Shastasaurus pacificus (Merriam 1895, Late Triassic, Norian, UCMP 9017) is a large basal ichthyosaur closely related to the odd and much smaller Hupehsuchus. S. pacificus is not related to Shastasaurus sikanniensis. however, S. alexandrae (see below) is indeed related and was found in nearby strata. These giant taxa are late representatives of a clade that is not only transitional from basalmost ichthyosaurs, like Xinminosaurus and the more derived Utatsusaurus, but their Early Triassic ancestors appear to have been ancestral to hupesuchids.
The rostrum is unknown in both of these Shastasaurus specimens. One of them, S. pacificus, is the holotype for the genus, but several unrelated specimens have also been attributed to this genus by Sander et al. 2011. These include Shastasaurus sikanniensis and two short-snouted specimens. The YGMR SPC V3108 specimen nests with the basal ichthyosaur Mikadocephalus. The YGMR SPC V3107 specimen and S. sikanniensis nested with the toothy Cymbospondylus.
Yet another specimen attributed to Shastasaurus, S. tangae (Shang and Li 2009; IVPP V11853) nests with the derived Ichthyosaurus and Ophthalmosaurus.