The braincase of Phantomosaurus neubigi (Sander, 1997), an unusual ichthyosaur from the middle triassic of Germany

摘要

The structure of the braincase and the posterior portion of the palate of the type and only known specimen of Phantomosaurus neubigi (Sander, 1997) from the Upper Muschelkalk (Late Anisian) of Franconia (Southern Germany) is redescribed. P. neubigi is unique among hitherto known ichthyosaurs in numerous features of braincase morphology. These include an anteroposteriorly compressed, elongate and slender processus paroccipitalis, a clear sutural connection between the basioccipital and opisthotic, a foramen metoticum that is completely enclosed by the exoccipital and opisthotic, fusion between exoccipital and opisthotic, the retention of rudimentary postparietals -which are recorded in ichthyosaurs for the first time- and a concave occipital condyle (the latter feature is also known in a limited number of other Triassic ichthyosaurs). These characters of the Phantomosaurus braincase differ from post-Triassic ichthyosaurs, which are characterized by a strong reduction of braincase ossification, particularly with regard to the otic capsule. A short survey of current knowledge on ichthyosaur braincase anatomy as well as comparison to well-known basal amniotes and basal diapsids suggests, that the features shown by Phantomosaurus must largely be regarded as plesiomorphic and that its braincase is the most plesiomorphic ichthyosaur braincase currently known. This leads to a new view on the primitive condition of the ichthyosaur braincase that has to be considered in any further studies on ichthyosaur origins and evolution.