When describing "Psittacosaurus mongoliensis" in 1923, Osborn also gave the name "Protiguanodon mongoliense" to another skeleton found nearby, believing it to represent an ancestor of the ornithopod "Iguanodon", in the new subfamily Protiguanodontinae. "Protiguanodon mongoliense", AMNH 6523, measured long, and was known from much of the skeleton, although at the time of description the neck vertebrae were still covered by matrix. Osborn diagnosed his taxa on the basis of features of the teeth and snout. However, modern taxonomists find these features insignificant, instead placing "Protiguanodon mongoliense" within "Psittacosaurus mongoliensis". When the skeleton was prepared further, it became clear that it was nearly identical to "Psittacosaurus mongoliensis". In 1958, Chinese paleontologist Yang Zhongjian (better known as C. C. Young) renamed the skeleton "Psittacosaurus protiguanodonensis". Today the specimen is generally referred to as the species "Psittacosaurus mongoliensis" and the names "Protiguanodon mongoliense" and "Psittacosaurus protiguanodonensis" are considered junior synonyms of the name "Psittacosaurus mongoliensis", which was coined first.