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Re: (origin of the palaeognathous palate)

> So, a diverse group of volant palaeognaths
> with a propensity for dispersal like rails, geese.

This seems to be the most parsimonious explanation. (Though the propensity
for dispersal was certainly smaller. For example, there are no ratites on
Fiji or even New Caledonia -- Fiji is of Eocene-Oligocene age, IIRC.)

> If this is what happened, it is interesting to think
> about why ratites sustained their success in a flightless niche whereas
> "terror birds", "killer ducks", and Gastornis(?) could not.

The latter three were seemingly top predators*, which made them more
vulnerable to just about anything. Besides, the end of the gastornithids
could well coincide with the end-Eocene mass extinction, while the other two
survived well into the late Pleistocene... hint, hint.

* Thus in different ecological niches from any ratite. If we are picky, each
and every species has an ecological niche of its own.

Then of course ratites fared quite badly in New Zealand and Madagascar.