The otic capsule of the Middle Triassic ichthyosaur Mixosaurus from Monte San Giorgio (Switzerland): New evidence on the braincase structure of basal ichthyosaurs

For the first time, the otic capsule (proötic and opisthotic) of a Triassic ichthyosaur is described, based on a specimen of Mixosaurus cf. cornalianus from the Grenzbitumenzone of Monte San Giorgio, Switzerland. The otic capsule of Mixosaurus differs from that of more highly derived ichthyosaurs by several features, which are mainly related to the much higher degree of ossification found in Mixosaurus. The proötic is extensively ossified and shows an anteroventral facet for contact with the basisphenoid, a crista prootica lateralis, a foramen completely enclosed by bone for the exit of the nervus facialis, and a posterior sutural contact with the opisthotic, thus closing off the lateral wall of the internal ear capsule. The proötic also forms the anterior margin of a well ossified fenestra vestibuli and has a posterodorsal articulation facet for the supraoccipital. The opisthotic has an elongate, anteroposteriorly compressed, plate-like processus paroccipitalis and shows a well ossified crista interfenestralis, forming the posterior margin of the fenestra vestibuli and the anterior margin of the foramen metoticum. The anterodorsal contact with the supraoccipital is well defined.