Horseshoe crabs are an ancient lineage with an evolutionary history stretching back 450 million years (Ordovician period) and are generally considered to be examples of ‘living fossils.’ A paleontologist from West Virginia University has described a new genus and species of true horseshoe crab from a Silurian-age specimen unearthed in Indiana, the United States. The species bridges the 80-million-year gap in the horseshoe crab fossil record and has an overall morphology similar to that of the Ordovician species.
Ciurcalimulus discobolus, holotype. Scale bars – 5 mm. Image credit: James C. Lamsdell, doi: 10.1098/rspb.2025.0874.
“Horseshoe crabs (order Xiphosura) are aquatic chelicerate arthropods defined by the fusion of their body segments into a thoracetron,” Dr. James Lamsdell of West Virginia University wrote in his new paper.
“Four living species are known and exhibit a disjunct geographical distribution, with one 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 northeast Indian Oceans (extending from the south of Japan to the east coast of India).”
“The group is famous as an example of an evolutionary conservative lineage and is considered to comprise archetypal ‘living 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 evolutionary history stretching 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) species reported but awaiting formal description from Morocco.”
“The origins and early evolution of horseshoe crabs are poorly known, however, with an 80-million-year gap between these Ordovician species and the first record of Xiphosurida (horseshoe crabs that have reduced their postabdomen to a single segment) in the Late Devonian (370 million years ago).”
“This lack of a fossil record for horseshoe crabs in the Silurian, a time during which other aquatic chelicerate groups were rapidly diversifying, makes it difficult to determine the timing of the origin of xiphosurids and to what extent the end Ordovician mass extinction and Silurian ecosystem recovery influenced horseshoe crab evolution.”
The newly-identified species of horseshoe crab lived during the Silurian, around 424 million years ago.
Named Ciurcalimulus discobolus, the animal is known from a single specimen found in 1975 by Samuel J. Ciurca, Jr in the Kokomo Member of the Wabash Formation in Indiana.
“The Kokomo Member comprises up to 30 m of finely laminated dark dolostones and is considered to be Silurian in age based on conodont data,” the paleontologist wrote.
“The Kokomo localities are primarily known for their endemic eurypterid fauna, which occur in a single horizon and are recognized to represent a mass extinction event, although a variety of algae co-occur with the eurypterids and brachiopods are found alongside corals near the top of the member in what is sometimes referred to as the brachiopod horizon.”
“Ciurcalimulus discobolus is derived from the eurypterid horizon and is preserved similarly to the eurypterids, which are compression fossils with carbonized cuticle.”
Ciurcalimulus discobolus is distinguished from other early horseshoe crabs through its unique combination of characteristics unknown among other species.
“Ciurcalimulus most closely resembles the Ordovician Lunataspis species in possessing a heavily rounded prosomal carapace and a semicircular thoracetron lacking lateral segment boundary expression or tergopleural projections as well as a multisegmented postabdomen,” the researcher wrote.
“However, the new genus is distinguished from Lunataspis in lacking axial nodes on the thoracetron and the absence of a thoracetron marginal rim defined dorsally by a furrow, both of which are present in the Ordovician species.”
“The Silurian age of Ciurcalimulus shows that Xiphosura retaining the general morphology observed among Ordovician species persisted beyond the end Ordovician mass extinction, suggesting the event had a limited impact on horseshoe crab evolution.”
“Over the course of their evolutionary history, horseshoe crabs attained a global distribution,” he added.
“However, the first horseshoe crabs are known from the paleocontinents of Laurussia and Siberia, while the oldest described horseshoe crabs are found in Laurentia.”
“The additional discovery of Ciurcalimulus suggests that Laurentia was an important site of early horseshoe crab evolution, although it must be recognized that there is a strong historical bias in paleontological research toward 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 given that the oldest currently known horseshoe crab is an undescribed species from Morocco.”
The paper was published on June 18 in the Proceedings of the Royal Society B.
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James C. Lamsdell. 2025. The first Silurian horseshoe crab reveals details of the xiphosuran ground plan. Proc. R. Soc. B 292 (2049): 20250874; doi: 10.1098/rspb.2025.0874