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| Lunataspis gundersoni and comparative material of Limulus polyphemus (B-C). (A) Holotype (specimen UWGM 4087). Expanded views of ovarian network are shown with eggs indicated by white arrows. (B) Developing eggs sampled from a single gravid Limulus polyphemus. (C) Ovarian network of Limulus polyphemus showing ovarian tubes. (D) Interpretive drawing of holotype with ovarian tissue (orange), fossil surface (beige), cuticle (russet), and burrow trace (brown). (E) Holotype counterpart. (F) Paratype (UWGM 6422). Lamsdell, Falk, Hegna & Meyer, 2025 Abbreviations: cu—cuticle; e—eggs; ft—free tergite; la—leperditid arthropod; le—lateral eye; mn—median node; on—ovarian network; ot—ovarian tubes; pc— prosomal carapace; pt—pretelson; te—telson; tf—trace fossil; th—thoracetron; |
Horseshoe crabs are an ancient lineage of aquatic chelicerate arthropods traditionally considered to be “living fossils” exhibiting limited morphological change throughout their evolution. This paradigm is challenged by the divergent morphologies of Paleozoic and Mesozoic xiphosurans in nonmarine environments, which show greater variation of prosomal carapace shape. Marine horseshoe crabs appear to exhibit a more conservative evolutionary history punctuated by marked increases in body size during the Carboniferous and Jurassic. However, interpretation of horseshoe crab evolutionary trends is complicated by uncertainty over whether the smaller Paleozoic species represent adults or juveniles. Here, we describe a diminutive 445-million-year-old horseshoe crab, Lunataspis gundersoni sp. nov., preserving details of ovarian structures in the prosoma. Unique among Xiphosura, the new species possesses an unusually prolongated prosoma, indicating early morphological variation within the group. However, the female reproductive system appears identical to that of extant Tachypleinae. Critically, the presence of eggs demonstrates that the specimens are adults and that horseshoe crabs were significantly smaller in size during the Paleozoic. Lunataspis gundersoni displays a mixture of divergent external and conserved internal anatomy that reveals mosaic evolutionary patterns and illustrates early morphological variation among marine Xiphosura.
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| Lunataspis gundersoni and comparative material of Limulus polyphemus. (A) Holotype (specimen UWGM 4087). Expanded views of ovarian network are shown with eggs indicated by white arrows. (B) Developing eggs sampled from a single gravid Limulus polyphemus. (C) Ovarian network of Limulus polyphemus showing ovarian tubes. (D) Interpretive drawing of holotype with ovarian tissue (orange), fossil surface (beige), cuticle (russet), and burrow trace (brown). (E) Holotype counterpart. (F) Paratype (UWGM 6422). Abbreviations: cu—cuticle; e—eggs; ft—free tergite; la—leperditid arthropod; le—lateral eye; mn—median node; on—ovarian network; ot—ovarian tubes; pc— prosomal carapace; pt—pretelson; te—telson; tf—trace fossil; th—thoracetron; |
Arthropoda Gravenhorst, 1843
Chelicerata Heymons, 1901
Xiphosura Latreille, 1802
Lunataspis Rudkin, Young & Nowlan, 2008
Lunataspis gundersoni sp. nov.
Etymology. Named in honor of the late Gerald Gunderson, co-discoverer of the Big Hill biota.
Diagnosis. Xiphosura with curved genal expansion of prosomal carapace; low ophthalmic ridges present posterior to lateral eyes; cardiac lobe comprising slight swelling without lateral furrows; opisthosoma with two freely articulating tergites anterior to thoracetron; thoracetron comprising six fused segments, segmental boundaries expressed axially; postabdomen comprising four freely articulating ankylosed segments, pretelsonic segment elongated; axial nodes present centrally on each opisthosomal segment excepting the pretelson; telson lanceolate. Differentiated from other Lunataspis species by the shovel-shaped anterior prolongation of the prosomal carapace.
James C. Lamsdell; Amanda R. Falk; Thomas A. Hegna and Ronald C. Meyer. 2025. Exceptionally preserved Ovaries in An ancient Horseshoe Crab. Geology. DOI: doi.org/10.1130/G53317.1 [May 29, 2025]
