 |
| Polonolimulus zaleziankensis Audycki, Bicknell, Niedźwiedzki & De Baets, 2026 Reconstruction by Jonatan Audycki |
Abstract
Xiphosurids are aquatic chelicerates widely viewed as examples of so-called ‘living fossils’ due to their apparent morphological conservatism and limited diversity since at least the Jurassic. However, earlier representatives were much more diverse and morphologically disparate. Particularly striking are hypertrophied genal spines and reduced thoracetrons of the Triassic austrolimulids, possibly related to their colonization of brackish or freshwater habitats. Here we describe Polonolimulus zaleziankensis gen. et sp. nov., a new austrolimulid genus from the Early Triassic of Holy Cross Mountains, Poland. Geometric morphometric analysis positions the new find among the morphologically most ‘extreme’ austrolimulids, extending the geographic range of those forms to Central Europe. A palaeobiogeographic reconstruction of Triassic xiphosurids reveals their surprisingly wide distribution already in Early Triassic, suggesting either an earlier dispersal in the Late Permian or a rapid diversification in the earliest Triassic. The reconstruction of most austrolimulid occurrences within or proximal to the shallow marine areas casts doubts on the hypothesis they inhabited fully freshwater palaeonvironments, which should be reinvestigated in the future. The new material further adds to the growing understanding of xiphosurid diversity and evolution in the early Mesozoic.
Keywords: Limulus, Horseshoe crab, Arthropoda, Buntsandstein, Holy Cross Mountains
 |
| Polonolimulus zaleziankensis gen. et sp. nov. The rock slab bearing Muz. PGI 1808.II.10 and another xiphosurid fossil. (A) The upper surface of the rock slab with Muz. PGI 1808.II.10 preserved as a concave natural mould. (B) The lower surface of the rock slab with the uncertain convex horseshoe crab-shaped structure, considered a possible trace fossil (see ‘Remarks’ for further discussion). (C) Close-up of Muz. PGI 1808.II.10 (D) Close-up of the convex structure on the lower surface coated with ammonium chloride sublimate. Scale bars: (A, B) 50 mm, (C, D) 20 mm. Photo credit: Jonatan Audycki. |
Polonolimulus zaleziankensis gen. et sp. nov.
 |
| Idealized reconstruction of Polonolimulus zaleziankensis gen. et sp. nov. Reconstruction by Jonatan Audycki. |
 |
| Palaeobiogeography of Triassic horseshoe crabs. Palaeogeography of the world is reconstructed at 245 Ma (Anisian, Middle Triassic) and displayed using Mollweide projection, with a close-up of the Central European Basin System. The extents of shallow seas (light blue), landmasses (pale yellow) and mountain ranges (orange) are reconstructed based on Cao et al. (2017). Symbols represent Triassic localities that have yielded horseshoe crab fossils, their ages, and taxonomic assignment. The palaecoordinates of the xiphosurid localities are also reconstructed at 245 Ma for consistency. Simplified reconstructions of austrolimulids are displayed for context and comparison: 1–Polonolimulus zaleziankensis from the Lower Triassic (uppermost Induan/lowermost Olenekian) Zalezianka-Gózd locality, Poland; 2–Psammolimulus gottingensis from the Lower Triassic (Olenekian) Solling Formation, Germany; 3–Batracholimulus fuchsbergensis from the Upper Triassic (Norian/Rhaetian boundary) Exter Formation, Germany; 4–Attenborolimulus superspinosus from the Lower Triassic (upper Olenekian) Petropavlovka Formation, Russia; 5–Vaderlimulus tricki from the Lower Triassic (lower Spathian, Olenekian) Thaynes Group, USA; 6–Austrolimulus fletcheri from the Middle Triassic (lower Anisian) Beacon Hill Quarry, Hawkesbury Sandstone, Australia; 7–Dubbolimulus peetae from the Middle Triassic (lower Anisian) Ballimore Formation, Australia; 8–Tasmaniolimulus patersoni from the Lower Triassic (lower Induan) Jackey Shale, Tasmania (Australia). Since the investigated xiphosurid localities differ stratigraphically, their geographical position relative to continental boundaries and shallow seashores are approximated. The display window on the map showing close-up boundaries is not square due to employed map projection. |
Jonatan Audycki, Russell D.C. Bicknell, Grzegorz Niedźwiedzki, Kenneth De Baets. 2026. 2026. A New Triassic austrolimulid from Poland presents insight into xiphosurid Evolution and Palaeobiogeography at the Dawn of the Mesozoic. PeerJ. 14:e20950. DOI: doi.org/10.7717/peerj.20950 [2026-03-25]
x.com/IBE_Warszawa/status/2036854712750780757
