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



Dann Pigdon <dannj@alphalink.com.au> wrote:


> It would seem that most of Australia's large flightless birds were extinct 
> well before dingos got into
> the country. Even with dingos-a-plenty for a couple of millennia, emus (and 
> cassowaries) are still
> with us though.


The mainland species are still with us, anyway.  Dwarf emus on two
smaller islands off the southern Australian mainland went extinct in
the 19th century.  One of these was a separate emu species (_Dromaius
baudinianus_, Kangaroo Island emu); the other (King Island emu) was a
subspecies of the mainland emu (_D. novaehollandiae_).


Which brings me to Brian Lauret's point:

> Generally, faunas fare poorly when continental or 'more continental' invaders 
> show
> up. This tells us more about the fragility of island ecosystems than of any 
> inherent inferiority
> of the islanders' lineage.


I agree with the first point.  For example, aside from being smaller,
there was nothing inherently inferior about island-dwelling emus
compared to the mainland species.  These insular emus were derived
from populations that were 'marooned' once they lost contact with the
mainland ~ 10,000 years ago due to rising sea levels.  It was the
vulnerability of their island habitats that brought about their
extinction when _Homo sapiens_ came along.


However, many island-dwelling birds lost the power of flight because
of the absence of predators that would otherwise target them.  So it
was the island habitat that fostered their loss of flight ability.
Because flight was essentially the only defense they had,
flightlessness left them highly exposed to the arrival of new
predators.  The dodo typifies this quite well - iconically, in fact.


But flightless birds such as fossil ratites, dromornithids,
gastornithids, phorusrhacoids, eogruids, etc were typically found
co-existing with predatory mammals.  I don't think there is any reason
to assume that their flightlessness necessarily arose in sheltered
insular or "semi-insular" environments prior to their continental
success (although this might have occurred for some, such as the
gastornithids).  Instead, these lineages could originally have been
quite adept on the ground, which promoted the loss of flight and which
contributed to their success.


For one flightless group, the dromornithids of Australia, their
post-Miocene decline has been tied to *competition* with herbivorous
mammals (especially the rising diprotodontids) rather than predation
by carnivorous mammals.


Similarly, the extinction of _Gastornis_ (_Diatryma_) at the end of
the Eocene has been linked to competition from emerging mammalian
predators, such as mesonychians and hyaenodonts.  Although in this
case, given that a herbivorous/folivorous diet rather than
predation/carnivory has also been argued for _Gastornis_ (e.g.,
Andors, 1992), it's an open question whether mammalian predators did
contribute to the latter's extinction.  European _Gastornis_ species
lived on an archipelago, and possibly evolved there before dispersing
into North America.






Cheers

Tim