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Re: neornithean size



It's an interesting question and something I've pondered a lot.
Mammals are another factor - both outcompetition and ovivory. Places
where birds seem to have got big (e.g. New Zealand, Madagascar) had
limited mammalian faunas. However, I think arguments about mammalian
innovations (e.g. vivipary etc.) are overcomplicated - I think this
pattern isn't necessarily an indicator that flightless birds are
competitively inferior (ostritches obviously aren't, after all), but
rather may just be an accident of history, with flightless birds
unable to invade already filled niches (just as archosaurs didn't
initially radiate until therapsid diversity crashed). It is also worth
pointing out that in the early part of the theropod radiation even
non-avian theropods weren't astronomically large. Thus, with low
diversity and relatively recent radiation, flightless birds might be
expected not to have achieved giant size due to factors other than
physiological constraints.

On Thu, Mar 15, 2012 at 4:57 PM, Augusto Haro <augustoharo@gmail.com> wrote:
> 2012/3/15  <brushes2@juno.com>:
>> One possibility that constrains size is related to flight. Above a
>> certain mass, they may just not be able to generate enough power to lift
>> off.  Living ratites, of course, have escaped this problem by increasing
>> total mass, reducing forearm size, and giving up flight.
>
> It may be that for most birds, which are volant, acquiring, say,
> elephant size is difficult because of aerodynamic reasons. Perhaps it
> is unusual for not-flying birds to acquire this size because they are
> very few (as the loss of flight may be a much minoritary event in bird
> lineages compared with keeping airborne). There likely were much fewer
> birds exceeding the 100 kg-500kg range size, than herbivorous mammals
> in the same range size, and if only a small proportion of animals in a
> given group is expected to increase in size in each group, then it
> would seem reasonable that a small number of more than 100 kg bird
> species is less likely to produce elephant-sized descent compared with
> a larger number of mammalian herbivores in the same range size.
>
>> There also may be metabolic limitations, eg ability to undergo sustained
>> anaerobic energy production, but that has not been demonstrated AFAIK.
>
> Their respiratory system seems to indicate they have advantage over
> mammals relative to extracting oxygen in order to keep aerobic
> metabolism. Hypothetical elephant-sized birds would have been at least
> as efficient as gigantic dinosaurs in aerobic capacity, and likely
> more, given that their air sacs excavate a largest share of their
> skeleton.
> Cheers.



-- 
Roland Sookias
Scientific assistant and doctoral student
Archosauromorph Research Group
GeoBio-Center
Ludwig-Maximilians-Universität
Richard-Wagner-Straße 10
80333 Munich
Germany

http://www.archosauromorpha.com/people/rolandsookias
r.sookias@gmail.com
r.sookias@lrz.uni-muenchen.de