Horseshoe crabs are an historic lineage with an evolutionary historical past stretching again 450 million years (Ordovician interval) and are usually thought of to be examples of ‘residing fossils.’ A paleontologist from West Virginia College has described a brand new genus and species of true horseshoe crab from a Silurian-age specimen unearthed in Indiana, the USA. The species bridges the 80-million-year hole within the horseshoe crab fossil report and has an general morphology just like that of the Ordovician species.
Ciurcalimulus discobolus, holotype. Scale bars – 5 mm. Picture credit score: James C. Lamsdell, doi: 10.1098/rspb.2025.0874.
“Horseshoe crabs (order Xiphosura) are aquatic chelicerate arthropods outlined by the fusion of their physique segments right into a thoracetron,” Dr. James Lamsdell of West Virginia College wrote in his new paper.
“4 residing species are identified and exhibit a disjunct geographical distribution, with one species occurring within the western Atlantic (starting from the east coast of Canada to the Gulf of Mexico) and three within the western Pacific and northeast Indian Oceans (extending from the south of Japan to the east coast of India).”
“The group is known for instance of an evolutionary conservative lineage and is taken into account to comprise archetypal ‘residing fossils,’ though newer work has demonstrated repeated ecological transitions throughout the group to be related to the event of maximum morphologies.”
“Horseshoe crabs have a protracted evolutionary historical past stretching again to the Late Ordovician (450 million years in the past) with two species described from North America and one other, barely older (Early Ordovician, 480 million years in the past) species reported however awaiting formal description from Morocco.”
“The origins and early evolution of horseshoe crabs are poorly identified, nevertheless, with an 80-million-year hole between these Ordovician species and the primary report of Xiphosurida (horseshoe crabs which have diminished their postabdomen to a single phase) within the Late Devonian (370 million years in the past).”
“This lack of a fossil report for horseshoe crabs within the Silurian, a time throughout which different aquatic chelicerate teams have been quickly diversifying, makes it troublesome to find out the timing of the origin of xiphosurids and to what extent the top Ordovician mass extinction and Silurian ecosystem restoration influenced horseshoe crab evolution.”
The newly-identified species of horseshoe crab lived in the course of the Silurian, round 424 million years in the past.
Named Ciurcalimulus discobolus, the animal is understood from a single specimen present in 1975 by Samuel J. Ciurca, Jr within the Kokomo Member of the Wabash Formation in Indiana.
“The Kokomo Member includes as much as 30 m of finely laminated darkish dolostones and is taken into account to be Silurian in age primarily based on conodont information,” the paleontologist wrote.
“The Kokomo localities are primarily identified for his or her endemic eurypterid fauna, which happen in a single horizon and are acknowledged to signify a mass extinction occasion, though quite a lot of algae co-occur with the eurypterids and brachiopods are discovered alongside corals close to the highest of the member in what is usually known as the brachiopod horizon.”
“Ciurcalimulus discobolus is derived from the eurypterid horizon and is preserved equally to the eurypterids, that are compression fossils with carbonized cuticle.”
Ciurcalimulus discobolus is distinguished from different early horseshoe crabs via its distinctive mixture of traits unknown amongst different species.
“Ciurcalimulus most intently resembles the Ordovician Lunataspis species in possessing a closely rounded prosomal carapace and a semicircular thoracetron missing lateral phase boundary expression or tergopleural projections in addition to a multisegmented postabdomen,” the researcher wrote.
“Nevertheless, the brand new genus is distinguished from Lunataspis in missing axial nodes on the thoracetron and the absence of a thoracetron marginal rim outlined dorsally by a furrow, each of that are current within the Ordovician species.”
“The Silurian age of Ciurcalimulus reveals that Xiphosura retaining the overall morphology noticed amongst Ordovician species endured past the top Ordovician mass extinction, suggesting the occasion had a restricted affect on horseshoe crab evolution.”
“Over the course of their evolutionary historical past, horseshoe crabs attained a worldwide distribution,” he added.
“Nevertheless, the primary horseshoe crabs are identified from the paleocontinents of Laurussia and Siberia, whereas the oldest described horseshoe crabs are present in Laurentia.”
“The extra discovery of Ciurcalimulus means that Laurentia was an vital web site of early horseshoe crab evolution, though it have to be acknowledged that there’s a robust historic bias in paleontological analysis towards localities in Europe and former European colonies.”
“As such, Laurentia is prone to be extra closely sampled than different paleocontinents corresponding to Gondwana, an vital consideration on condition that the oldest at the moment identified horseshoe crab is an undescribed species from Morocco.”
The paper was revealed on June 18 within the Proceedings of the Royal Society B.
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James C. Lamsdell. 2025. The primary Silurian horseshoe crab reveals particulars of the xiphosuran floor plan. Proc. R. Soc. B 292 (2049): 20250874; doi: 10.1098/rspb.2025.0874
