Shonisaurus is a very large genus of ichthyosaur. At least 37 incomplete fossil specimens of the marine reptile have been found in the Luning Formation of Nevada, USA. This formation dates to the late Carnian age of the late Triassic period, about 237–227 million years ago. Description Shonisaurus lived during the Carnian stage of the late Triassic period. With a large skull about long, S. popularis and measured around in length and in body mass. A second species from British Columbia was named Shonisaurus sikanniensis in 2004. S. sikanniensis was one of the largest marine reptiles of all time, measuring and weighing . However, phylogenetic studies later showed S. sikanniensis to be a species of Shastasaurus rather than Shonisaurus. A new study published in 2013 reasserted the original classification, finding it more closely related to Shonisaurus than to Shastasaurus. In the 2019 study, S. sikanniensis was pertained within the genus Shastasaurus. In the 2021 analysis, S. sikanniensis forms a clade with Shonisaurus, indicating that it is closer to Shonisaurus than to Shastasaurus. Specimens belonging to S. sikanniensis have been found in the Pardonet Formation British Columbia, dating to the middle Norian age (about 210 million years ago). Shonisaurus had a long snout, and its flippers were much longer and narrower than in other ichthyosaurs. While Shonisaurus was initially reported to have had socketed teeth (rather than teeth set in a groove as in more advanced forms), these were present only at the jaw tips, and only in the very smallest, juvenile specimens. All of these features suggest that Shonisaurus may be a relatively specialised offshoot of the main ichthyosaur evolutionary line. More recent finds however indicate that Shonisaurus possessed teeth in all ontogenetic stages. It was historically depicted with a rather rotund body, but studies of its body shape since the early 1990s have shown that the body was much more slender than traditionally thought. S. popularis had a relatively deep body compared with related marine reptiles. Although it is not known if Shonisaurus had one, a more basal ichthyosaur Mixosaurus had a dorsal fin. History Fossils of Shonisaurus were first found in a large deposit in Nevada in 1920. Thirty years later, they were excavated, uncovering the remains of 37 very large ichthyosaurs. These were named Shonisaurus, which means "lizard from the Shoshone Mountains", after the formation where the fossils were found. S. popularis, was adopted as the state