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.

[10] The ribs are shaped like a figure eight in cross section near their top ends, but this is less obvious closer to the bottom; such a morphology is typical among ophthalmosaurids, except for Acamptonectes[14] and Mollesaurus.

[1] The clavicle, which is not fused to other elements in the pectoral girdle, bears a thickened process on its frontal bottom edge which points towards the midline of the torso.

[1] The ischium and the pubis are fused together into a single, continuous, solid trapezoid-shaped element known as the ischiopubis, with the wider edge (1.4 times the width of the other end, which is shorter proportionally than that of Aegirosaurus,[11] Ophthalmosaurus, and Athabascasaurus[8]) being at the midline of the body.

This complete fusion is also seen in Sveltonectes,[16] Athabascasaurus,[8] Aegirosaurus,[11] Caypullisaurus,[19] and possibly Platypterygius australis;[10] meanwhile, Ophthalmosaurus, Undorosaurus,[4] and Nannopterygius retain a small hole in the ischiopubis.

[3] This specimen was notable for presenting unusual braincase morphology including the opening for the internal carotid artery on the posterior surface of the parabasisphenoid, which is a diagnostic trait of Arthropterygius, which was at the time only known from Canada.

From these, numerous new genera and species of ichthyosaur were described including Cryopterygius kristiansenae (later recognized to be synonymous with Undorosaurus gorodischensis[25]), Palvennia hoybergeti (named for PalVenn, the Friends of the Palaeontological Museum in Oslo, whose expedition led to the discovery of the type specimen),[4] and Janusaurus lundi (named for the mountain Janusfjellet, where the specimen was found).

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.

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.

[2] 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.

[25] While Zverkov and some others treat Janusaurus, Keilhauia and Palvennia within Arthropterygius, a study on the Jurassic Ichthyosaurs performed by Delsett et al. suggests the synonymy is not supported by the presence evidence.

The Slottsmøya Member of the Agardhfjellet Formation, from which fossils of A. chrisorum, A. lundi, and A. hoybergeti have been recovered, provides perhaps the best glimpse into the ecosystems and habitats this genus was a part of.

It consists of a mix of shales and siltstones and was deposited in a shallow water methane seep environment, near a patch of deeper marine sediment.

[29] Though direct evidence from Slottsmøya is currently lacking, the high latitude of this site and relatively cool global climate of the Tithonian mean that sea ice was likely present at least in the winter.

[31][32] In addition to Arthropterygius, the Slottsmøya Member presents a diverse assemblage of other marine reptiles, including the ichthyosaurs Undorosaurus gorodischensis, Nannopterygius borealis, and a partial skull attributed to Brachypterygius sp..[25][27][33] Additionally, 21 plesiosaurian specimens are also known from the site, including two belonging to the large pliosaurid Pliosaurus funkei, which would have been the apex predator of the ecosystem, three to Colymbosaurus svalbardensis, one to Djupedalia engeri, one to Ophthalmothule cryostea, and one each to Spitrasaurus wensaasi and S. larseni.

Many of these specimens are preserved in three dimensions and partially in articulation; this is correlated with high abundance of organic elements in the sediments they were buried in, as well as a lack of invertebrates locally.

[29] Material belonging to four species of Arthropterygius, A. chrisorum, A. volgensis, A. lundi, and A. hoybergeti, has been reported from the Volga region of Russia, which gives the Volgian stage its name.

[25] In addition to these three genera, fossils of the ichthyosaur Grendelius, the pliosaur Pliosaurus rossicus, and indeterminate remains belonging to a metriorhynchid, as well as a high diversity of ammonites including the large-bodied taxon Titanites, are also known from the Volgian-aged sediments of this region.

[35] It also features a uniquely diverse array of metriorhynchids, with species belonging to the genera Dakosaurus, Cricosaurus, Purranisaurus, and Geosaurus all having been reported.

[36][37] The extent to which the fauna reported from the Vaca Muerta Formation resembles that known from sites in the Northern Hemisphere, such as the Kimmeridge Clay, suggests that many marine reptile genera may have had near cosmopolitan distributions in the Late Jurassic.