The evolution of cranial display structures in hadrosaurian dinosaurs

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 matingswithinspecies,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 kritosaurBrachylophosaurusfighting 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.

Morphometric observations on hadrosaurid ornithopods

“…The posteroventral landmarks tend to be far more conservative. The greater average distances among the lambeosaurines (Table 12.3) is to be expected due to high variability in forms thought to exhibit strong social behavior and sexual dimorphism (see Dodson 1975;Hopson 1975;Molnar 1977;Chapman et al 1981).…”

“…This unusually high taxonomic diversity is a consequence of the interplay between the taxonomic philosophies of the many researchers studying hadrosaurs, the high level of real taxonomic diversity, the unusually abundant material available, and the high degree of morphological variability within populations and between age groups. The latter is the result of allometric and ontogenetic effects over a wide range of sizes (see Dodson 1975;Hopson 1975;Molnar 1977). Herein, we will present the results of a series of preliminary shape analyses of hadrosaur crania and pelves, and discuss these in the context of hadrosaur taxonomy, phytogeny, and identification.…”

“…In addition to this abundance, hadrosaurs exhibit a high degree of morphological variability, especially in cranial structures. These are thought by some to play a role in social behavior (Dodson 1975;Hopson 1975;Molnar 1977). Dodson (1975), for example, analyzed morphometric data for the crania of 36 specimens of lambeosaurine hadrosaurs referable to three genera {Corythosaurus, Lambeosaurus, and "Procheneosaurus") and 12 species.…”

A new saurolophine hadrosaurid (Dinosauria: Ornithopoda) from the Upper Cretaceous of Shandong, China

“…This lineage of dinosaurs includes some basal forms, also known as non-hadrosaurid hadrosauroids, and the monophyletic Hadrosauridea (PrietoMárquez 2010b, Wang et al 2013b). Hadrosaurids are characterized by duck-like bills and complex dentitions organized into dental batteries (Horner et al 2004); many species had hypertrophied nasal passages associated with a wide variety of supracranial crests, the nature of which forms the basis of their classification (Ostrom 1962, Hopson 1975, Horner et al 2004, Prieto-Márquez 2010b. Studies have shown that the Hadrosauridae can be divided into two main clades, the Saurolophinae on the basis of a sacrum and right ilium found in the same place as Tsintaosaurus (Zhen 1976).…”

Pararhabdodon isonensis and Tsintaosaurus spinorhinus: a new clade of lambeosaurine hadrosaurids from Eurasia

“…They are unique among ornithischians in having posterodorsally located nasal passages (Weishampel, 1981;Wagner, 2004) enshrouded by hollow supracranial crests. These crests manifest a bewildering diversity of shapes and sizes; their function may have been species recognition and sexual display (Hopson, 1975;Evans, 2006). The fossil record of the Lambeosaurinae in the uppermost Cretaceous (CampanianMaastrichtian) of North America is extensive (Wagner, 2001;Lund and Gates, 2006).…”