Which arthropods have feet and why? Addressing an argument for aquatic fossil scorpions

Abstract Scorpions are the first arguably terrestrial animals identifiable in the fossil record, with important implications for arachnid evolution and terrestrial ecosystem development. However, debate persists on whether some fossil scorpions from the Silurian and Devonian were, in fact, aquatic. This study assesses the claim that a digitigrade (‘footless’) posture implies an aquatic habitat for early scorpions by evaluating the distribution of feet among ambulatory arthropods and how their presence correlates with factors including aquatic versus terrestrial habitat as well as body size, number of legs, cuticle mineralization, and time since terrestrialization. The results demonstrate that these variables in isolation are poor predictors of leg posture, but become highly statistically significant in certain combinations. However, with as many lineages diverging from the usual pattern of plantigrady as adhering to it, predictive power is weak. Therefore, all factors influencing plantigrady must be accounted for when discussing its implications in fossil arthropods, and digitigrady alone does not provide compelling support for an aquatic habitat in fossil scorpions. We further argue that other lines of evidence for an aquatic habitat in Palaeozoic scorpions are equivocal and that the weight of phylogenetic and anatomical evidence supports a terrestrial origin for total group Scorpiones.