The skull of a large Lower Triassic ichthyosaur from Spitzbergen and its implications for the origin of the Ichthyosauria

An incomplete ichthyosaur skull from the Lower Triassic (Spathian) Sticky Keep Formation of Spitzbergen is one of the earliest ichthyosaurs known and by far the largest of the most basal ichthyosaurs. The specimen shows an extremely plesiomorphic cheek region of the skull and allows for the first time a confident reconstruction of the grundplan condition of the lower cheek region of ichthyosaurs. It unequivocally demonstrates the presence of a deep emargination of the lower cheek, which is bordered dorsally by the quadratojugal, squamosal and jugal, and therefore a plesiomorphic lack of contact between jugal and quadratojugal. The specimen also allows an evaluation of recent skull reconstructions of several Lower Triassic taxa which probably contain severe misinterpretations. The structure of the temporal and cheek region of the specimen can be most parsimoniously explained by the initial presence of a lower temporal fenestra. This provides important new evidence that the ancestry of ichthyosaurs might have lain within the Diapsida or their stem-group.