Calvarial suture interdigitation in hadrosaurids (Ornithischia: Ornithopoda): Perspectives through ontogeny and evolution

Abstract

Lambeosaurine hadrosaurids exhibited extreme modifications to the skull, where the premaxillae, nasals, and prefrontals were modified to form their iconic supracranial crests. This morphology contrasts with their sister group, Hadrosaurinae, which possessed the plesiomorphic arrangement of bones. Although studies have discussed differences between lambeosaurine and hadrosaurine skull morphology and ontogeny, there is little information detailing suture modifications through ontogeny and evolution. Suture morphology is of particular interest due to its correlation with the mechanical loading of the skull in extant vertebrates. We quantify and contrast the morphology of calvarial sutures in iguanodontians and ontogenetic series of Corythosaurus and Gryposaurus to test whether the evolution of lambeosaurine crests impacted the mechanical loading of the skull. We found that suture interdigitation (SI) increases through ontogeny in hadrosaurids, although this increase is more extreme in Corythosaurus than Gryposaurus, and overall suture complexity (i.e., overall shape) remained constant. Lambeosaurines also have higher SI than other iguanodontians, even in crestless juveniles, suggesting that increased sinuosity is unrelated to the structural support of the crest. Hadrosaurines and basal iguanodontians did not differ. Similarly, lambeosaurines have more complexly shaped sutures than hadrosaurines and basal iguanodontians, while the latter two groups do not differ. Taken together, these results suggest that lambeosaurine calvarial sutures are more interdigitated than other iguanodontians, and although suture sinuosity increased through ontogeny, the suture shape remained constant. These ontogenetic and evolutionary patterns suggest that increased suture complexity in lambeosaurines coincided with crest evolution, and corresponding modifications to their facial skeleton altered the distribution of stress while feeding.

A new small duckbilled dinosaur (Hadrosauridae: Lambeosaurinae) from Morocco and dinosaur diversity in the late Maastrichtian of North Africa

“…6 a), the laterosphenoid ventrally. Although the sutures are visible, the frontals are tightly joined to each other and the parietal by a strongly interdigitating contact, similar to that seen in other lambeosaurines 52 . The degree of interdigitation is stronger than in Arenysaurus 50 , but not developed to the same degree as seen in Corythosaurus 52 .…”

“…Although the sutures are visible, the frontals are tightly joined to each other and the parietal by a strongly interdigitating contact, similar to that seen in other lambeosaurines 52 . The degree of interdigitation is stronger than in Arenysaurus 50 , but not developed to the same degree as seen in Corythosaurus 52 . In places the frontoparietal suture is well-developed, but in others is difficult to see, due either to tight knitting of bones or perhaps partial fusion of bones.…”

“…A similar morphology is seen in Arenysaurus 50 ; in Jaxartosaurus the parietal has a broad, quadrangular contribution to the dome. A similar triangular process contributing to the frontoparietal dome occurs in juvenile Corythosaurus but is reduced in adults 52 . In the basal hadrosauroid Eotrachodon , a triangular process projects between the frontals 55 , but does not contribute to the dome; the saurolophines Edmontosaurus 56 and Gryposaurus 52 lack this triangular process.…”

“…The parietal is broadly arched between the supratemporal fenestrae, with a narrow sagittal crest as in Hypacrosaurus 40 ; the parietal of Arenysaurus is narrower 50 . The parietal is moderately elongate, resembling Arenysaurus , Lambeosaurini 52 and Parasaurolophini 54 ; the parietal of Tsintaosaurus is shorter; the parietal of Saurolophinae tends to be more elongate 52 , 56 . Ventrolaterally, the parietal curls down to contact the orbitosphenoids, laterosphenoids, and prootics.…”

“…A similar morphology occurs in Arenysaurus , Jaxartosaurus , the lambeosaurin Corythosaurus 54 and the parasaurolophin Charonosaurus . The anterolateral processes project laterally in Saurolophinae 52 , 56 and Eotrachodon 55 . The parietal has a small, triangular parietal table and behind this a parietal sagittal crest; in Arenysaurus 50 the parietal lacks a parietal table and the sagittal crest extends further forward towards the frontals.…”