[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: Vegavis gen. nov. - new anseriform in today's Nature
> > Predation/competition could theoretically lead to small and large
> > extinctions. Physical calamities ditto. So, no...I don't accept your
> > premise.
David M. replied:
> Why shouldn't there be an upper limit to the effects of predation and
> competition? It's really hard to imagine a predator or group of predator
> species that could have an effect the size of the K-T mass extinction --
A good review of the effect of foxes, cats, rabbits, camels, goats, pigs on
Australian native faunal diversity can be found in: Edwards, G.P., _et al_,
2004 Introduced mammals in Australian rangelands: Future threats and the
role of monitoring programmes in management strategies. _Austral Ecology_
Vol 29. #1 pp40-50.
It's a bleak picture...and without management would lead to close to a mass
extinction. Not only _not_ hard to imagine...it's happening! Meanwhile
look to the sky for danger. Reminds me (vaguely) of Diamond's new book,
Collapse...the Easter Islanders looking to the Gods while they grind their
home to dust.
> Sorry for forgetting predation -- but what are those adaptations of
> and birds...
Behavioral flexibilty, smarts, sensory acuity, vertical take-off, aerial
agility, greater protection and nutritional support of fetus, etc., etc.
>...and why should azhdarchids first survive for some 60 million
> years and then die out at the same time as everything else does and a huge
> planetoid crashes down?
That's an easy one...it was predation/competition from rapidly evolving
neornithines. Either that or it was the planetoid.
> > 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.
> Then why do you think something evolved precisely at the boundary that was
> able to plunder those remote locations as well?
Borrowing your logic style for a second: because that's when they
> >> ...What if, because of their
> >> different ecological niches or whatnot, the ratio of black to white
> >> 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
> >> across. And what if the total number of balls per box is unequal (which
> >> is
> >> of course highly likely)?
> Has it gotten my point across? :-)
Sorry I left this bit out. No. What you are saying is that there is a
higher susceptibility to extinction in Enantiornithines. Then you have to
posit a reason for this. "Whatnot" _does not_ do it.
> > ostriches and rheas survive just fine...
> > Even after suffering greater than 90% nest/chick
> > predation to the first year.
> Yes, even then. They are r-strategists. Like, for example, sauropods.
...that were replaced on some continents by the relatively K strategists,
hadrosaurs. Are we looking for an out-of-planet experience for this?
>> > 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*).
Things you probably already know:
Population density in one area does not indicate continental density.
Australia's soil and increasingly arid climate was not condusive to
continent-wide high predator density.
Australia's flora was pre-adapted to tolerate arid and salty soils.
Mihirungs were pre-adapted to be able to process leaves with a high salt
concentration (they had salt glands) and therefore, as probable browsers,
could take advantage of this crop.
Putting these together, these birds could make a living in very hostile
environments when they needed to (nesting time).
Now, maybe they didn't do this. Perhaps they nested colonially _amid_ the
nasties. Evidence from ostriches argues against this--when ostrich nests
are discovered by black-backed jackals, they are destroyed by them! These
mammals are smaller than the marsupials you mentioned...but they are able to
drive the ostriches off the nests and break the eggs. Now, either the
marsupials were not competent in this respect, or the dromornithids were
superior defenders. In my view, the critical determinant here is the
ability to operate at night (jackals cannot attack nests in the day time!).
Was this visual asymmetry between pred/prey also a part of the Australian
case. Dunno. Dromornithids, if they were like most birds, could not see at
> > What is your explanation for this astounding diversity of non-ratite big
> > birds on these southern continents?
>...dromornithids never got out of Australia. :-|
But, if dromornithids are geese they had a global distribution! The
question is: why didn't geese evolve this morphology in other
parts...specifically, in Northern continental parts.
> *Titanis*? In swamps? This was a grassland runner, with leg proportions
> an ostrich. Plus, it was a top predator, certainly capable of defending
> nest at least as efficiently as an ostrich (...known to kill lions with
See above comments on nest defense. I'm trying to argue a general
principal. You are refuting it with an exception...a short-lived relict of
a once grand adaptive radiation. This shouldn't take our focus away fromthe
central question: what caused this radiation to implode.
> > 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.
> Up to the, say, 70s, people believed in continent-scale drying periods...
> do you happen to have a ref?
I think it was this: Andors, A.V. 1988 Reappraisal of the Eocene groundbird
Diatryma [Aves: Anserimorphae]. Pages 109-125in K. E. Campbell, Jr., ed.
Papers in avian paleontology. Honoring Pierce Brodkorb No. 36 Science
Series Natural History Museum of Los Angeles County.
> We don't _need_ to know, (exact population survival data) and this is an
important point. We know that
> _enough_ survived. Therefore all hypotheses that allow the survival of
> minimum number, while allowing the survival of less than that minimum
> number of other clades, are a priori acceptable. The impact hypothesis
> these brackets.
This is indeed the ideal hypothesis! A Teflon hypothesis.
> We certainly know, within error margins, things like the kinetic energy of
> the impactor. We also have some good clues on what that amount of energy
OK...but relating all of this to specific survivorship is a weakness.