At present, very little is known regarding the diversity and morphological disparity of long-necked plesiosaurs in Tithonian-aged (latest Jurassic) units globally. Here, we describe two species of a new, long-necked plesiosaur genus Spitrasaurus from the Upper Jurassic Slottsmøya Member of the Agardhfjellet Formation on Svalbard. The holotype species of the genus, S. wensaasi, is the most complete long-necked specimen found in this unit to date and is readily diagnosed on the basis of having at least 60 cervical vertebrae possessing a prominent lateral longitudinal ridge, as well as the presence of a column of well-developed preaxial accessory ossicles in the limbs.
A second taxon, S. larseni, includes a partial skull that broadly resembles the Kimmeridgian taxon Kimmerosaurus, but differs in the morphology of its basioccipital, and in having a distinctive lower jaw with a greatly elongate and strongly dorsally inflected retroarticular process, among other characteristics. Each species of Spitrasaurus can be differentiated on the basis of cervical vertebral proportions and in the morphology of the cervical ribs, rib facets and neural arches, in addition to being stratigraphically separated. The high number of cervical vertebrae in Spitrasaurus significantly exceeds that for described Middle to Late Jurassic plesiosaurs, but is comparable to some Cretaceous elasmosaurids. The Middle Volgian age of this material helps to bridge the temporal and morphological gap between better known Middle and Late Jurassic plesiosaurians from Europe and Late Cretaceous taxa primarily known from North America.