The skull and taxonomy of Mixosaurus (Ichthyopterygia)

Mixosaurus, a Middle Triassic ichthyopterygian, has traditionally been recognized as a typically primitive ichthyopterygian, but this view was recently questioned. Examination of the skull of Mixosaurus revealed many features to confirm this doubt. Its cranial characters, such as the parietal ridge and short supratemporal process of the parietal are shared with derived ichthyopterygians from the Jurassic. Mixosaurus also has features that are derived within the Ichthyopterygia and unique to the genus, such as a long sagittal crest reaching the nasal, and an expanded anterior terrace of the upper temporal fenestra that also reaches the nasal. Phalaradon, Contectopalatus, and Sangiorgiosaurus are all considered as junior synonyms of Mixosaurus. The supratemporal, squamosal, and quadratojugal are all present in Mixosaurus atavus, as in basal ichthyopterygians. Because all basal ichthyopterygians, and even some derived ichthyopterygians from the Jurassic, have these three elements, the absence of the squamosal in Ichthyosaurus and Platypterygius should be considered derived if it is not preservational.