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



----- 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.


_Of course_ the impact theory predicts that dumb luck mattered. It predicts
continent-scale wildfires -- something may have been in places that didn't
burn. It predicts humongous tsunamis -- something must have been in places
where the tsunamis didn't reach. And so on.

But no enantis.

There could well have been some that survived all this. They would then have entered the above scenario.


What are the odds?

Good question.

The best comparison might be: Line up a lot of people along a wall. Then
give the execution squad imprecise shotguns that strew a large amount of
small bullets in rather random directions. Give them one shot. Someone is
going to survive because no pellet happened to hit a vital organ of theirs.

But you can't have it both ways.

I must have it in more than just two ways because the impact of such a huge rock has more than two effects. Some of those kill everything with a more or less equal probability, some don't. See above.


> I think species interactions are common and well-observed phenomena in
> extant communities.  they cause extinction, etc., etc.

Problems are:
- They ae much less well observed and studied than you seem to think.

What does this mean?

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.


- They have never led to _mass_ extinctions in known cases.

Are you sure mass extinctions are that different. Or do they grade into ordinary extinctions?

The question is wrong here. I'm sure you agree that some causes can only lead to small extinctions while others can only lead to large extinctions. 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.


C'est la vie.  Pterosaurs were apparently a hermetically sealed component
of past ecology.

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.


No, it is not, because your calculation has a serious thinking error. It
isn't "how probable are 5 out of 5 vs 0 out of 5". It is "how probable are 5
out of many millions vs 0 out of many millions". There's practically no
difference there.

Doesn't it reduce to the same thing? Say, a million enantis and a million neos randomly distributed. Chance of getting a neo is still .5.

Hm... I'll try to visualize:
Imagine a box with half a million black and half a million white balls in it. You take 5 balls out; if you draw a black ball, it symbolizes a bird species that goes extinct, while a white ball is a surviving species.
How probable is it that all 5 are black and that all 5 are white? I'm too lazy to do the actual math now, but in any case both probabilities are equal.
And now the interesting things start. 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)?


They do it by simply knocking the eggs together!

OK. Anyway, I shouldn't have started this in the first place; ostriches and rheas survive just fine...


I guess this is how ostriches survive in the presence
of _high_ predator diversity.

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?

And then come the success stories of dromornithids,

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*).


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.
And had survived the local mass extinction of South America that happened shortly _before_ the Interchange, at the same time as an impact off Argentina.


perhaps gastornithids count, too.

Possibly denizens of swamp lands, traditional refuge from mammalian predators--became extinct after continental wetlands drained.

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?


Read Pond for advantages enjoyed by placentals, for example.

Could you give me a more specific citiation? (And should I list some advantages of marsupials over placentals?)


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 _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.