Cranial anatomy of the Late Jurassic dwarf sauropod Europasaurus holgeri (Dinosauria, Camarasauromorpha): ontogenetic changes and size dimorphism

Abstract

Sauropods were the most successful herbivorous group of dinosaurs during the Mesozoic era. Despite their supremacy as reflected in the fossil record, sauropod skulls are very rare and current knowledge of skull anatomy is based on just a few taxa. Juvenile skull bones are even rarer than adult skulls; thus, our understanding of their morphology and ontogenetic changes is limited. The recent discovery of several adult and juvenile specimens of a Late Jurassic taxon from Germany, Europasaurus holgeri, extends our knowledge of sauropod skull anatomy. A total of 123 skull bones, representing at least 14 skulls, were examined, described and compared to other taxa. The skull material includes several individuals of various ontogenetic stages. Because size alone is not sufficient to determine the morphological ontogenetic stage (MOS), size-independent characters were used to stage the bone elements. Detailed studies of the skull bones proved that the material represents two morphotypes, independent of ontogenetic stage. Since the original description of Europasaurus, new skull material has been found, and an updated skull reconstruction of an adult individual is presented here. All the autapomorphic characters of Europasaurus recognized in the skull (i.e. anteroposteriorly long and lateromedially narrow frontal; presence of postparietal fenestra; large participation of the jugal to the ventral rim of the skull and the orbit; presence of a postparietal foramen and single optic foramen) are plesiomorphic characters of basal sauropodomorphs and/or present in embryos and juvenile sauropods. Therefore, we consider that in Europasaurus these characters evolved through paedomorphosis, which resulted in the dwarf condition of this taxon.

Keywords:

• Macronaria
• ontogeny
• maturity
• dimorphism
• dwarfism
• island rule
• heterocrony
• Late Jurassic

Acknowledgements

Part of this study comes from the Master's thesis of JSM, which would not have been possible without the generous support of many people. Early versions of this manuscript were improved by M. D'Emic, T. Martin and H. Petermann. J. Wilson is thanked for comments on the Abydosaurus skull. R. Romero for drawing the braincase for the figure 13. V. Régent helped with the skull reconstructions. O. Dülfer, P. Göddertz, G. Oleschinski and A. Schmitt are thanked for technical assistance, and R. Kosma (Staatliches Naturhistorisches Museum Braunschweig, Braunschweig, Germany) for granting JSM access to the material housed in their collection. Comments made by P. Mannion, an anonymous reviewer, and the editors helped improve early versions of this paper. J. Mitchell is thanked for English corrections. The authors gratefully acknowledge the financial support provided by the Deutsche Forschungsgemeinschaft (DFG) for this research (Project RE 2874/1–1 and SA 469/33-1). This is contribution no 160 of the DFG Research Unit 533 ‘Biology of the Sauropod Dinosaurs: The Evolution of Gigantism’. Comparisons with relevant material were possible thanks to collection access provided by: A. Kramarz (MACN); B. Pabst and H-J Siber (SMA); J. Ove R. Ebbestad (PMU); M. Brett-Surman (USNM); M. Brett-Surman (CM); D. Schwarz-Wings and F. Witzmann (MB.R.). Author contributions: NK, JSM, and JLC collected the data, all authors analysed the data, while JSM and JLC wrote the manuscript.