At least 37 incomplete fossil specimens of the type species, Shonisaurus popularis, have been found in the Luning Formation of Nevada, USA.

All of these features suggest that Shonisaurus may be a relatively specialised offshoot of the main ichthyosaur evolutionary line.

[8] Robust sectorial teeth and gut contents indicate that Shonisaurus was a macrophagous raptorial predator which fed on vertebrates and shelled mollusks like cephalopods, possibly even large-bodied prey.

[13] A subsequent study by Ji and colleagues published in 2013 reasserted the original classification, finding it more closely related to Shonisaurus than to Shastasaurus.

[15][16] Specimens belonging to S. sikanniensis have been found in the Pardonet Formation British Columbia, dating to the middle Norian age.