Two distinct hadrosaurian lineages can be delineated, each showing changes from possible hypsilophodont ancestors. The Hadrosaurinae are typified by moderately vaulted palates, broad pterygoid flanges, large anterior maxillary processes, and non-folded premaxillae, with direct narial passages. The Lambeosaurinae are characterized by highly vaulted palates, narrow pterygoid flanges, reduced anterior maxillary processes, and folded premaxillae and nasals with looped narial passages.Reconstruction of the palate indicates that the internal nares were displaced rearward, thus producing a structure similar to the secondary palate of mammals.The Constrictor Dorsalis group-muscles were well developed, allowing some minor lateral movement of the palate by slight torsion of the pterygoid. The large pterygoid flange would have prevented fore-and-aft movement of the palate.A reappraisal of the relationships between the lambeosaurine crests and the respiratory system indicates that the crests may have been used in developing individually recognizable calls. Previously unrecognized behavioural relationships may have been typical of these possibly endothermic animals.