A new genus of ichthyosaur from the Late Triassic Pardonet Formation of British Columbia: bridging the Triassic-Jurassic gap

摘要

Both the genus Shastasaurus and the family Shastasauridae have long been hard to define due to the fragmentary nature of the type specimens. Consequently, recent interpretations of the genus have been based almost entirely on Shastasaurus neoscapularis from the Late Triassic Pardonet Formation of British Columbia. Two new specimens of this taxon, from Pink Mountain, British Columbia, demonstrate that it does not belong in the genus Shastasaurus. This paper describes the new specimens, and refers the species to Metashastasaurus gen nov. Post-cranially, the skeleton of Metashastasaurus resembles that of shastasaurids, differing primarily only in the shape of the scapula and fibula. However, the skull has a unique combination of characters, including large diamond-shaped frontals that enter the supratemporal fenestrae, and very narrow posterior extensions of the nasals, which contact the postfrontals. It also differs from the skull of Shastasaurus in the presence of both a parietal ridge and postparietal shelf. This is a combination of derived characters previously known only in Jurassic forms. The front limb has four proximal carpals and four digits, indicating that previous reconstructions were based on incomplete material. Shastasaurus pacificus Merriam 1895, the type species of the genus Shastasaurus, must be considered a nomen dubium, making the genus Shastasaurus invalid. Until this problem is clarified, the use of the generic name Shastasaurus should be restricted to Merriam's type specimens, of which only Shastasaurus alexandrae and Shastasaurus osmonti are based on adequate material.