Azhdarchidae, but they are also similar enough to be a basis for the synonymy of Hatzegopteryx and Quetzalcoatlus. However, this is likely due to the relatively non-diagnostic nature of the humerus in giant azhdarchid taxonomy, and the lack of a detailed description for the elements of Q. northropi. However, the neck and jaw anatomy of Hatzegopteryx is quite clearly distinct from the smaller Q. sp., which warrants the retention of Hatzegopteryx as a taxon separate from Quetzalcoatlus. The neck vertebra referred to Hatzegopteryx sp. contains a number of traits that allow for it to be definitely identified as that of an azhdarchid. The centrum is relatively low, the zygapophyses are large and flattened, and the preserved portions of the neural spine indicate that it is bifid, or split into two. A phylogenetic analysis conducted by Nicholas Longrich and colleagues in 2018 had recovered Hatzegopteryx within the Azhdarchidae in a derived position, sister taxon to both Arambourgiania and Quetzalcoatlus. The cladogram of their analysis is presented below: Paleobiology Bone structure While the skull of Hatzegopteryx was unusually large and robust, its wing bones are comparable to those of other flying pterosaurs, indicating that it was not flightless. Buffetaut et al. suggested that, in order to fly, the skull weight of Hatzegopteryx must have been reduced in some way. The necessary weight reduction may have accomplished by the internal structure of the skull bones, which were full of small pits and hollows (alveoli) up to long, separated by a matrix of thin bony struts (trabeculae). The wing bones also bear a similar internal structure. This unusual construction differs from that of other pterosaurs, and resembles more closely the structure of expanded polystyrene (used to manufacture Styrofoam). This would have made the skull sturdy and stress-resistant, but also lightweight, enabling the animal to fly. A similar internal structure is also seen in the cervical vertebra referred to Hatzegopteryx. Neck biomechanics As a consequence of its robust, thick-walled vertebrae, the neck of Hatzegopteryx was much stronger than that of Arambourgiania. This can be quantified using relative failure force, which is the bone failure force of a vertebra divided by the body weight of the pterosaur that it belongs, estimated at for Arambourgiania and Hatzegopteryx. While Arambourgiania'''s neck vertebrae fail at about half of its body weight, the posterior neck vertebrae of Hatzegopteryx can withstand anywhere between five to ten body weights, depending on the