Continuing the analysis:
"By the early Cretaceous, some other pterosaurs (by the process of cladogenesis), had developed into flying enantiornithes.
And by the late Cretaceous, many of these flying enantiornithes had developed into a variety of modern flying birds (Neognaths).
Concurrently, during the Cretaceous, some of the flying enantiornithes settled on the land/water, and developed (by the process of cladogenesis), into primitive aquatic birds, such as the hesperornithes.
And by the Paleocene, these primitive aquatic birds had developed into modern aquatic birds, such as loons, grebes and penguins."
All of this follows the same types of processes analyzed earlier, related to dromaeosaurids etc.
Note, that because dromaeosaurids and troodontids developed from pterosaurs by cladogenesis, there were still pterosaurs extant, so the cladogenesis of enantionithes from pterosaurs, could also occur, during the Cretaceous.
Again this is perfectly acceptable cladistic thinking.