| Summary: | A partial postcranial plesiosaurian skeleton uncovered on Svalbard in the winter of 1930-31 was described and named Tricleidus svalbardensis Persson, 1962. However, the precise geographic location and stratigraphic unit in which the skeleton was found were not published. Recent fieldwork on Svalbard has uncovered two more plesiosaur skeletons that are demonstrated herein to be conspecific with the 1931 taxon. These new specimens, along with recently discovered historic documents produced by the excavation members of the 1930-31 expedition, reveal that the holotype specimen, PMO A 27745, was recovered from the Upper Jurassic Slottsmøya Member of the Agardhfjellet Formation, which is Middle Volgian in age. Collectively, the Svalbard material is neither morphologically nor stratigraphically consistent with the Callovian taxon Tricleidus Andrews, 1909 and is here referred to Colymbosaurus Seeley, 1874 from the Kimmeridge Clay Formation (Kimmeridgian to Tithonian) of the UK.
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