Body of a mounted Pteranodon, showing the notarium between the shoulder blades:
"Notarium is a term used for the fused vertebra of the shoulder in birds and some pterosaurs. The structure helps brace the chest against the forces generated by the wings.".
Fig. 22. The notarium of large short-tailed pterosaurs, after O. Abel(1912). By fusion of shoulderbones this area was intensively reinforced in order to be able to transmit the increased forces due to higher beat frequency.
"In some later pterosaurs, the backbone over the shoulders fused into a structure known as a notarium, which served to stiffen the torso during flight, and provide a stable support for the scapula (shoulder blade)."
"Fusion of vertebral ribs to dorsal vertebrae, and of these vertebrae to one another to form a notarium , occurred in many (possibly all) large pterodactyloids (e.g. Pteranodon, Dsungaripterus, Tupuxuara (Fig. 4)), and likely reflects a response to the structural demands placed on this region by stresses transmitted through the body during flight ,".
See this article for more detail.
"The fusion of two or more thoracic vertebrae, independent of the synsacrum, is more widespread in the Passeriformes than has previously been reported. The bone thus formed is known as a “notarium.”"
"The fusion of two or more thoracic vertebrae, independent of the synsacrum, is more widespread in the Passeriformes than has previously been reported. The bone thus formed is known as a “notarium.”""Mapping of character evolution on a supertree suggests that a fully fused notarium has evolved independently at least 12 times in the oscine passerines and that notaria with less extensive fusion of the vertebrae (only the spinous processes fused, for example) are even more widespread phylogenetically.Phenotypic expression of a notarium is fixed in some species and higher taxonomic groups but varies within the species in others. Ontogenetically, the fully fused notarium forms when the bird is immature. The evolutionary development of notaria probably depends on mutations that alter expression patterns of transcription genes (Pax and Hox genes are likely candidates) that control embryological differentiation of the vertebrae."
The holotype [of Mesadactylus] is BYU 2024, a synsacrum of seven sacral vertebrae, featuring a unique -- for a pterosaur -- complete fusion of the spinae into a supraneural blade, a character, as the specific name indicates more typical for birds, at first leading Jensen to assign the fossil to a bird, Paleopteryx.