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

Abstract

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 suturai 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. For all these features, Mixosaurus is much closer to basal amniotes and basal diapsids, such as Captorhinus, Procolophon or Youngina than to more highly derived ichthyosaurs. The new data on the otic capsule of Mixosaurus are corroborated by the braincase of another Middle Triassic ichthyosaur, Phantomosaurus. It can be assumed that the reported features of the Mixosaurus braincase, which are unusual compared with other known ichthyosaurs, could be plesiomorphies retained from an unknown ichthyosaur ancestor, that is representative of the grundplan of ichthyosaurs. However, in the absence of further material the possibility that some features of the mixosaurid otic capsule are autapomorphic within ichthyosaurs can not be excluded.