The hadrosaurid Nipponosaurus sachalinensis Nagao, 1936, was the first dinosaur discovered in Japan. Here we establish its ontogenetic stage, diagnostic characters, and systematic position by redescribing and phylogenetically analyzing the type and sole specimen of Nipponosaurus. The unfused neurocentral sutures of all vertebrae suggest that the type specimen of Nipponosaurus is subadult. Furthermore, it can be diagnosed by the following characters: (1) the coronoid process of the surangular is robust, (2) the neural spine of the axis is only slightly developed, and (3) the lateral margin of the first phalanx on digit IV is strongly deflected. The phylogenetic analysis, using 78 characters for seven ingroup and six outgroup taxa, indicates that Nipponosaurus belongs within Lambeosaurinae as the sister taxon to Hypacrosaurus altispinus.
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