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| Ciurcalimulus discobolus Lamsdell, 2025 |
Abstract
Horseshoe crabs are an ancient lineage with an evolutionary history stretching back 450 million years and are generally considered to be examples of ‘living fossils’ exhibiting slow rates of evolution. Despite this reputation, relatively little is known of the early evolution of the group, with only two species described from the Ordovician and a subsequent 80-million-year gap in their fossil record until xiphosurids appear in the Late Devonian. Furthermore, all described Ordovician species are assigned to a single genus, with their close phylogenetic relatedness rendering it unclear whether their morphology is representative of the horseshoe crab ground pattern or an independently derived condition. Here, a new species of horseshoe crab is described from the Silurian of Indiana, USA. The new species bridges the temporal gap in the xiphosuran fossil record and has an overall morphology similar to that of the Ordovician taxa. These new data provide critical information on the ancestral morphology of horseshoe crabs, showing that xiphosurids evolved from forms with a fused thoracetron exhibiting axial segment boundaries, and demonstrate the persistence of basal Xiphosura into the Silurian. Laser-stimulated fluorescence is also shown to be an effective method for studying and imaging arthropod fossils exhibiting challenging preservation.
Keywords: Ciurcalimulus, ground plan, horseshoe crab, laser stimulated fluorescence, Silurian, Xiphosura, Xiphosurida
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| Ciurcalimulus discobolus gen. et sp. nov., holotype (YPM IP 548961). (a) Imaged dry under polarized light, showing specimen relief. (b) Imaged immersed in ethanol under polarized light, providing greater contrast of the carbonized cuticle. (c) Interpretive drawing of specimen. Carbonized cuticle is shown in brown, damaged or incomplete margins by dashed lines. (d) Imaged under ultraviolet light, exhibiting fluorescence of both the carbonized cuticle and to a lesser extent the exfoliated regions of the fossil, affording the clearest view of the segment boundaries in the thoracetron axis. (e) Imaged under 447 nm blue laser, with carbonized cuticle and exfoliated regions again both fluorescing. The outline of the fossil is most readily apparent here as is the morphology of the cardiac lobe. (f) Imaged under 532 nm green laser, clearly showing the location of carbonized cuticle via strong fluorescence. Scale bars = 5 mm. |
Arthropoda Gravenhorst, 1843
Chelicerata Heymons, 1901
Xiphosura Latreille, 1802
Ciurcalimulus discobolus gen. et sp. nov.
Etymology. The genus is named for the late Sam Ciurca, a prolific collector and avocational palaeontologist who discovered the specimen in 1975. The species name refers to the ancient Greek sculpture by Myron in reference to the extremely rounded, discus-like form of the prosoma and thoracetron.
Holotype. YPM IP 548961 (Yale Peabody Museum, Invertebrate Paleontology), complete specimen in dorsal view preserving the prosomal carapace, thoracetron, postabdomen and telson (figures 1 and 2).
Diagnosis. Xiphosuran with semicircular prosomal carapace bearing curved genal spines that reach almost to thoracetron posterior; prosomal carapace bearing crescentic lateral eyes located centrimesially; prosomal cardiac lobe quadrate with rounded anterior, well expressed with margins defined by furrows; anterior two opisthosomal segments short and freely articulating; thoracetron semicircular, approximately equal in length to the prosomal carapace, comprising up to six fused body segments with segmental boundaries expressed axially; postabdomen comprising up to five free articulating segments including an elongated pretelson; telson equal in length to entire body, lanceolate in shape with bifurcate termination.
James C. Lamsdell. 2025. The first Silurian horseshoe crab reveals details of the xiphosuran ground plan. Proc. R. Soc. B. 292: 20250874. DOI: doi.org/10.1098/rspb.2025.0874 [18 June 2025]
