In the early 2000s, local fossil collector Mohamed “Ou Said” Ben Moula discovered numerous fossils at the Fezouata Shale, a site in Morocco known for its well-preserved fossils from the Early Ordovician, about 480 million years ago. Recently, a team of researchers from the University of Lausanne (UNIL) studied 100 of these fossils and identified one as the earliest ancestor of modern chelicerata, a group that also includes spiders, scorpions and horseshoe crabs.
The fossil preserves the species Setapedites abundantisa small animal that crawled and swam on the bottom of a 100-200 meter deep ocean near the South Pole 478 million years ago. It was 5 to 10 millimeters long and fed on organic material in the sediments of the sea floor. “Fossils of what is now known as S. abundant have been found early on – a specimen mentioned in the 2010 paper recognizing the importance of this biota. However, this creature has not been studied in detail before, simply because scientists focused on other taxa first,” Pierre Gueriau, one of the researchers and a junior lecturer at UNIL, told Ars Technica.
The study by Gueriau and his team is the first to describe this S. abundant and the connection with modern chelicerates (also called euchelicerates). It is of great importance, because “the origin of chelicerates has been one of the most complicated knots in the arthropod tree of life, since there was a lack of fossils between 503 and 430 million years ago”, Gueriau added.
An ancestor of spiders
The study authors used X-ray scanners to reconstruct the anatomy of 100 fossils from the Fezouata Shale in 3D. When they compared the anatomical features of these ancient animals with those of chelicerates, they noticed several similarities between S. abundant and several ancient and modern arthropods, including horseshoe crabs, scorpions, and spiders.
For example, the nature and arrangement of the head appendages or 'legs' in S. abundant were homologous to those of modern horseshoe crabs and Cambrian arthropods that lived between 540 and 480 million years ago. In addition, like spiders and scorpions, the organism exhibited body tagmosis, in which the body is organized into distinct functional sections.
“Setapedites abundantis contributes to our understanding of the origin and early evolution of two key euchelicerate traits: the transition from biramous to uniramous prosomal appendages and body agglomeration,” the study authors note.
Currently there are two arthropods from the Cambrian period, Mollisonia plovenenatrix And Habelia optata are generally considered to be the earliest ancestors of chelicerata (not all scientists accept this idea). Both lived about 500 million years ago. When we asked how these two differ from S. abundantGueriau replied, “Habelia and Mollisonia represent at best early branching lineages in the phylogenetic tree. While S. abundant appears, along with a few other fossils, to represent the earliest branching lineage inside chelicerates.”
This means that Habelia and Mollisonia are relatives of the ancestors of modern chelicerates. On the other hand, S. abundant represents the first group to split after the chelicerate clade was established, making it the earliest member of the lineage. “These findings bring us closer to unraveling the origin story of arthropods, because they allow us to close the anatomical gap between Cambrian arthropods and early-branching chelicerates,” Gueriau told Ars Technica.
S. abundant connects other fossils
The researchers faced many challenges during their study. For example, the small size of the fossils made observations and interpretations difficult. They overcame this limitation by examining a large number of specimens, fortunately S. abundant fossils were abundant in the samples they studied. However, these fossils have not yet yielded all their secrets.
“Some of S. abundant“Anatomical features allow a deeper understanding of the early evolution of the chelicerate group and may even link other fossil forms, whose relationships are still highly debated, to this group,” Gueriau said. For example, the study authors noted a ventral bulge at the back of the organism. Such a feature is observed for the first time in chelicerates, but is known in other primitive arthropods.
“This feature could thus bring together many other chelicerate fossils and further resolve the early branches of the arthropod tree. So the next step for this research is to investigate this feature more deeply across a broad range of fossils and its phylogenetic implications,” Gueriau added.
Nature Communications, 2023. DOI: 10.1038/s41467-024-48013-w (About DOIs)
Rupendra Brahambhatt is a seasoned journalist and filmmaker. He covers science and culture news and over the last five years has been actively working with some of the most innovative news agencies, magazines and media brands operating in different parts of the world.
