Braincase redescription of Dysalotosaurus lettowvorbecki (Dinosauria, Ornithopoda) based on computed tomography

ABSTRACT

Computed tomography (CT) has become a major tool to elucidate the anatomy of fossil taxa, including the braincase and inner ear. However, sample size is still limited and studies have concentrated mainly on saurischian dinosaurs. Here we report on the braincase anatomy of the Upper Jurassic ornithopod Dysalotosaurus using high-resolution X-ray micro-CT (μCT). The main trunk of the vena capitis dorsalis was found between the parietal and supraoccipital, leading to a nomenclatural revision of cephalic veins and reidentification of their bony correlates. Contrary to the condition in hadrosaurid ornithischians, the posterior semicircular canal is the shortest canal in Dysalotosaurus, whereas the lateral is the longest, lying dorsal to the vestibule, as in basal archosaurs. The anteroventral inclination of the lateral semicircular canal suggests that Dysalotosaurus must have oriented its head dorsally while alert. Because the cochlea is relatively short, Dysalotosaurus had reduced ability to discriminate between high- and low-frequency sounds, as in sauropods, basal ceratopsians, and other basal archosaurs. The metotic foramen, however, was fully subdivided into a fenestra pseudorotunda and a vagus foramen, a derived character for archosaurs that is usually associated with a more refined sense of hearing. Lack of a refined capacity for discrimination of sound frequencies due to a short cochlea together with the equal-sized and thick semicircular canals represent plesiomorphic characters for Dinosauria. The middle and inner ears of Dysalotosaurus bear a mosaic of primitive and derived features, pointing to a more complex evolutionary history of these structures.

ACKNOWLEDGMENTS

We would like to thank O. Rieppel for discussion, E. Maxwell for discussion and help with the figures, D. Gower for critically reading an earlier version of the manuscript, D. Schwarz-Wings for access to the specimens, and C. Radke for photography of the material. We likewise thank The Royal Society and the Paläontologische Gesellschaft for the help with the figure copyrights. We are also very grateful to the Senior Editor P. Barrett, A. T. McDonald, and an anonymous reviewer for comments and suggestions that helped improve the manuscript. This project is supported by the DAAD and CAPES program with funding provided by CAPES (BEX 3474/09-7) to G.S.

Handling editor: Paul Barrett