Shonisaurus lived during the Carnian stage of the late Triassic period. S. popularis measured around long. A second species from British Columbia was named Shonisaurus sikanniensis in 2004. S. sikanniensis was one of the largest marine reptilesof all time, measuring. 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. Specimens belonging to S. sikanniensis have been found in the Pardonet Formation British Columbia, dating to the middleNorian age. 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, 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.
History
s 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 Charles Camp and Samuel Welles of the University of California, Berkeley, were continued by Camp throughout the 1960s. It was named by Charles Camp in 1976. The Nevada fossil sites can currently be viewed at the Berlin-Ichthyosaur State Park.
Bonebed interpretation
The Nevada bonebed represents a large assemblage of Shonisaurus which died at varying times and became preserved on the sea floor in a curiously regular arrangement of bones. The lack of fossil invertebrates encrusting the remains indicates that the carcasses sank in relatively deep water poor in oxygen.
