Reanalysis of the phylogenetic status of <italic>Nipponosaurus sachalinensis</italic> (Ornithopoda: Dinosauria) from the Late Cretaceous of Southern Sakhalin.

<italic>Nipponosaurus sachalinensis</italic> 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 <italic>Nipponosaurus</italic> 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 <italic>Nipponosaurus</italic> are ontogenetically variable characters based on the different ontogenetic stages of <italic>Hypacrosaurus stebingeri</italic>. Our phylogenetic analysis reveals that <italic>Nipponosaurus</italic> is a basal lambeosaurine hadrosaurid, much further down in the tree than previously suggested, and shows a polytomy with <italic>Blasisaurus</italic> and <italic>Arenyisaurus</italic>. This study also indicates that <italic>Nipponosaurus</italic> 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. [ABSTRACT FROM AUTHOR]