Horseshoe-shaped crabs are ancient creatures with an evolutionary history that stretches back 450 million years (during the Ordovician period) and are often regarded as “living fossils.” Paleontologists from West Virginia University have identified a new genus and species of true horseshoe-shaped crabs from Silurian specimens found in Indiana, USA. This species fills an 80 million-year gap in the fossil record of horseshoe-shaped crabs and exhibits a morphology reminiscent of those from the Ordovician period.
Ciurcalimulus discobolus holotype. Scale bar – 5 mm. Image credit: James C. Lamsdell, doi: 10.1098/rspb.2025.0874.
“Horseshoe-shaped crabs (order Xiphosura) are aquatic arthropods characterized by the fusion of their body segments in the thoracic area,” stated Dr. James Ramsdell from West Virginia University in his recent publication.
“Currently, there are four known living species, each exhibiting isolated geographical distributions: one inhabiting the Western Atlantic (from the East Coast of Canada to the Gulf of Mexico) and three found in the Western Pacific and Northeast Indian Oceans (from southern Japan to the East Coast of India).”
“This group is widely recognized as a classic example of an evolutionarily conservative lineage, often referred to as ‘living fossils.’ However, recent studies indicate that they undergo ecological transitions tied to significant morphological changes within the group.”
“The evolutionary history of horseshoe-shaped crabs dates back to two species from North America (450 million years ago) and one slightly older species (early Ordovician, 480 million years ago) from Morocco, which is pending formal description.”
“The origins and early evolution of horseshoe-shaped crabs remain largely unknown, with an 80 million-year gap between these Ordovician species and the first record of Xiphosurida (horseshoe-shaped crabs with a reduced retroabdomen, dating back 370 million years).”
“The absence of Silurian horseshoe-shaped crab fossils occurs during a period of rapid diversification of other aquatic groups, complicating efforts to pinpoint the timing of the origins of Xiphosurids.”
The new species of horseshoe-shaped crab thrived during the Silurian period, approximately 424 million years ago.
It has been designated as Ciurcalimulus discobolus, known from a single specimen discovered in 1975 by JR Samuel J. Sieuca, found in the Kokomo member of the Wabash Formation in Indiana.
“Kokomo members consist of finely stacked dark drostons reaching up to 30 meters, and their age is considered Silurian based on Conodont data,” the paleontologist noted.
“The Kokomo region is primarily recognized for its endemic Euripterid fauna, which exists on a single horizon and is linked to significant extinction events. In this area, various algae of Euripterid and Brachiopod coexist, sometimes alongside corals, above the corals at the upper levels of the sub-arm phyla.”
“Ciurcalimulus discobolus is derived from Euripterid-rich horizons and is preserved similarly to Euripterids, featuring a compressed fossil with well-defined cuticles.”
Ciurcalimulus discobolus differentiates itself from other early horseshoe-shaped crabs by a distinctive combination of traits that are not found in other species.
“Ciurcalimulus bears resemblance to the Ordovician Lunataspis, characterized by a distinctly rounded prosomal shell and a semicircular thorax that lacks lateral segment boundaries or prominent projections, along with a multisphere retroabdominal region,” the researchers explained.
“Nonetheless, the new genus Ciurcalimulus is set apart from Lunataspis due to the absence of axial nodes on the chest and the marginal edge of the thorax being defined dorsally by fur.”
“The Silurian era Ciurcalimulus maintains the common morphology observed in Ordovician species, suggesting its survival beyond the Ordovician mass extinction had a limited impact on the evolution of horseshoe-shaped crabs.”
“Throughout their evolutionary journey, horseshoe-shaped crabs have achieved a global distribution,” he continued.
“However, the first known horseshoe-shaped crabs hail from ancient Roursia and Siberia while the oldest can be traced back to Laurentia.”
“The discovery of Ciurcalimulus in Laurentia indicates it may be a crucial area for the evolution of early horseshoe crabs, but it is essential to acknowledge the strong historical bias in paleontological studies focused on European and former colonial regions.”
“This suggests that Laurentia may have been sampled more intensively than other ancient continents, such as Gondwana. This is a vital consideration given that the oldest horseshoe-shaped crabs currently identified are undescribed species from Morocco.”
The paper was published on June 18 in Proceedings of the Royal Society B.
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James C. Ramsdell. 2025. The first Silurian horseshoe-shaped crab reveals insights into the ground plans of Xiphosurans. Proc. R. Soc. B 292 (2049): 20250874; doi: 10.1098/rspb.2025.0874
Source: www.sci.news
