In the early 2000s, local fossil collector Mohamed “Ou Said” Ben Moura discovered numerous fossils in Morocco’s Fezwata Shale, a site known for well-preserved fossils from the Early Ordovician period, about 480 million years ago. Recently, a team of researchers from the University of Lausanne (UNIL) examined 100 of these fossils and identified one as the oldest ancestor of modern chelicerates, which include spiders, scorpions, and horseshoe crabs.
Fossils preserve species Setapedites avantis478 million years ago, a small animal crawled near the bottom of the ocean at a depth of 100 to 200 meters near the Antarctic. It was 5 to 10 millimeters long and fed on organic matter in the seafloor sediments. S. avantis “There were early discoveries, including specimens mentioned in a 2010 paper that recognized the importance of this group of organisms, but the organism was not studied in detail until now because scientists first focused on other taxa,” one of the researchers, Pierre Guerriau, an associate lecturer at UNIL, told Ars Technica.
The research by Gerriau and his team S. avantis And it also revealed a connection to modern chelicerates (also called euchericerates), which is significant: “The origin of the chelicerates is one of the most complicated knots in the arthropod family tree, with very few fossils found between 503 and 430 million years ago,” Geriau added.
Ancestors of spiders
The researchers used an X-ray scanner to create a 3D reconstruction of the anatomy of 100 fossils from the Feswata Shale. They compared the anatomical features of these ancient animals with those of chelicerates and noticed several similarities. S. avantis It is also home to a variety of ancient and modern arthropods, including horseshoe crabs, scorpions and spiders.
For example, the nature and arrangement of the head appendages and “legs” S. avantis It was similar to the present Horseshoe crab It is very similar to Cambrian arthropods that lived between 540 and 480 million years ago, and like spiders and scorpions, the organism exhibits somatotagmosis, a body structured into different functional sections.
“Setapedites avantis “This study contributes to our understanding of two important euptian features: the transition from bibranched to unibranched prosomal appendages and the origin and early evolution of body tagmosis,” the study authors said. Note.
Currently, there are two Cambrian Arthropods, Morrisonia prenovenatrix and Javeria optata They are generally considered to be the oldest ancestors of chelicerates (though not all scientists accept this idea). Both lived about 500 million years ago. When asked how they are different from other animals, S. avantisGeriau responded: “Haveria and Morrisonia simply represent early branching lineages in the phylogenetic tree. S. avantis Along with several other fossils, it was discovered to represent one of the oldest branching lineages. At the inner “Arachnids”
This means that Habelia and Morrisonia are ancestral relatives of modern chelicerates. S. avantis They represent the first group to branch off after the chelicerate lineage was established, making them the earliest members of this lineage. “These discoveries allow us to bridge the anatomical gap between Cambrian arthropods and early-diverging chelicerates, bringing us closer to unravelling the arthropod origin story,” Gelliau told Ars Technica.
S. avantis Connecting to other fossils
The researchers faced many challenges during their study. For example, the fossils were small, which made them difficult to observe and interpret. The researchers overcame this limitation by examining a large number of specimens. Fortunately, S. avantis The samples they studied were rich in fossils, but these fossils have yet to reveal all their secrets.
“Several S. avantis“The anatomical features allow us to better understand the early evolution of chelicerates and may also link other fossil forms to this group whose relationships are still hotly debated,” Gelliau said. For example, the study authors noticed a ventral protrusion on the rear of the creature, a feature that has never been observed in a chelicerate before. Other primitive arthropods.
“This feature connects many other fossils, including chelicerates, and may further shed light on earlier branches of the arthropod family tree. The next step in this study is therefore to investigate this feature and its phylogenetic implications in a wider range of fossils in more depth,” Gelliau added.
Nature Communications, 2023. DOI: 10.1038/s41467-024-48013-w (About DOIs)
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