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Re: Vegavis gen. nov. - new anseriform in today's Nature




On Tue, 25 Jan 2005, David Marjanovic wrote:

> ----- Original Message -----
> From: "John Bois" <jbois@umd5.umd.edu>
> Sent: Tuesday, January 25, 2005 12:54 AM
>
> >> "Enough different birds" means one viable population per species, of at
> >> least 5 species. This can amount to _very_ few individuals. Especially if
> >> you take the postapocalyptic world into account -- practically no
> >> predators,
> >> no competition.
> >
> > ...and no food.
>
> This is only a factor at the start of that epoch -- and it is precisely
> where the ability to subsist on seeds and/or insects comes into play.

Makes a good story, anyway.

> > But no enantis.
>
> There could well have been some that survived all this. They would then have
> entered the above scenario.

When does a good narrative become good science?

> Firstly, the big cases of what were once presumed to be cases of
> competition-caused extinction are not all that well studied. The Great
> American Interchange is an example. Secondly, competition on a small scale
> has not been observed _so_ often either.

But predation has.  And as per recent discussions on this list, thinking
about the connection/inseparatability of competition and predation is at
best fuzzy as it relates to extinction.  I agree with you that study of
extant species is wanting--however, studies that have been done offer
tantalizing clues...clues that may have application to past ecologies.

> ...I'm sure you agree that some causes can only
> lead to small extinctions while others can only lead to large extinctions.

Predation/competition could theoretically lead to small and large
extinctions.  Physical calamities ditto.  So, no...I don't accept your
premise.

> Whether there are middle extinctions or not is irrelevant here. I think that
> competition can (on a global scale) only lead to rather small extinctions;
> see below.

I don't know on what basis you could make the claim that
competition/predation couldn't have as huge an impact as a huge impact
could have.  This is an active field of research as far as I know.

> I'm just talking about the azhdarchids: *Quetzalcoatlus*, *Hatzegopteryx*,
> *Arambourgiania* and the unnamed Spanish monster. No known bird was able to
> compete with them. Sure, we may see "the ghost of competition past" here;
> birds may have been the reason for why these pterosaurs got so huge; but
> this is a different question.

I don't know why you're stuck on _competition_...as if there is a race off
for every food morsel in the K.  If pterosaurs did decline (if!) as a
result of species interactions, I would hypothesize that the predatory
environment got more intense over the K as a result of adaptations of
mammals and birds.  Some species were more able to withstand this
environment, ptereosaurs weren't.  In this view, large body size has
little to do with it--except that, large bodied pterosaurs were more
likely to be able to nest in more remote locations..and that this
afforded them some protection.

> ...What if, because of their
> different ecological niches or whatnot, the ratio of black to white balls in
> the box is indeed 1 : 1 for Neornithes, but 1 : 4 for Enantiornithes? Just
> pulling numbers out of thin air (obviously), but I think it gets my point
> across. And what if the total number of balls per box is unequal (which is
> of course highly likely)?

> OK. Anyway, I shouldn't have started this in the first place.

You big tease.

> ostriches and
> rheas survive just fine...

Even after suffering greater than 90% nest/chick predation to the
first year.

> > Indeed, ostriches nest in regions of _low_ predator density--arid
> > grasslands...one nest per km^2...very difficult to find...islands of low
> > predator density.
>
> Islands?

Nesting animals must conceal or otherwise prevent other animals from
eating their eggs/hatchlings.  For big nesting animals two options: hide
or defend.  Ostriches depend on concealment.  This succeeds for them
because they are adapted to arid grassland conditions where there are
fewer predators and great concealment.  In that sense, they exist in a
island of low predator density--analogous to islands like New
Zealand, etc.

> > An argument can be made that the predatory regime was milder in Australia.
>
> Explain this to *Thylacoleo carnifex* (relevant for the last known
> dromornithid, *Genyornis*)... and to the many large dasyurid and thylacinid
> species found in Riversleigh (relevant for the famous *Bullockornis*).

What is your explanation for this astounding diversity of non-ratite big
birds on these southern continents?
capable of global travel.  Why not on Northern continents?

> >> phorusrhacoids...
> >
> > That suffered extinction after the immigration of continental predators.
>
> Explain this to *Titanis*, an immigrant to North America and the largest
> known phorusracid, and to its unnamed contemporary in Uruguay. Both lived
> some 3 million years _after_ the Great American Interchange.

OK...I'm not arguing it was surgical.  I'm sure there were good reasons
for the survival _and_ the extinctions.  Perhaps _Titanis_ enjoyed
immunity in the swamps.  Hey, perhaps gators inadvertently protected their
nests as they do other species today.

> Ah, the continent-sized wetlands that people used to imagine fifty years
> ago. One could become nostalgic. -- The distribution of gastornithids across
> all three northern continents seems to be at odds with such a restricted
> ecology. Besides, don't they co-occur with several "creodonts" each?

Were gastornithids not thought to be wetland species?  The story I read
only about ten years ago was that these species became extinct in NA and
Europe at different times, but both coincident with drying periods on
their respective continents.

> > And I have explained that there is _no_ information on population sizes
> > from the K/T.  For all we know, the surviving populations of "ducks" and
> > "chickens" were huge!
>
> What about "for all we know, they were tiny"?

I think we are coming together on this!  Exactly.  We _don't_ know.

> I _really_ think we can
> _exclude_ the possibility that they were "huge", because, after all, the
> impact of a 10-km rock _is_ The End Of The World As We Know It. Sometimes I
> think you're trying to non-discuss the entire impact away.

Oh, no...we _do_ know.