Euchelicerata is a large group of arthropods comprising horseshoe crabs, scorpions, spiders, mites and ticks, as well as the extinct sea scorpions and chasmataspidids. Setapedites abundantis — a new euchelicerate species that lived in what is now Morocco during the Early Ordovician epoch, 478 million years ago — fills the gap between modern species and those from the Cambrian period.
“Modern scorpions, spiders and horseshoe crabs belong to the vast lineage of arthropods, which appeared on Earth nearly 540 million years ago,” said University of Lausanne paleontologist Lorenzo Lustri and colleagues.
“More precisely, they belong to a subphylum that includes organisms equipped with pincers used notably for biting, grasping prey, or injecting venom — the chelicerae, hence their name chelicerates (Euchelicerata+Pycnogonida). But what are the ancestors of this very specific group?”
“This question has puzzled paleontologists ever since the study of ancient fossils began.”
“It was impossible to identify with certainty any forms among early arthropods that shared enough similarities with modern species to be considered ancestors.”
“The mystery was further compounded by the lack of fossils available for the key period between 505 and 430 million years ago, which would have facilitated genealogical investigation.”
Setapedites abundantis. Image credit: Lustri et al., doi: 10.1038/s41467-024-48013-w.
Dr. Lustri and co-authors euchelicerate fossils from the 478-million-year-old Fezouata Shale of Morocco and identified a new species that links modern euchelicerates to those from the Cambrian period (505 million years ago).
Scientifically named Setapedites abundantis, the species was between 0.5 and 1 cm in length.
“This animal makes it possible, for the first time, to trace the entire lineage of euchelicerates, from the appearance of the earliest arthropods to modern spiders, scorpions and horseshoe crabs,” Dr. Lustri said.
“Initially, we only intended to describe and name this fossil.”
“We had absolutely no idea that it would hold so many secrets.”
“It was therefore an exhilarating surprise to realize, after careful observations and analysis, that it also filled an important gap in the evolutionary tree of life.”
“Still, the fossil has yet to reveal all its secrets,” he added.
“In fact, some of its anatomical features allow for a deeper understanding of the early evolution of the euchelicerate group, and perhaps even link to this group other fossil forms whose affinities remain highly debated.”
The results appear in the journal Nature Communications.
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L. Lustri et al. 2024. Lower Ordovician synziphosurine reveals early euchelicerate diversity and evolution. Nat Commun 15, 3808; doi: 10.1038/s41467-024-48013-w
