Re: Extinction

[Tommy Tyrberg <tommy.tyrberg@norrkoping.mail.telia.com>]

At 20:11 2000-03-24 -0500, John Bois wrote:
>
>
>On Fri, 24 Mar 2000, Tommy Tyrberg wrote:
>
>> This is just quibbling. An impact that is large enough form a 150 km crater
>> and to emplace a centimeter-thick deposit world-wide would certainly be
>> extremely devastating (shockwave, heat radiation, ejecta blanket, tsunamis,
>> dust blanking out sunlight, injecting vast quantities of rock vapour, water
>> vapour, carbon dioxide, sulfur dioxide and nitrogen oxides into the
>> atmosphere just to mention the most obvious effects). To what extent these
>> effects these effects are visible in the geologic record is really
>> immaterial since there is no way a Chicxulub-type impact could happen
>> *without* having these effects.
>
>In my life time, science has determined that there is and that there isn't
>enough matter to contract the universe. Suuch pronouncements are usually
>made with a parenthetic: "...but we're not exactly certain."  I am sorry
>to say that many in the astro field lack such humility and state as fact
>that which they cannot know, viz, the effect of an asteroid on planet
>Earth's ecology.
>Now, you can _see_ the universe expanding, you can see the moon move from
>one side of the sky to the other and so on.  We have never seen that which
>you claim.  Indeed, the variables are so massively complex it must be
>guesswork.
>
Do You mean that You can think of a scenario where a large asteroid impact
on a continental shelf can occur *without* having the effects I mention? If
so I would be interested in details (You can skip the carbon dioxide and
sulfur dioxide since they are dependent on the geology of the impact area,
though the geology is right at Chicxulub).

>> >Specifically, among terrestrial vertebrates the ONLY group that
>> >experienced true extinctions were the dinosaurs!  And, by the way, the
>> >"boundary" at sea has not been exactly matched to the terrestrial
>> >boundary.
>> >And temporal clusters of extinction may occur as a purely statistical
>> >phenomenon (shoot blindfold at the side of a barn and you will make
>> >clusters).
>> >
>> How about enantiornithine birds?
>
>There is not enough evidence to distinguish between the two main
>hypotheses: enantiornithines were diverse and numerous right up to the
>K/T; and, enantiornithines were gradually outcompeted by neornithines
>culminating at the K/T.  I will say this though.  I think it is incredibly
>far-fetched to say (as does Feduccia) that all neornithines are the
>ancestors of a few lucky shore-bird survivors.  Such a claim suggests,
>a) enantiornithines could not compete in the shorebird niche (but we know
>they were a diverse taxon); or, b) the asteroid had smart-bomb
>capability.
>
No, I don't think only a few shorebirds survived. But I do think that the
present-day avifauna is derived from a quite limited number of late
Cretaceous species. And it is a fact that enantiornithines are strikingly
rare (but not quite absent) in coastal and marine environments.


>> And do You imply that the iridium anomaly
>> at sea and on land aren't the same age? And that mass extinctions are just
>> random fluctuations, even the Permo/Triassic one?
>
>Just parroting what I read in a recent Lillegraven article (ref. if
>needed).  I'll check it.
>
>> >sea-level regression, mountain building, climate change, novel
>> >speciation--these things really did happen!!
>
>> Yes, they happen all the time, and usually with no drastic effect on the
>> biosphere.
>
>Archibald has argued persuasively that there was a great confluence of all
>these forces just at this time.
>
>> As far as I know Carnivora do not turn up until well into the Paleocene.
>
>Earliest Paleocene (Fox and Th... 1994) ref. if needed)
>
>> Also You are making a common mistake in constructing an explanation which
>> might work in North America, but not elsewhere. Carnivores can hardly have
>> exterminated dinosaurs in South America which they did not reach until the
>> Miocene, in Australia where they only arrived 4,000 years ago (the dingo),
>> or New Zealand (nineteenth century). Also I have never been able to
>> understand how a regression is supposed to cause habitat fragmentation,
>> normally it's the other way around, *rising* sea levels fragment habitats
>> and isolate populations.
>
>The Western Interior seaway drained thus taking away a huge area of
>coastal plain.
>
Yes, but there would still be large areas of coastal plains left arond the
periphery of the continent. Also dinosaurs did live inland in upland areas
(like in Mongolia). Also note that dinosaurs did survive for at least
several tens of millions of years (until the Maastrichtian) in India and
Madagascar and on New Zealand showing that they did not require
continental-size landmasses for long-term survival.

>The biogeography of these events is a challenge to any hypothesis since we
>only have a record of the transition in North America--hence my 
>invoking Carnivora.  However, if it is true that today's predatory guilds
>limit the distribution of large egg layers to grasslands, wetlands, and
>places with low predator density (e.g., New Zealand), it is worth looking
>toward the point where these guilds first came into existence.  I believe
>birds are a very important limiting factor in this--ostriches and rhea are
>both suffer tremendous mortality of chicks from birds.  Plus, marsupials
>were probably effective, too.  If this is the answer to the question
>of: why no reconvergence to the dinosaur-size form? then it's worth
>looking more closely for the time when this first started being true.
>
Since I happen to have field experience of all extant ratites (except Dwarf
Cassowary) I know that this is simply not true. A quick summary of habitat
preferences:

Ostrich: avoids closed forest, otherwise almost ubiquitous. I've seen it
(breeding) in e. g. mopane woodland, dense acacia woodland, fairly extreme
desert and coastal fynbos.

Lesser Rhea: puna, steppe and scrub steppe

Greater Rhea: grassland (campo), cerrado, chaco woodland

Southern Cassowary: Lowland rainforest, in New Guinea also in swamp forest
and savanna woodland

Northern Cassowary: Lowland rainforest, swamp forest

Dwarf Cassowary: Highland rainforest

Emu: not in rainforest or completely waterless desert but otherwise almost
ubiquitous, including closed forest. 

Brown Kiwi, Lesser Spotted Kiwi, Greated Spotted Kiwi: All forest birds,
though the Brown Kiwi does venture into open habitat to some extent.

Thus, ratites occur in almost all habitats except taiga and tundra (though
ostriches did live in steppe-tundra in Mongolia and Siberia up till ca
10,000 years ago). I suppose You might consider Australia to have "low
predator density", but this was hardly true 50,000 years ago with
Thylacinus, Thylacoleo and Meiolania, and it's hardly true today with
humans, dingos, cats, foxes and wild pigs. Actually Cassowaries and
especially Emus survive these a great deal better than do most australian
mammals, marsupials *or* placentals. Note also that ostriches do very well
in what is probably the most predator-dense biome in the world, the african
veldt.


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