Postcranial axial skeleton of Europasaurus holgeri (Dinosauria, Sauropoda) from the Upper Jurassic of Germany: implications for sauropod ontogeny and phylogenetic relationships of basal Macronaria

Abstract

Neosauropods are well represented in the Late Jurassic fossil record, both in Laurasia and Gondwana. Among Macronaria, Europasaurus represents one of the most basal forms of this group. In addition to its systematic importance, Europasaurus is also the first unequivocal dwarf sauropod from which adult and juvenile material is available. Despite the abundance of sauropods in the fossil record, early juvenile specimens are rare, limiting knowledge about sauropod ontogeny. Therefore, the great amount of material of Europasaurus provides an excellent opportunity to improve our knowledge on the early evolution of Macronaria, as well as to shed light on some morphological changes through ontogeny. The postcranial axial skeleton of sauropods is extremely modified with respect to the anatomy observed in its ancestors, the ‘prosauropods’, proving to be one of the most informative regions of the body. Here we provide a detailed description of the axial skeleton of Europasaurus, including adult and juvenile elements, discussing its systematic and ontogenetic importance. We also analyse the phylogenetic position of Europasaurus through a cladistic analysis using TNT, which retrieves this taxon in a basal position among Camarasauromorpha. Additionally, the presence/absence of discrete characters and the comparison of juvenile elements with adult specimens allowed us to recognize different morphological ontogenetic stages (MOS). Whereas early stages lack derived characters (e.g. spinodiapophyseal lamina and prespinal lamina on dorsal vertebrae), all derived characters (including autapomorphies) are present in late immature specimens. Therefore, while late immature specimens provide the same phylogenetic signal as adult specimens of Europasaurus, more immature stages are recovered in a basal position among sauropods. Finally, we apply the MOS to other maturity criteria (e.g. neurocentral closure, sexual maturity) in a search for a wider definition of maturity.

Keywords:

• Macronaria
• ontogeny
• maturity
• phylogeny
• dimorphism
• Late Jurassic

Acknowledgements

We especially want to thank N. Knötschke for complete access to and preparation of the material. Comments and suggestions made by L. Salgado, D. Pol, K. Stein and S. Marpmann and other members of the Bonn working group during the preparation of this manuscript are greatly appreciated. Jessica Mitchell is thanked for English corrections. Comments made by P. Mannion, an anonymous reviewer and the editor helped improve an early version of this paper. 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), and FoncyT PICT 2006–1756 (to D. Pol). This paper is contribution number 134 of the DFG Research Unit 533 ‘Biology of the Sauropod Dinosaurs: The Evolution of Gigantism’. Comparisons with relevant material was possible thanks to the collection access provided by: R. Coria (MCF-PVPH); A. Kramarz (MACN); E. Ruigomez (MEF); A. Garrido (MOZ); J. Calvo and J. Porfiri (MUCPv); J. Powell (PVL); L. Giraldez (CHMO); B. González Riga, (IANIGLA); I. Cerda (MCS); R. Royo Torres (MPZ, CPT, CL and CLH); O. Mateus (MIGM and ML); B. Pabst and S. Hans-Jakob (SMA); J. Nour-Eddine (CPSGM); N. Knötschketo (DFMMh); D. Schwarz and F. Witzmann (HMN); J. Ove R. Ebbestad (PMU); F. Torcida (MPS-RV-II); B. Battail (MNHN); X. Xu (IVPP); G. Peng and S. Jiang (ZDM); C. Ito and M. Brett-Surman (USNM); W. F. Simpson (FMNH); A. Henrici (CM); and J. Canale and A. Haluza (MMCH).

Supplementary material

Supplementary material is available online DOI: 10.1080/14772019.2013.764935