UNIL) recently identified a significant specimen as the earliest ancestor of modern-day chelicerates, which include spiders, scorpions, and horseshoe crabs.
The fossil, known as Setapedites abundantis, dates back 478 million years ago when it lived in a 100–200-meter-deep ocean near the South Pole. This tiny creature, measuring 5 to 10 millimeters in length, scavenged on organic matter in the seafloor sediments. While fossils of S. abundantis were mentioned in a 2010 paper, this recent study by Pierre Gueriau and his team is the first to provide a detailed analysis of this ancient creature and its connection to modern chelicerates.
By using X-ray scanners to reconstruct the anatomy of 100 fossils from Fezouata Shale in 3D, the researchers found striking similarities between S. abundantis and various ancient and modern arthropods. The arrangement of head appendages and body organization in S. abundantis resembled those of horseshoe crabs, scorpions, and spiders. This discovery sheds light on the early evolution of chelicerates and fills the gap in the arthropod tree of life between 503 to 430 million years ago.
While previous Cambrian-era arthropods like Mollisonia plenovenatrix and Habelia optata are considered early ancestors of chelicerates, S. abundantis represents the earliest branching lineage within chelicerates. This distinction places S. abundantis as a pivotal link in understanding the origin story of arthropods and resolving the relationships between various fossil forms.
Despite the challenges posed by the small size of the fossils, the abundance of S. abundantis specimens allowed researchers to make significant observations. Anatomical features such as a ventral protrusion at the rear of the organism provide valuable insights into the early evolution of chelicerates and their relationships with other primitive arthropods. Further research into these features and their phylogenetic implications will deepen our understanding of the arthropod tree of life.
The study by Gueriau and his team, published in Nature Communications, marks a crucial milestone in paleontological research. It not only uncovers the earliest ancestor of modern chelicerates but also highlights the importance of continued exploration and analysis of fossil specimens to unravel the mysteries of ancient life forms.
