The braincase anatomy of <i>Simosaurus gaillardoti</i> (Diapsida: Sauropterygia) revealed with X-ray micro-computed tomography

Sauropterygia is a clade of Mesozoic marine reptiles that includes the eosauropterygian Simosaurus gaillardoti Von Meyer, 1842, classically considered to be a member of Nothosauroidea. The braincase of this species has thus far only been studied in acid-prepared specimens. Acid preparation is a destructive technique prone to information loss, e.g., through the dissolution of thin braincase bones. Here, one well-preserved skull (SMNS 16363) that remains partially embedded in matrix has been visualised using X-ray micro-computed tomography, and the braincase region has been virtually extracted. This braincase provides valuable information on the general shape of the endocast, the existence and shape of epipterygoids, which were previously considered absent in the taxon, the course of cranial nerves and the bifurcation of the internal carotid arteries along an expanded and broad parabasisphenoid, the latter extending in a tapering cultriform process to the level of the external and internal narial openings. The arrangement of the semicircular canals of the inner ear confirms previously hypothesised adaptations for near-shore aquatic life in the species. The anatomical similarities of the braincases between Simosaurus gaillardoti and Nothosaurus marchicus, including a jugular foramen that is framed by the exoccipital medially and by the opisthotic laterally, support the current phylogenetic placement of the former as an early branching member of Nothosauroidea. The cranial flattening observed in nothosaurs relative to the less dorsoventrally flattened skull of Simosaurus reflects diverging feeding strategies. Most nothosaurs were fish-trap ambush predators, whereas Simosaurus gaillardoti had durophagous, as well as (opportunistic) piscivorous capacities. These results might indicate that specialised piscivorous predation using fish-trap dentition could be independently derived in nothosaurs and in pistosauroids (including plesiosaurs).