Exceptionally preserved ovaries in an ancient horseshoe crab

Abstract 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.