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Re: Moa-Tinamou Clade Found Within Ratites



That is certainly a neat hypothesis.  Flight (powered and sustained
flight) comes at a cost.  It requires a great deal of energy, and
stops the bird from getting too big.  For ground-dwelling herbivorous
birds that feed on low-energy foodstuffs, flight can be very
constraining.  A flightless herbivorous bird can subsist on low-energy
browse, and get larger to help process this fodder.  Palaeognaths may
have conceded the air to the neognaths.

There are quite a few lineages of large, secondarily flightless birds
clustered at or near the base of Neornithes.  Not just palaeognaths
such as the various ratite lineages, but also basal neognath lineages
such as dromornithids, gastornithids and sylviornithids that were big,
flightless birds.  Most were herbivores - including the gastornithids
(diatrymids) it now seems.

The proliferation of large, herbivorous flightless birds might have
something to do with the time that these lineages first appeared.  Or
it may have been something to do with basal neornithean anatomy that
was lost in Neoaves. Large flightless birds do pop up elsewhere in
crown Aves - phorusrhacoids, eremopezids, aptornithids, etc.  But they
seem to be more common closer to the base of crown Aves, and more
often than not these big and flightless palaeognaths/basal neognaths
were herbivores.



Cheers
Tim


On Wed, May 21, 2014 at 2:57 PM, Dann Pigdon <dannj@alphalink.com.au> wrote:
> On Wed, May 21st, 2014 at 11:15 AM, Tim Williams <tijawi@gmail.com> wrote:
>
>> Then again, it may be that the most recent common ancestor of ratites
>> and tinamous was a perfectly decent flier after all, and
>> flightlessness (and near-flightlessnes in the tinamous) arose several
>> times within the palaeognath clade.  Palaeognaths may have begun as
>> rather small, volant birds with excellent flight abilities (unlike the
>> tinamous).  Key to this is _Proapteryx_, the fossil kiwi (apterygid)
>> from the early Miocene of New Zealand, which was smaller than extant
>> kiwis and possibly volant (Worthy, 2013).  If basal apterygids were
>> indeed volant, it improves the chances for kiwi ancestors to have
>> arrived in NZ via overwater dispersal.
>
> It may be the case that although palaeognaths were once all volant, they may 
> have lacked certain
> adaptations that made it difficult to compete with neognaths. Their flight 
> abilities may have been
> good enough in the absence of neognath competition, but it may not have cut 
> the mustard where
> the two groups co-existed. That might explain why several palaeognath 
> lineages lost flight
> independantly - once neognaths expanded into most environments on the planet, 
> it just wasn't
> worth it for palaeognaths to retain their flight abilities when their new 
> competition did it far better
> than they ever could. It may have been better to abandon that niche in favour 
> of becoming
> flightless; a niche that neognaths may have also lacked the predisposing 
> qualities to easily exploit.
> In the face of competition, niche partitioning is often the name of the game.
>
> --
> _____________________________________________________________
>
> Dann Pigdon
> Spatial Data Analyst               Australian Dinosaurs
> Melbourne, Australia               http://home.alphalink.com.au/~dannj
> _____________________________________________________________
>