The large deltoid process in pterosaur humeri show a great degree of variation between species and families. Early Rhamphorhynchoid species tend to have deep, long and flat deltoid processes whilst later species show a narrowing and extending of this process. Later pterodactyloid species tend towards a curved deltoid process on the Humerus.
This shows an example of some of the types. Two later pterodactyloid species on the left and three earlier Rhamphorhynchoid species on the right.
These examples of early Rhamphorhynchoid humeri show the simple joint structures and the flat profile of the deltoid crest
By contrast, the humeri of larger pterodactyloid species show a different structure. The joint surfaces are more complex and the deltoid crest is far from flat, showing a curved shape with a pronounced articular surface.
The curved shape of the deltoid crest can be seen in these images.
Pterosaurs were a very diverse order of animals, showing a vast variation of forms across millions of years of time. As they became more specialised, the skeletal structures showed more complexity. Many of the structural bones of the skeleton demonstrate variation in this way, less so in the wing phalanges.
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