A new Triassic austrolimulid from Poland presents insight into xiphosurid evolution and palaeobiogeography at the dawn of the Mesozoic

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.