The oldest well-known hadrosaurid fauna (from Iren Dabasu, People's Republic of China) includes undescribed cranial material, primarily the prefrontals, which indicates the presence of a true lambeosaurine that bore a hollow supracranial crest. The lambeosaurine taxon Bactrosaurus johnsoni Gilmore (1933) is redefined based on this material, while the hadrosaurine skull elements formerly attributed to this form are now assigned to Gilmoreosaurus mongoliensis.
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Weishampel et al. (1986) studied this question.
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