In this study, tooth-bearing bones and vertebrae of Omphalosaurus from the Early Triassic of Spitsbergen have been examined to better understand the morphology, lifestyle and systematic affinities of this enigmatic marine reptile. The tooth-bearing bones consist of 18 dentaries, 7 premaxillae and 30 bones with uncertain position in the jaw. In total 337 vertebrae were collected, together with more than 400 vertebrae fragments. Only the best preserved bones are described in this thesis. The material was collected at Marmierfjellet from the Grippia Niveau and Lower Saurian Niveau, Vikinghøgda Formation, Vendomdalen Member, dated as Spathian. The Omphalosaurus material described from Spitsbergen is unique in the amount of material, size range and 3D preservation. Even though ichthyopterygian affinity of Omphalosaurus has been controversial, such an association is proposed here based on the vertebral column described for the first time in this study. Four of the characters by Ji et al. (2015) used to define the ichthyopterygia is suggested present in Omphalosaurus; nasal anteriorly extending beyond external naris, neural spine articulation in tail present, caudal peak present and tail stem count ½ or more that of the presacral count. Omphalosaurus is probably more derived than the most basal ichthyopterygians, due to the presence of discoidal vertebrae centra and a caudal peak. Based on the vertebral column it is suggested as a possible transition between anguilliform and thunniform swimming mode, adapted to an open marine environment.