Azhdarchid pterosaur diversity in the Bayanshiree Formation, Upper Cretaceous of the Gobi Desert, Mongolia

Pterosaur remains are very rare in Mongolian Mesozoic deposits, in stark contrast with the great abundance of dinosaur fossils in the region. This contribution presents a reassessment of the azhdarchid pterosaur remains from the Bayanshiree Formation's "upper beds" (Turonian-Santonian), represented by two specimens coming from two distinct localities: the Burkhant and the Bayshin Tsav azhdarchids. These specimens, collected by the Japanese-Mongolian Joint Paleontological Expedition and originally described in 2009, have been previously interpreted as indeterminate azhdarchids. Under the light of current knowledge on the morphological diversity of azhdarchid cervical vertebrae, as well as on the taxonomic and phylogenetic signals these skeletal elements carry, we herein identify diagnostic features and reassess the phylogenetic affinities of the Bayanshiree azhdarchids in further detail. Our results suggest that the Burkhant azhdarchid, hereby named Gobiazhdarcho tsogtbaatari gen. et sp. nov., represents a medium-sized (3.0-3.5 meters in wingspan) basal member of a Quetzalcoatlus-Arambourgiania lineage. The Bayshin Tsav azhdarchid, Tsogtopteryx mongoliensis gen. et sp. nov., is recovered as a basal member of a Hatzegopteryx-lineage and, surprisingly, seems to represent a small form under 2 m in wingspan. Our results shed fresh light on the diversity and phylogeny of azhdarchid pterosaurs, and reinforce the reoccurring pattern of coexistence between multiple, differently-sized azhdarchid species from a same deposit.