Too new to name: investigating the quality and expression of Opiliones traces using neoichnology

Neoichnological experiments are essential for understanding arthropod interactions with sediments and in refining ichnotaxonomic interpretations. This study presents the first experimental investigation using Eusarcus (Opiliones, Arachnida) as a tracemaker under controlled substrate conditions. We tested twelve sediment mixtures with varying proportions of illite, calcite, halite, and sand—under dry and humid conditions—to evaluate how grain size, mineral composition, and moisture influence trace preservation. Results indicate that fine-grained, illite- or calcite-rich substrates with high moisture content (>85%) best preserve detailed track morphologies, including bifid tarsal impressions, opisthosomal drag marks, and staggered sets of three tracks produced by an alternating tripod gait. These experimental trackways display morphological similarities with ichnotaxa such as Lithographus, Paleohelcura, and Octopodichnus, though they exhibit distinct features—especially the anterior elongation of the central track. Such affinities support the hypothesis that some Palaeozoic trace fossils, often attributed to insects, may have been made by arachnids, particularly Opiliones. This possibility is reinforced by the ancient terrestrial origins of arachnids and the lack of ichnological studies involving harvestmen. We also identify sedimentological factors that influence preservation quality, including the plasticity of clay minerals and post-depositional changes in carbonate- and halite-rich substrates. By replicating Opiliones trackways experimentally, this study broadens the neoichnological framework for arachnids and aids in more accurate tracemaker attribution in the fossil record. Ultimately, our findings highlight the importance of conducting further experiments with underrepresented arthropod groups to refine behavioural and environmental interpretations of trace fossils.