Nipponosaurus (meaning "Japanese lizard") is a lambeosaurine hadrosaur from sediments of the Yezo Group, in Sinegorsk on the island of Sakhalin, which was part of Japan at the time of the species' classification. The type and only species is N. sachalinensis, known only from a single juvenile specimen discovered in 1934 and named in 1936, by Takumi Nagao, with further material of the same individual found in 1937. Since then, the taxon has been largely ignored, and its validity has been doubted, with synonymy with other Asian hadrosaurs or status as a nomen dubium being suggested. Redescriptions from 2004 and 2017, however, have supported recognition as a distinct species. Dating the only specimen has been difficult, but based on associated mollusc taxa, the species likely lived sometime in the upper Santonian or lower Campanian, around 80 million years ago.
History of study
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). It was disarticulated when discovered. N. sachalinensis was named and described in 1936 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. Although being relatively complete at the time of discovery, the specimen was missing much of the skull and limbs. A second expedition was organised in the summer of 1937, in which limb material pertaining to the holotype was recovered. The next year, Nagao authored a second article on the species describing these additional remains. Nipponosaurus was described as a member of the Trachodontidae; however Trachodon is now considered an invalid nomen dubium. The family is now known as the Hadrosauridae. Among its relatives, Nagao thought it was possibly closest to the taxa Cheneosaurus and Tetragonosaurus, even suggesting that his species might later prove to be congeneric with one of them. However, these were later determined to be juvenile forms of Hypacrosaurus, Corythosaurus and Lambeosaurus, and not distinct species. For the remainder of the 20th century, the taxon was scarcely mentioned. While they were still considered valid species, the relationship with the supposed "cheneosaurs" Cheneosaurus and Procheneosaurus was accepted; after this it was rarely assigned past the Lambeosaurinae. A humerus from a pit near Hashima Island, Japan was assigned to Nipponosaurus, variously referred to as Trachodon, in 1967. No further studies have acknowledged the specimen, and the referral has been ignored, with the species still considered to be represented by a single specimen. In 1977, a review of Asian dinosaur palaeontology mentioned it as a possible close relative of Mandschurosaurus without comment. Later in 1989, a study noted that features of its metacarpals could indicate a basal position within the Hadrosauridae. Near the turn of the century, in 1994, a review of Japanese dinosaurs noted the incomplete nature of many Asian hadrosaurs could mean some of them, including Nipponosaurus, may in fact be representatives of the same species. More extensive research has been conducted in the 21st century. In 2004, the species was redescribed, with focus on its growth stage. They concluded it was a valid taxon, and that the holotype was immature. More recently, a 2017 study provided further descriptions of the specimen, and conducted a microscopic examination on sections of the left femur, a rib, and an isolated chevron, which re-affirmed the age of the holotype.
Description The immature holotype specimen measures roughly 4 metres (13 feet) in length. When it was originally described, it was considered to be an adult, because of its co-ossified sacral vertebra. Future authors doubted this, merely citing its small size. In the 2004 redescription, it was examined more closely and several characteristics were found that identified it as an juvenile individual. Later, in 2017, another team re-evaluated the specimen, and determined through investigating the fossils at a microscopic level (in a process called histology) that this was correct, although they doubted the significance of some of the characters identified in the former study. Though the quality of bone preservation is generally poor, the holotype skeleton is estimated to be 60% complete. It 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, lower portions of both humeri, most elements of the lower forelimbs, an ischium, left ilium, and most of the hindlimbs.
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