"S. sikanniensis" was originally described in 2004 as a large species of "Shonisaurus". However, this classification was not based on any phylogenetic analysis, and the authors also noted similarities with "Shastasaurus". The first study testing its relationships, in 2011, supported the hypothesis that it was indeed more closely related to "Shastasaurus" than to "Shonisaurus", and it was reclassified as "Shastasaurus sikanniensis". However, a 2013 analysis supported 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 middle Norian age (about 210 million years ago).