Pygostyle

We can track the development of the pygostyle.

https://en.wikipedia.org/wiki/Epidexipteryx

The tail of Epidexipteryx also bore unusual vertebrae towards the tip which resembled the feather-anchoring pygostyle of modern birds and some oviraptorosaurs.^[2]

https://en.wikipedia.org/wiki/Ambopteryx

it had a short tail that lacked a transitional point and ended in a pygostyle, a set of fused tail vertebrae.

 

In the last step comes Pygostylia. 

Pterodactylid:

https://en.wikipedia.org/wiki/Pteranodon

Other distinguishing characteristics that set Pteranodon apart from other pterosaurs include narrow neural spines on the vertebrae, plate-like bony ligaments strengthening the vertebrae above the hip, and a relatively short tail in which the last few vertebrae are fused into a long rod.[17] The entire length of the tail was about 3.5% as long as the wingspan, or up to 25 centimeters (9.8 in) in the largest males.[17]