The oldest peracarid crustacean reveals a Late Devonian freshwater colonization by isopod relatives

Peracarida (e.g. woodlice and side-swimmers) are, together with their sister-group Eucarida krill decapods), the most speciose group of modern crustaceans, suggested to have appeared as early Ordovician. While eucarids' incursion onto land consists mainly freshwater littoral grounds, some peracarids evolved fully terrestrial ground-crawling ecologies, inhabiting even our gardens in temperate regions pillbugs sowbugs). Their fossil record extends back Carboniferous marine occurrences. Here, we provide a complete re-analysis arthropod— Oxyuropoda— reported 1908 from Late Devonian floodplains Ireland, left unresolved systematic affinities despite century attempts at identification. Known single specimen preserved two dimensions, analysed its anatomy using digital microscopy multispectral macroimaging enhance contrast morphological structures. The new anatomical characters completeness Oxyuropoda , phylogenetic analysis representatives all major Eumalacostraca groups, indicate that is crown peracarid, part clade including amphipods isopods. As such, oldest known species Peracarida, provides evidence derived had an into environments Famennian, more than 360 Ma.