Unraveling the identity of Plesiosaurus posidoniae, the first plesiosaur described from the Posidonia Shale of Germany.

Plesiosaurus posidoniae, described by Quenstedt in 1865, is the first plesiosaur taxon to be named from the lower Toarcian Posidonia Shale (Posidonienschiefer Formation) of Germany. The holotype consists of a fragmentary left hind limb, comprising the femur, tibia, tarsal and metatarsal elements, and phalanges, found at Reutlingen-Ohmenhausen in southwestern Germany. Despite its historical significance, the taxonomic affinities of P. posidoniae remain unexplored. Although fragmentary, the specimen shows characters useful for its taxonomic assessment, including the proportions of the femur and tibia, the uniformly rounded distal femoral end, and the lack of an intermedium facet at the tibia. Osteological comparisons, combined with multivariate analyses incorporating continuous and discrete data obtained from paddle elements of Posidonia Shale plesiosaurs, indicate great similarities and probable affinities with the early-diverging pliosaurid Hauffiosaurus. However, differences can also be observed, such as the greater slenderness of the femora of Hauffiosaurus. These differences may reflect intraspecific variability, but the fragmentary nature of P. posidoniae prevents an unequivocal taxonomic assignment.