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Re: [dinosaur] Evolution of giant flightless birds (free pdf)

Ronald Orenstein <ron.orenstein@rogers.com> wrote

> I have always assumed that one of the reasons for the evolution of at least
> some large flightless birds was lack of competition from mammals. This seems
> highly likely for moas, elephant-birds, giant flightless Hawaiian geese,
> large flightless galliforms on New Caledonia and Fiji, dodos etc.

Yes, I think this is certainly true of birds in insular environments.
But I don't think this necessarily applies to all birds that become
flightless.  Based on current evidence, _Gastornis_ originated in
Europe during the Paleocene, and spread to Asia and North America
where it's found in the Eocene.  _Gastornis_ (including _Diatryma_ and
_Zhongyuanus_) co-existed with coryphodontids (including _Coryphodon_
itself).  So gastornithids were quite successful for a time, alongside
herbivorous mammals (and predators).

In Australia, dromornithids and casuariiforms evolved alongside a
range of large marsupial herbivores and carnivores.  Dromornithids
survived until the end of the Pleistocene, and the latter are still
around (though they haven't fared as well on islands).

> it may be
> have been less of a factor for large birds on continents, but I wonder if
> the evolution of large size early giant birds such as Gastornis might have
> been facilitated by a depauperate mammal fauna after the C-T?

There might be some truth to this, but I don't think it should be
overstated.  I agree that conditions immediately following K/Pg might
have favored loss of flight in many bird lineages.  But these lineages
persisted alongside the radiation of mammals.  The Messel ecosystem
(Eocene) is remarkable for its diversity of large flightless (or
near-flightless) birds (_Palaeotis_, _Strigogyps_, _Dynamopterus_,
_Gastornis_), despite the existence of placental predators
(_Paroodectes_, _Messelogale_, _Lesmesodon_) that could have targeted
chicks and eggs.  These flightless bird lineages did go extinct,
possibly due to the arrival of new kinds of predatory mammals or
competition with new kinds of herbivorous mammals.  But many lineages
of volant Messel birds also went extinct (especially arboreal
lineages).  So the reason(s) that the flightless birds of Europe went
extinct might have had little to do with the inability to fly.