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

甲壳纲（例如潮虫和侧游纲）及其姊妹类真甲纲（例如磷虾和十足目）是现代甲壳类动物中最特殊的类群，据推测早在奥陶纪就已出现。虽然真核虫侵入陆地的区域主要是淡水和沿海地区，但一些甲虫已经进化出完全陆地爬行的生态系统，甚至栖息在我们温带地区的花园中（例如药丸虫和母猪蝽）。它们的化石记录可以追溯到石炭纪，主要由海洋化石组成。在这里，我们对 1908 年在爱尔兰晚泥盆世洪泛区发现的一种节肢动物化石——尖足纲——进行了完整的重新分析，尽管经过一个世纪的鉴定尝试，但其系统亲缘关系仍未解决。从二维保存的单个标本中得知，我们使用数字显微镜和多光谱宏观成像分析了其解剖结构，以增强形态结构的对比度。尖足动物门的新解剖特征和完整性，以及对所有主要真肢动物类群代表的系统发育分析，表明尖足动物是冠甲虫纲，是包括端足类和等足类在内的进化枝的一部分。因此，尖足纲是已知最古老的甲壳类物种，并提供了证据表明衍生的甲壳类早在法门纪（距今 360 Ma 多）就已侵入淡水和陆地环境。