Volgian (which roughly corresponds to the Tithonian-early Berriasian) based on ammonite biostratigraphy. From these, numerous new genera and species of ichthyosaur were described including Cryopterygius kristiansenae (later recognized to be synonymous with Undorosaurus gorodischensis), Palvennia hoybergeti (named for PalVenn, the Friends of the Palaeontological Museum in Oslo, whose expedition led to the discovery of the type specimen), and Janusaurus lundi (named for the mountain Janusfjellet, where the specimen was found). Janusaurus and Palvennia were both named off of less extensive material the Cryopterygius, but both were recognizably distinct to that genus and were deemed to be distinct genera. An additional ichthyosaur specimen was prepared at the University of Oslo and subsequently described in 2016: PMO 222.655, the holotype of Keilhauia nui, discovered from the Berriasian portions of the Slottsmøya Member in 2010. This specimen comprises an articulated partial skeleton, which was preserved lying on its left side, including part of the snout, the dorsal and anterior caudal vertebrae, the right forelimb and pectoral girdle, the majority of the pubic girdle, and both of the femora. The genus name Keilhauia honours the Norwegian geologist Baltazar Mathias Keilhau, who conducted an expedition to Spitsbergen in 1827. Meanwhile, the species name nui is derived from the acronym of the environmental organization Natur og Ungdom, the fiftieth anniversary of which occurred in 2017. An extensive revision of Arthropterygius conducted by Zverkov and Prilepskaya in 2019 revealed Janusaurus and Palvennia to be junior synonyms of that genus. They were reassigned as the species A. lundi and A. hoybergeti respectively. Additionally, they determined that the holotype and only known specimen of Keilhauia is referrable to Arthropterygius as well. However, the poor preservation of the material made it difficult to identify it at the species level, and thus they referred the specimen simply to A. sp. cf. chrisorum and designated the name Keilhauia nui as a nomen dubium. They also found that the specimen PMO 222.669, which was referred to Palvennia by Delsett et al. (2018), shares all identifying features with A. chrisorum and that there are no sufficient differences between them to warrant it belonging to A. hoybergeti. This means that there were at least three species of Arthropterygius present in the Slottsmøya Member, and significantly expands the geographic range of the species A. chrisorum specifically. The recognition that these taxa are referable to Arthropterygius also makes this now one of the best understood ichthyosaurs of