Here is what I said in the earlier post:
In the idea that I am presenting, the advanced pterosaurs (pterodactyls) developed into primitive flying birds (with feathers and all) during the Cretaceous. A branch of those primitive flying birds settled on the land and developed into the primitive flightless birds of the Cretaceous, such as the Ornithomimosauria and the Oviraptorosauria.
Those primitive flightless birds then developed into the modern flightless birds such as the ostrich etc.
For example, Struthiomimus is a genus within the family of Ornithomimosauria. It was long-legged and ostrich-like. It developed into the modern ostrich. So that is the pattern of the development of flightless birds.
Pterodactyls developed into primitive flying birds (with feathers and all). Those flying primitive birds then split into those that continued flying and those that settled on the land and gave up flying (like the ostrich).