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

David Marjanovic <david.marjanovic@gmx.at> wrote:

> I wasn't talking about the size but about the flightlessness. Why would a
> bird become flightless in the presence of terrestrial predators?

Yes, it's an interesting question.  But I think we can turn the
question around.  If a large flighted bird could deal with those
terrestrial predators - why *not* become flightless (and therefore
even larger)?  Losing the ability to fly saves a lot of metabolic

> The
> ratites, gastornithids, dromornithids and others now seem to have become
> flightless in the Paleocene, which was almost devoid of large terrestrial
> predators.

There does certainly seem to have been a proliferation of large and
flightless lineages at or near the base of the Neornithes: ratites,
dromornithids, gastornithids (diatrymids) - and brontornithids and
sylviornithids too, if (like dromornithids and gastornithids) these
are basal neognaths.  Did they *all* lose the power of flight courtesy
of large terrestrial predators being absent?  This is likely true for
the sylviornithids of New Caledonia.  But it's an open question for
the rest - as well as other more derived flightless lineages, such as
the phorusrhacids and whatever _Eremopezus_ is.

> *Gargantuavis* may have become flightless before it met the
> predators, but where?

The implication here is that you need an absence of predators in order
to foster loss of flight in the first place; but once the flightless
bird becomes big enough, it can fend for itself against anything (or
most things) the terrestrial biota dishes up - especially predatory

But this might be putting the cart before the horse. Large
island-dwelling birds have a poor track record when they encounter new
mammalian predators.  Rather, the ancestors of big flightless birds
might have evolved ways of dealing with non-avian terrestrial
predators *before* they became flightless.  This promoted the loss of
flight, rather than the other way around.

In other words... one hypothesis is that large, but still volant,
terrestrial birds evolved ways to become better able to evade or fend
off mammal predators - and *then* lost the ability to fly.  Flight is
energetically expensive, and so a terrestrial bird that didn't need
flight ability would abandon it - particularly if it was a herbivore
feeding on nutrient-poor plant fodder.

The flightless phorusrhacids ("terror birds") were carnivores, but the
same principle likely applied.  Today, their closest relatives, the
seriemas, are long-legged terrestrial birds that rarely fly.

> That said, of course, there's *Patagopteryx*, which not only seems to have
> done that, but stayed small in the process. That's a mystery -- and it's a
> mystery that's independent of *Gargantuavis*, as the latter seems to be a
> quite crownward ornithurine according to Buffetaut.

_Patagopteryx_ is especially mysterious, because it was neither large
or fleet-footed - and it lived in a continental environment inhabited
by snakes, crocodilians (notosuchids) and quick predatory theropods
like _Velocisaurus_.  The stocky hindlimbs of _Patagopteryx_, although
poorly adapted for speed, appear to be have been quite strong.  So
maybe _Patagopteryx_ burrowed, like a similar-sized kiwi?
_Patagopteryx_'s strategy was to hide in the face of danger...?

_Gargantuavis_ was much larger than _Patagopteryx_, although likely
not cursorial; Buffetaut and LeLoeuff (2010) suggest it was
graviportal.  In defending itself against theropod predators,
_Gargantuavis_ may have been no worse off than a smallish "rotund"
ornithopod like _Zalmoxes_.