>> Secondly, failure of any recent group of flightless birds to
>> spread over the world is in part due to the recent positioning of
>> the continents (ie there are oceans in the way). Presumably
>> flightless birds have only dispersed over a land connection (I
>> can't think offhand of an example of a flightless bird spreading
>> by, say, rafting).
>
> Gondwana and Lauarsia were separated, but they shared several
> groups of maniraptorans in common.
>> Third, the fauna of mammalian predators and nest-robbers has been
>> quite different since the K-T from what it was earlier, and the
>> barriers to radiation of flightless birds on continents may not
>> therefore be identical to what they might have been in the
>> Mesozoic.
>
> Different, certainly, but there is no reason to think the
> differences were consequential. Mammals were feeding on dinosaurs
> in the Mesozoic, they could glide, they could swim, no reason to
> think they couldn't rob nests.
> But, in the case of rails, the morphological changes
> associated with flightlessness were so recent and superficial that
> the resulting species did not move into new families, unlike with
> the ratites. In maniraptorans we had, I guess, Troodontidae and
> Dromaeosauridae that Paul might say descended from Archaepterygids,
> though he may group them all in the same family.