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Re: Island-dwelling dinosaurs (was Re: Gargantuavis neck vertebra)

 Do we really know this?

Of course not. :-)

 Especially for the ratites: how good is our timing of the origin of
 flightlessness in this bunch?

The oldest rhea, *Diogenornis*, is Paleocene. Because it's recognizable as a rhea, I suppose it's known to be flightless.

The ostriches may be related to some (particularly *Palaeotis*) or all of the European Eocene and Paleocene (like *Eleutherornis*) supposed ratites.

The other ratites aren't known from that far back, but neither are possible volant ancestors... except, as was presented at the conference this week, there's now a kiwi from the Miocene of New Zealand that was so small and so incompletely known that it might have been able to fly (according to the presenter, Trevor Worthy).

The other large flightless birds (gastornithids, brontornithids, dromornithids) reach down into the Eo- or Paleocene as well, and likewise lack a fossil record before that.

Finally, this isn't my idea; IIRC, it comes from one of the big 2008 papers on neornithean phylogeny that found ratite paraphyly.

 Furthermore, crocodylomorph and squamate terrestrial predators were
 present on both Laurasia and Gondwana at this time.

Not clear how far down they reach, except for *Titanoboa* which wasn't exactly cursorial.

 Additionally, phorusrhacids seem to have developed flightlessness in
 the presence of marsupial terrestrial carnivores.

Perhaps not. The oldest known phorusrhacoids are as old as *Diogenornis*, IIRC.