The evolution of cranial display structures in hadrosaurian dinosaurs

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Abstract
A theory is presented that cranial crests of hadrosaurs were visual and acoustical display organs. Facial morphology and phylogeny of the Hadrosauridae and earlier theories of crest function are reviewed. The following hypothesis is presented: cranial crests, whether hollow or solid, served as visual signal structures, and hollow lambeosaur crests were also vocal resonators; all crests promoted successful matings within species, i.e., they served as premating genetic isolating mechanisms. The following predictions are tested and found to support the hypothesis: (1) hadrosaurs had well-developed eyes and ears; (2) external features of crests varied independently of internal structure; (3) crest variations were species-specific and sexually-dimorphic; (4) crest distinctiveness correlates with species diversity; (5) crest size tended to increase through time. The circumnarial depression on the side of the face in hadrosaurines housed an inflatable diverticulum of the nasal passage which served as a visual display organ. Primitive hadrosaurs (kritosaurs) possessed a small nasal horn used as a butting weapon in intraspecific combat. Because the weapon was also used in intimidative displays, narial diverticula evolved to draw attention to it. In the kritosaur Brachylophosaurus fighting was modified to ritualized head-pushing using the flat nasal “shield”. Saurolophines expanded the diverticula on to the elongated nasal horn, converting the weapon to a dominance rank symbol. In non-crested edmontosaurs, enlarged diverticula assumed a vocalization function. Lambeosaurs created resonators by enclosing the diverticula in bone; they further enhanced the resonator function of the nose by forming elongated “organ pipes” in the premaxillae. This “pushed” the olfactory region above the eyes as a conspicuous dome which then was modified to form species-specific visual display organs.