Ecology, morphology and ontogeny of Paleolimulus kunguricus--a horseshoe crab from the Kungurian (Cisuralian) of the Cis-Urals, Russia

Paleolimulids represent a group of fossil horseshoe crabs that are morphologically comparable to the extant limulids. Recent work has highlighted that, contrary to traditional opinions, Paleolimulidae has a limited taxonomic and morphological diversity at the species and generic level. In the light of this new perspective, it is imperative that material documenting novel facets of species within this Paleozoic group be explored. Here, we revisit Paleolimulus kunguricus Naugolnykh, 2017 from the Cisuralian of Russia and illustrate distinct ontogenetic stages of the species. The so-called ‘trilobite’ stage, two juveniles and adult stages are presented. We document morphological transitions within the prosomal and opisthosomal sections, and changes to the lateral compound eye position. These shifts may reflect distinct life modes experienced during ontogeny. Documentation of Selenichnites ichnofossils associated with P. kunguricus specimens present insight into how the xiphosurid functioned during the later ontogenetic stages and illustrate the first evidence for shell-crushing in fossil xiphosurids. Finally, we demonstrate how occupation of lagoonal palaeoenvironments limited competition with other organisms and aided the survival of Xiphosurida.