Neoteny

For reference:
http://en.wikipedia.org/wiki/Palaeognathae
"Other authors have questioned the monophyly of the Palaeognathae on various grounds, suggesting that they could be a hodgepodge of unrelated birds that have come to be grouped together because they are coincidentally flightless. One point is that unrelated birds have developed somewhat ratite-like anatomies multiple times around the world through convergent evolution. McDowell (1948)) asserted that the similarities in the palate anatomy of paleognathes might actually be neoteny, or retained embryonic features. He noted that there were other feature of the skull, such as the retention of sutures into adulthood, that were like those of juvenile birds. Thus, perhaps the characteristic palate was actually a frozen stage that many carinate bird embryos passed through during development. The retention of early developmental stages, then, may have been the mechanism by which various birds became flightless and came to look similar to one another.[19]"


Notice that this relates to not just flightlessness, but also the palate:
"Thus, perhaps the characteristic palate was actually a frozen stage that many carinate bird embryos passed through during development. "