A morphometric approach to the taxonomic dilemma of Zonozoe drabowiensis Barrande, 1872 and Zonoscutum solum Chlupáč, 1999 (Upper Ordovician, Czech Republic)

Zonozoe drabowiensis Barrande, 1872 and Zonoscutum solum Chlupáč, 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, lack of knowledge related to post-cephalic morphology and the absence of clear diagnostic features. They were previously considered as members of Aglaspidida, an extinct group of arthropods from the Cambrian and Ordovician, within Vicissicaudata, a branch of the larger arthropod clade Artiopoda. Herein, we analysed the cephalic outlines of Zonozoe and Zonoscutum to determine whether their shapes align more closely with vissicaudatans than with other early Palaeozoic arthropods, offering a new morphological perspective on their systematics. We assembled a data set of cephalic outlines each representing one of thirty-three early Palaeozoic species, including Zonozoe , Zonoscutum , nine 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 within the vicissicaudatan morphospace, and Zonoscutum in their proximity, clearly distinguishing them from euchelicerates. These data add support to 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 aiding systematic hypotheses when discrete characters are unavailable or scarce, offering a reproducible tool for re-evaluating other problematic fossils.