Osteology and phylogeny of Late Jurassic ichthyosaurs from the Slottsmoya Member Lagerstätte (Spitsbergen, Svalbard)

Phylogenetic relationships within the important ichthyosaur family Ophthalmosauridae are not well established, and more specimens and characters, especially from the postcranial skeleton, are needed. Three ophthalmosaurid specimens from the Tithonian (Late Jurassic) of the Slottsmøya Member Lagerstätte on Spitsbergen, Svalbard, are described. Two of the specimens are new and are referred to Keilhauia sp. and Ophthalmosauridae indet. respectively, whereas the third specimen consists of previously undescribed basicranial elements from the holotype of Cryopterygius kristiansenae. The species was recently synonymized with the Russian Undorosaurus gorodischensis, but despite many similarities, we conclude that there are too many differences, for example in the shape of the stapedial head and the proximal head of the humerus; and too little overlap between specimens, to warrant synonymy on species level. A phylogenetic analysis of Ophthalmosauridae is conducted, including all Slottsmøya Member specimens and new characters. The two proposed ophthalmosaurid clades, Ophthalmosaurinae and Platypterygiinae, are retrieved under some circumstances, but with little support. The synonymy of three taxa from the Slottsmøya Member Lagerstätte with Arthropterygius is not supported by the present evidence.