Reanalysis of the phylogenetic status of Nipponosaurus sachalinensis (Ornithopoda: Dinosauria) from the Late Cretaceous of Southern Sakhalin

Abstract

Nipponosaurus sachalinensis is the only definitive lambeosaurine hadrosaurid from Sakhalin Island of Russia. Previous studies suggested it was a member of Lambeosaurini (derived lambeosaurines). However, its phylogenetic status within Lambeosaurini remains controversial. In addition, some studies argued the juvenile ontogenetic stage of the holotype and regarded Nipponosaurus as an invalid taxon. In order to solve these problems, its definite growth stage is determined through histological studies. Absence of a line of arrested growth, presence of osteons with large vascular spaces, and presence of primary bone remnants even in the highly modified regions of the femur confirm that the holotype was a juvenile. More than a hundred of the 350 characters used to determine the phylogenetic position of Nipponosaurus are ontogenetically variable characters based on the different ontogenetic stages of Hypacrosaurus stebingeri. Our phylogenetic analysis reveals that Nipponosaurus is a basal lambeosaurine hadrosaurid, much further down in the tree than previously suggested, and shows a polytomy with Blasisaurus and Arenyisaurus. This study also indicates that Nipponosaurus is a valid taxon because it possesses unique characters within the Lambeosaurinae (presence of massive surangular anterodorsal process, presence of lateral shelf of the dentary, and a relatively short ulna), which are independent of ontogeny.

Keywords:

• Dinosaur
• Ornithopoda
• Hadrosauridae
• Nipponosaurus
• ontogeny
• histology

Acknowledgments

The authors are grateful to B. Strilisky of TMP, J. Scanella and J. Horner of MOR, M. Manabe and C. Sakata of NSM-VP, K. and Tsogtbaatar and C. Tsogtbaatar of MPC-D for access to collections and specimen information. The authors also thank I. Takahashi of Gobi Support Japan Co., Ltd for the creation of molds and casts, and to Hidehiko Nomura and Kousuke Nakamura of Hokkaido University Department of Science Technical office for preparation of thin sections. The first author is grateful to D. Suzuki of Sapporo Medical University Department of Anatomy, Japan for providing anatomical notions and for valuable discussions.