A newly excavated locality yielding plesiosaurs in the Late Jurassic Slottsmøya Member of the Agardhfjellet Formation, Spitsbergen shed new light on earlier theories of functional morphology. The propodials of the Slottsmøya Member have the reverse proportional relationship than previously seen in the Late Jurassic plesiosaurs, with the femora being significantly larger in length and distal width than the humeri. This is presumably implied by a functional adaption to the ecology of the Boreal Sea. The preservated propodials of eight plesiosaurs (clade Sauropterygia, order Plesiosauria, superfamily Plesiosauroidea), all from mid-Volgian are used in this study. The preserved material consists of seven femora, five humeri, threeulnae, two radii, two tibiae and four fibulae. In the field each specimen was found in an articulated or associated state allowing humeri and femora to be clearly recognizable. These results are supported by measurements, statistics and comparisons with contemporary species from the Oxford Clay Formation in England.