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 Shastasaurus 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. 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. 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.
Shonisaurus was also traditionally depicted with a dorsal fin, a feature found in more advanced ichthyosaurs. However, other shastasaurids likely lacked dorsal fins, and there is no evidence to support the presence of such a fin in Shonisaurus. The upper fluke of the tail was probably also much less developed than flukes found in later species.
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 fossil of Nevada in 1984. Excavations, begun in 1954 under the direction of Dr. Charles Camp and Dr. Samuel Welles of the University of California, Berkeley, were continued by Camp throughout the 60s. It was named by Charles Camp in 1976.
The Nevada fossil sites can currently be viewed at the Berlin-Ichthyosaur State Park.