Xiphosurans have often been considered as archaic appearing cheliceratan arthropods, with a rich fossil record. We describe here parts of the post-embryonic ontogeny of the 300 million year old xiphosuran Euproops danae (Xiphosura sensu stricto, Euchelicerata), from the Mazon Creek Lagerstätte (Upper Carboniferous), USA. Recently, the ontogeny of a closely related species, Euproops sp. from the Upper Carboniferous Piesberg quarry, Osnabrück, Germany (informally called 'Piesproops'), has been reconstructed. This analysis has drawn characters into question that were used to differentiate E. danae from another species occurring at the same time, Euproops rotundatus from the British Middle Coal Measures. More precisely, early post-embryonic stages of Piesproops resemble E. danae; later stages resemble E. rotundatus. Based on this earlier study, the here-described reinvestigation of E. danae has been performed as the ontogenetic sequence itself may yield more reliable characters for differentiating species of Euproops. We could identify eight different growth stages for E. danae. This ontogenetic sequence shows a comparable growth to that of Piesproops, but differs markedly in the development of the opisthosomal flange. This character may serve as a basis for reliably differentiating these species. Additionally, analysing the ontogeny of further species may offer the basis for identifying heterochronic shifts in the evolution of xiphosurans.
Keywords: Euchelicerata; Fossilised ontogeny; Hapantotype; Heterochrony; Xiphosura sensu stricto.