On the pterosaur Aerotitan sudamericanus (Neuquén Basin, Upper Cretaceous of Argentina), with comments on azhdarchoid phylogeny and jaw anatomy

Aerotitan sudamericanus, from the Upper Cretaceous of the Neuquén Basin (Patagonia, Argentina), is known from a partial jaw fragment which has been interpreted as either an azhdarchid upper jaw, azhdarchid lower jaw, or thalassodromine upper jaw (as the sister-group of Alanqa). Here, we compare it in detail to upper and lower jaws of taxa belonging to all azhdarchoid lineages. It possesses a lateral angle (angle of divergence between occlusal and apex margins in lateral view) that is too low for an upper jaw of any azhdarchoid group. It further differs from thalassodromine upper jaws in exhibiting a convex occlusal margin (in lateral view), a sulcate occlusal surface, and lacking a sagittal crest. Furthermore, Aerotitan differs from Alanqa in 5 aspects: (1) occlusal margin shape in lateral view (convex in Aerotitan, straight in Alanqa), (2) median dentary eminence shape (slender in Aerotitan, posteriorly expanded in Alanqa), (3) median dentary eminence position (anterior in Aerotitan, close to the posterior end of the symphysis in Alanqa), (4) tomial edges shape (thick and blunt in Aerotitan, thin and sharp in Alanqa), and (5) occlusal surface anterior to the median eminence (cross-section concave in Aerotitan, slightly convex in Alanqa). We also conclude that the holotype of A. sudamericanus is a match for an azhdarchid lower jaw, being extremely similar to that of Mistralazhdarcho. When scored as a lower jaw in our phylogenetic analysis, it is recovered as a close relative of Mistralazhdarcho, in a polytomy that also includes Arambourgiania. In contrast, Alanqa is recovered as the sister-group of Keresdrakon, both located at the base of a broader clade of long-snouted azhdarchoids that also includes chaoyangopterids and azhdarchids, to the exclusion of tapejarines and thalassodromines.