Giant Flying Jaws: Aerodynamic Effects and Constraints on Cranial Hypertrophy in Pterosaurs
Authors: Habib Michael 
Published: April 2020

Pterosaurs are most well‐known for their expanded and altered forelimbs, which supported muscular membrane wings. Indeed, pterosaurs possessed one of the most derived hand morphologies of any vertebrate animal. However, several pterosaur lineages also possessed enormously expanded cranial and cervical proportions. The most extreme cases occur within ornithocheiroid and azhdarchoid pterosaurs, for which the cranial length, even excluding display crests, regularly exceeded 3.5 times the shoulder to hip distance.The enlarged heads and necks of many pterosaurs had implications for their flight and ecology. Understanding the costs and mechanisms of cranial hypertrophy in pterosaurs also has implications for understanding the evolution of the vertebrate skull, more generally. Investigating this problem is made particularly challenging by the fact that pterosaurs have no living descendants. A numerical modeling approach, using first principles, is likely the most effective way to investigate the evolution of cranial hypertrophy in pterosaurs.I addressed the performance impacts of cranial expansion in pterosaurs through a modeling approach combining aeromechanics and beam mechanics. I focused particularly on azhdarchids. These include some of the most extreme cases of cranial hypertrophy and possessed cervical anatomy amenable to beam theory applications (azhdarchids possessed highly derived cervical vertebrae with an elongate, tubular morphology). I used CT imaging to resolve cortical bone thickness in key elements. I created updated soft tissue reconstruction estimates, and I then applied both volumetric and scaling methods to estimate body masses and mass distribution.The results of this analysis provide new insights regarding the soft tissue morphology, flight dynamics, and wing positions of large pterosaurs. While the skulls were relatively light, the neck of azhdarchoid pterosaurs may have been much more robust than previously suggested. The Relative Cantilever Failure Force (RCFF) values of the cervical vertebrae range from roughly equal to nearly double that of the humeri. The expanded skull and neck of azhdarchid pterosaurs did have a substantial impact on center of mass (COM). To align the center of lift with the forward center of mass, a forward sweep of the wings was required. Rather than being a costly wing position, this would have improved low speed performance by delaying stall of the wingtip. Stability could be achieved dynamically through leading edge adjustments of the propatagium and passively with a wing dihedral and inboard reflex camber. Pterosaurs therefore may have obtained flight advantages from their expanded cranial and cervical proportions, in addition to any advantages to signaling or feeding.I suggest that cranial hypertrophy in pterosaurs is best explained as a case of constraint release, rather than an unusual selective regime. Other flying animals would probably obtain similar advantages from a forward center of mass, but developmental, anatomical, and ecological constraints most likely prevent extreme increases in head size in other flying taxa.Support or Funding InformationI wish to thank Gretchen Augustyn and Family for their financial support of research activities in the Dinosaur Institute of the NHMLAInstallation of two life sized Quetzalcoatlus sculptures by Blue Rhino Studio, demonstrating the giant size and extreme cranial proportions of these pterosaurs.Figure 1