A new juvenile specimen of Yunnanosaurus robustus (Dinosauria: Sauropodomorpha) from Early to Middle Jurassic of Chuxiong Autonomous Prefecture, Yunnan Province, China

Abstract

An almost complete skeleton with partial cranial material (ZMNH-M8739) is recovered from the Early or Middle Jurassic of southwest China. ZMNH-M8739 is identified as a juvenile individual of basal sauropodomorph dinosaur, Yunnanosaurus robustus Young, 1951. The revised diagnoses are as follows: absence of anteroposterior expansion on the medial end of astragalus and dorsoventrally compressed medium shaft of the metatarsal IV. Unfused neural arch and finely grooved long bone surface texture indicate that this individual is in the immature growth stage. ZMNH-M8739 possesses the tooth–tooth wear facet on its mesial maxillary and dentary teeth. However, the distal maxillary teeth have coarse serrations. Such a characteristic dentition could represent a unique feeding mechanism of this animal. Finally, ZMNH-M8739 constitutes a monophyletic group with Y. robustus (holotype), and Y. huangi is nesting this clade in the phylogenetic tree of the present analysis. Comparison of juvenile and adult specimen reveals distinctive growth changes of Y. robustus. This clade is positioned in an unnamed clade at a sister taxon of Sauropoda. Finally, some members of the so-called prosauropod dinosaurs constitute a monophyletic group in the present result.

Keywords::

• Sauropodomorpha
• Yunnanosaurus robustus
• juvenile
• Early to Middle Jurassic
• Chuxiong
• Yunnan

Acknowledgements

The authors thank to Ms Ding Ming and Mr Wang Qiang (ZMNH) for the preparation and arrangement of our observation, and Prof. Guangzhao Peng for partial economical support for the observation of the materials. Special thanks are due to personnel of Chuxiong Autonomous Prefecture for their field activities. We are grateful to Dr A Yates, Dr P. Mannion and the editors to evaluate the manuscript.