Lustri, L.; Laibl, L.; Collantes, L.; Bruthansova, J.; Nohejlova, M.; Liu, Y.; Pates, S.

Zonozoe drabowiensis Barrande, 1872 and Zonoscutum solum Chlupa[s] 1999 are rare and incompletely preserved arthropods from the Upper Ordovician of the Czech Republic. Their classification has been a subject of debate for over a century due to the limited number of specimens and the absence of clear diagnostic features. They were previously thought to belong to Aglaspidida, an extinct group of arthropods from the Cambrian and Ordovician, within Vicissicaudata, a branch of the larger arthropod clade Artiopoda that also includes trilobites. Herein, we used analyses of cephalic outline to quantify whether Zonozoe and Zonoscutum are morphologically more similar to vicissicaudates than other early Palaeozoic arthropods. We assembled a dataset of twenty-six cephalic outlines representing twenty-six early Palaeozoic species, including Zonozoe, Zonoscutum, five euchelicerates, six aglaspidids, three cheloniellids, and a selection of other artiopodans. We quantified their shape using elliptical Fourier analysis. Principal component analysis (PCA) and linear discriminant analysis (LDA) place Zonozoe and Zonoscutum within and close to the vicissicaudatan morphospace, clearly distinguishing them from euchelicerates and non-vicissicaudatan artiopodans. These data support the most conservative classification of Zonozoe and Zonoscutum within Artiopoda, while strengthening the case for a more specific affinity with Vicissicaudata, helping to resolve a 150-year-old taxonomic uncertainty. More broadly, this study demonstrates the value of outline-based morphometrics in testing systematic hypotheses when discrete characters are unavailable or scarce, offering a reproducible tool for re-evaluating other problematic fossils.