Arthropterygius is a widespread genus of ophthalmosaurid ichthyosaur which existed in Canada, Norway, Russia, and Argentina from the late Jurassic period and possibly to the earliest Cretaceous. Description Arthropterygius appears to have been a relatively large ichthyosaur, with all species measuring between long. The partially preserved specimen PMO 222.655 has been estimated at based on comparisons to the contemporary ophthalmosaurid Undorosaurus. This specimen was probably mature or close to maturity at time of death, judging by the convex head of the humerus and the smooth texture of the humeral shaft. Skull and axial skeleton The skull of Arthropterygius has become well known due to the discovery of a well preserved skull from Svalbard. It is unusual among ichthyosaurs in having a very short, yet also robust rostrum for its skull length. As a result, the orbit appears very large (about 0.34× the skull length). The skull bears a very large pineal foramen, which is larger proportionately than that of any other ophthalmosaurid. This may potentially be an adaptation to the high latitude environment of Arthropterygius, but it is not yet well understood how the size of the pineal eye relates to latitude in ichthyosaurs. The presence of the foramen for the internal carotid artery (a major artery that supplies blood to the brain) on the posterior surface of the parabasisphenoid is a distinguishing trait of this genus. The teeth are relatively large, with robust, conical crowns, and are largely straight or slightly recurved. It is difficult to estimate the exact number of vertebral segments that were present in front of the sacrum; the preserved centra suggest that there are at least 43. This is more than the 42 in Athabascasaurus, the 41 in Nannopterygius, and the 37 in Platypterygius americanus, but less than the 46 in Platypterygius australis and the 50 or more in Undorosaurus, Aegirosaurus, and Platypterygius platydactylus. The posterior dorsal vertebrae are wider and taller than those near the front, and their front and rear faces are rounded. The first few caudal vertebrae are the proportionally tallest and widest of all the vertebrae, but the vertebrae quickly become shorter vertically prior to the bend in the tail that supported the tail fluke. Each neural spine is about the same length as the underlying centrum, which is similar to Undorosaurus but not Ophthalmosaurus (in which they are proportionally longer in the posterior cervical vertebrae). Similar to Gengasaurus, the first