The braincase and inner ear of Placodus gigas (Sauropterygia, Placodontia)—a new reconstruction based on micro-computed tomographic data

ABSTRACT

Placodus gigas is an unarmored placodont marine reptile (Diapsida, Sauropterygia) known from the Middle Triassic of Europe, most commonly found in the shallow marine carbonate facies of the German Muschelkalk (lower Anisian to middle Ladinian, approximately 243–235 Ma). Generally, the morphology of the skull is well understood, with the exception of the braincase, which is partially obscured by dermatocranial bone. Two skulls that display well-preserved and intact chondrocranial elements were scanned using industrial micro-computed tomography (μCT), thus revealing parts of the braincase that were previously obscured and allowing a new three-dimensional reconstruction of the region to be constructed. This includes a complete osteological description, the first reconstruction of a sauropterygian vestibular apparatus, and a new virtual cranial endocast. The morphology of the braincase and sphenoid region has been revised, revealing the position of the hypophyseal pit. The enigmatic ‘alisphenoid bridge’ has been reinterpreted as being a dorsally extended dorsum sellae. The vestibular apparatus is shown to have strongly dorsoventrally compressed vertical semicircular canals, a commonly observed morphology of other marine reptiles, and an ‘alert’ head position of about 20°, indicating a highly aquatic lifestyle. Because placodonts are the sister group to all other sauropterygians, these new data are of great comparative and phylogenetic significance, providing insight into some of the morphological and functional changes that occurred on the stem leading to the more derived sauropterygians.

ACKNOWLEDGMENTS

Sincere thanks go to J. Rabold and S. Eggmaier (UMO, Bayreuth) as well as M. Moser and O. Rauhut (BSPG, Munich) for allowing access to the specimens and giving permission for them to be scanned. We are indebted to S. Nosotti, N. M. Gardner, F. R. O’Keefe, and an anonymous reviewer for their helpful comments that improved the quality of this publication. Our thanks also go to L. Rager (PIMUZ, Zurich) and A. Caromel (University of Bristol) for help with model segmentation, and the members of the PIMUZ for useful discussions and technical support. We are also grateful to W. Leis (Hochschule Aalen) for aiding us with the CT scanning itself. This study was funded by the Swiss National Science Foundation (grant no. 31003A 127053 to T.M.S.).

Handling editor: Robin O'Keefe