Shonisaurus is a genus of ichthyosaur. At least 37 incomplete 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 215 million years ago. Shonisaurus lived during the Norian stage of the late Triassic period. S. popularis measured around 15 metres (49 ft) long. A second species from British Columbia was named Shonisaurus sikanniensis in 2004. S. sikkanniensis was one of the largest marine reptiles of all time, measuring 21 metres (69 ft). However, phylogenetic studies later showed S. sikanniensis to be a species of Shastasaurus rather than Shonisaurus. Shonisaurus had a long snout, and its flippers were much longer and narrower than in other ichthyosaurs.
Contributions by Dinoguy2, Circulationsys, and J. Spencer.