Horse -iron crabs are an old line with an evolutionary history which dates back 450 million years (ordovician period) and are generally considered as examples of “living fossils”. A paleontologist from the University of Virginia-Western has described a new genre and species of real horseshoe crab of a specimen of age of the naked silurian in Indiana, in the United States. The species fills the gap of 80 million years in the fossil file of horseshoe crab and has a global morphology similar to that of the ordovician species.
Ciurcalilulus discobobolusHolotype. Scale bars – 5 mm. Image credit: James C. Lamsdell, DOI: 10.1098 / RSPB.2025.0874.
“Horseshoe crabs (Order Xiphosura) are chélépéré aquatic arthropods defined by the merger of their bodily segments in a thoracetron,” wrote Dr. James Lamsdell of the University of Virginia-Western in his new article.
“Four living species are known and present a disjunctive geographic distribution, with a species occurring in the Western Atlantic (ranging from the east coast of Canada to the Gulf of Mexico) and three in the Western Pacific and the northeast of the Indian oceans (extending from southern Japan to the east coast of India).”.
“The group is famous as an example of an evolving conservative line and is considered to be comprising” archetypal live fossils “, although more recent work has demonstrated repeated ecological transitions within the group to be associated with the development of extreme morphologies.”
“Horseshoe crabs have a long scalable history that dates back to the late Ordovician (450 million years ago) with two species described from North America and another, slightly older (early ordovician, 480 million years ago) reported an official description of Morocco.”
“The origins and the early evolution of horseshoe crabs are however poorly known, with a gap of 80 million years between these ordovician species and the first registration of Xiphosurida (horseshoe crabs that reduced their postabdomen to a single segment) at the end of the Devonian (370 million years ago).”
“This lack of fossil assessment for horseshoe crabs in the Silurian, at a time when other aquatic chéulent groups diversified quickly, which makes it difficult to determinate the origin of the XIPHOSURIDS and to what extent the evolution of the mass of the Ordovician and the Silurian influenced the crab crab.”
The newly identified species of horseshoe crab lived during the Silurian, about 424 million years ago.
Appointed Ciurcalilulus discobobolusThe animal is known in a single specimen found in 1975 by Samuel J. Ciurca, JR in the Kokomo member of the Wabash formation in Indiana.
“The member of Kokomo includes up to 30 m of finely laminated dark dolostones and is considered an age of the Silurian based on Conodont data,” wrote the paleontologist.
“Kokomo’s localities are mainly known for their endemic fauna Euryptrid, which occur in a single horizon and are recognized to represent a mass extinction event, although a variety of algae co-ocaling with Euryptry and Brachiopods are alongside the corals near the Summit of the Member in what is sometimes referred as the horizon of Brachiopod.” ».
“”Ciurcalilulus discobobolus is derived from the Euryptrid horizon and is preserved in a similar way to Euryptries, which are compression fossils with a charred cuticle. »»
Ciurcalilulus discobobolus Distinguished other early horseshoe crabs through its unique combination of unknown characteristics among other species.
“”Ciracalimulus The most closely looks like the ordovician Lunatavis Species in possession of a strongly rounded prosomal shell and a semi-circular thoracetron devoid of expression limit of the lateral segment or tergopleural projections as well as a postabdomine with multisegmentation, “wrote the researcher.
“However, the new genre is distinguished from Lunatavis In the lack of axial nodes on the Thoracetron and the absence of a marginal edge Thoracetron defined dorsally by a furrow, which are both present in the species of the Ordovician. »»
“The age of the Silurian of Ciracalimulus shows that Xiphosura retaining the general morphology observed among the ordovician species persisted beyond the mass extinction of the end ordovician, suggesting that the event had a limited impact on the evolution of the horseshoe crab. »»
“During their scalable history, horseshoe crabs have reached global distribution,” he added.
“However, the first horseshoe crabs are known to the paleocontinents of Laurusse and Siberia, while the oldest horseshoe crabs are found in Laurentia.”
“The additional discovery of Ciracalimulus suggests that Laurentia was an important site of the early evolution of horsesome crab, although it must be recognized that there is a strong historic bias in paleontological research towards localities in Europe and former European colonies. »»
“As such, Laurentia is likely to be more heavily sampled than other paleocontinents such as Gondwana, an important consideration since the oldest known horseshoe crab is an undecreemed species in Morocco.”
The document was published on June 18 in the Acts of the Royal Society B.
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James C. Lamsdell. 2025. The Silurian’s first horseshoe crab reveals details of the Plan on the ground Xiphosuran. Proc. R. Soc. B 292 (2049): 20250874; DOI: 10.1098 / RSPB.2025.0874
