Temporal range: Late Cretaceous
The holotype (UHR 6590, University of Hokkaidō Registration) was discovered in November 1934 during the construction of a hospital for the Kawakami colliery of the Mitsui Mining Company on Karafuto Prefecture (now Sinegorsk, Sakhalin, Russia), and additional material belonging to the specimen was recovered in the summer of 1937. The type species Nipponosaurus sachalinensis was in 1936 named and described by Professor Takumi Nagao of the Imperial University of Hokkaido. The generic name refers to Nippon, the Japanese name of Japan; it was the first dinosaur named based on a find made on Japanese territory — though South Sakhalin was annexed by the Soviet Union in 1945. The specific name refers to Sakhalin. In 1938 Nagao dedicated a second article to the species.
The specimen was collected from Upper Yezo Group ("Upper Ammonites Bed) (late Santonian — early Campanian). UHR 6590 consists of a left maxilla and dentary, parietal, various isolated skull elements, thirteen cervical vertebrae, six dorsal vertebrae, two sacral vertebrae, a series of 35 caudal vertebrae, a left scapula, distal portions of both humeri, most elements of the lower forelimbs, an ischium, left ilium, and most of the hindlimbs. Though the quality of bone preservation is generally poor, the skeleton is estimated to be 60% complete. It nevertheless remains one of the most poorly known lambeosaurine dinosaurs.
Nipponosaurus was recently redescribed by Suzuki et al. (2004), who diagnosed the taxon as follows: "Robust coronoid process of the surangular, only slight development of the neural spine of the axis, and strong deflection of the lateral margin of the first phalanx of digit IV."
Suzuki et al. (2004) have determined that the holotype represents a subadult individual, measuring roughly four meters (13 feet) in length. A cladistic analysis (Suzuki et al., 2004; pp. 160–161) places Nipponosaurus sachalinensis very close to the well-known North American Hypacrosaurus altispinus within the Lambeosaurinae. They denied the taxon was a nomen dubium as has been concluded by several authors.
- Nagao, T. 1936. Nipponosaurus sachalinensis - A new genus and species of trachodont dinosaur from Japanese Saghalien. J. Faculty Sci. Hokkaido Imperial Univ. Ser. IV (2): 187-220
- Nagao, T. 1938. On the limb bones of Nipponosaurus sachaliensis Nagao, a Japanese hadrosaurian dinosaur. Annot. Zool. Japan 17(3/4):312-318.
- Suzuki D., Weishampel D. B., and Minoura N. 2004. Nippponosaurus sachalinensis (Dinosauria; Ornithopoda): anatomy and systematic position within Hadrosauridae. Journal of Vertebrate Paleontology 24(1):145-164.