Evolutionary Relationships and Flight Capacity of Late Cretaceous Pterosaurs Elucidated by New Azhdarchoid Pterosaur Remains From Afro-Arabia

Abstract

Pterosaurs were the earliest and largest vertebrates to evolve powered flight, but they are the only major volant group that has gone extinct. Attempts to understand pterosaur flight mechanics have relied on aerodynamic principles and analogy with extant birds and bats. Both of these lines of inquiry rely on the size, three-dimensional shape, and internal structure of flight bones, which in pterosaurs are surprisingly rare. Remarkably, two new large-bodied pterosaur individuals with three-dimensionally preserved wing elements were recently recovered from the Late Cretaceous (Maastrichtian) of Jordan. Both specimens represent azhdarchoid pterosaurs and are described in this study; one is referrable to the giant species Arambourgiania philadelphiae (ca. 10 m wingspan) and the second to a new, smaller species Inabtanin alarabia gen. et sp. nov. (ca. 5 m wingspan). The pterosaur fossil record is highly variable, which confounds attempts to understand pterosaur diversity and relationships particularly for the groups that lasted to the latest Cretaceous, like the azhdarchids and their closest relatives. Recent studies challenge the traditional understanding that the azhdarchids were the only pterosaur lineage surviving into the Maastrichtian. This complicates our ability to identify new remains and to understand changes in diversity leading up to the Cretaceous-Paleogene extinction. Here, we address this problem by systematically assessing existing character matrices and taxon sampling to be more inclusive of the latest Cretaceous pterosaur fossil record. A parsimony analysis conducted in TNT results in a topology that recovers Inabtanin alarabia as a member of the clade Azhdarchidae. Our topology implies a narrower membership for that group than recovered by previous analyses, with some previously suggested azhdarchids falling outside the phylogenetic definition of the group. In contrast, Azhdarchiformes includes all taxa previously included with in that group. Traditional characters defining Azhdarchidae, such as elongation of cervical vertebrae and long wingspan, appear to be gained independently in multiple groups; the structure of the humerus is recovered as a synapomorphy of Azhdarchiformes. With this updated understanding of the evolutionary relationships of pterosaurs, we will have a stronger framework for the identification of new latest Cretaceous pterosaur remains. Finally, we use high-resolution micro-computed tomography scans to reconstruct and compare the internal osteology of the humeri of these two differently sized species to that of extant birds, for which internal bone structure correlates with flight behavior. The humerus of Arambourgiania exhibits a series of helical ridges formed along the cortical bone, whereas Inabtanin exhibits a denser pattern of hollow struts. Variation in internal structure for these individuals likely reflects responses to mechanical forces applied on the wings of pterosaurs. Results indicate that Inabtanin has internal bone morphology similar to that of continuously flapping birds, whereas the internal morphology of Arambourgiania is most similar to that of flapping-soaring birds.