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Combined answer 1 Re: The extinction of small dinosaurs
Original Message by Daniel Bensen
Tuesday, 10 December 2002 15:32
> It might help to define "small". I remember hearing something like "all
> terrestrial animals over X kilograms were killed at the end-K", but I
> can't remember what the X was.
About 25 is the literature value. All terrestrial animals above that
threshold _and most below it_ died out.
Original Message by Stephan Pickering
Tuesday, 10 December 2002 16:54
> BRIEF REPLY: Current research -- by J.R. Carey in
> particular -- indicates extinctions are more complex
> than simplistic scenarios of one asteroid impact.
Actually I don't know what's simplistic about an impact scenario, with
consequences ranging from the shattering itself (magnitude 13 on the Richter
scale as far away as California... will destroy lots of forests and knock
over lots of big terrestrial animals... maybe it's enough for the extinction
of Sauropoda, who knows) over acid rain and darkness & cold to a greenhouse
effect, from global firestorms to tsunami and and and.
> Using bifurcation/evolutionary game theory, and
> computer simulations of fast-slow dynamical systems,
> an image is beginning to manifest itself:
Now don't tell me anyone has done a computer simulation of the
end-Maastrichtian biosphere. Tell me I've misunderstood you. :-)
> a combination of factors (volcanoes, climates,
> rising/falling sea levels
Just falling. Maybe not everywhere, and slowly, beginning 2 Ma before and
ending 2 Ma after the K-T.
> depleted gene pools,
Can only arise as a consequence of one or more of the other factors.
> lethal diseases [the
> opening of landbridges may have exposed breeding
> populations to extinctions])
May have. Well, certainly did. But firstly there's no evidence for a sudden
_global_ emergence of landbridges at the K-T anywhere I know of, nor are
there pathogens that are so extremely deadly to such an enormous diversity of
species. And then the whole thing is next to untestable anyway. We can find a
crater, a fern spike, a layer with iridium and plenty of extraterrestrial
isotope ratios and so on; we won't find a fossil pathogen.
> Some temperature-sensitive taxa (amphibians,
Looks like some clarification about frogs is necessary. Over here frogs don't
freeze as some North American species do. Over here most if not all species
spend the winter -- as if it were a tropical dry season! -- in the
mud/detritus on lake bottoms, where breathing water through the skin is
enough to sustain resting metabolism at a temperature of 4 °C.
> survived for reasons unclear, although burrowing
> may have played a part.
Burrowing and slow metabolism are good candidates. Another -- more for
mammals and birds -- is the ability to eat carrion, insects and/or plant
seeds (which themselves can often survive for decades and germinate then).
Another is being far away from the impact, such as in Antarctica, which
apparently played a role in the survival of Neornithes. Then there is body
size & population size & reproductive rate.
Similar things seem to have been important in the seas. Ammonites had
planktonic larvae that could hardly have survived a Strangelove ocean;
nautiloids have big, yolk-rich eggs and spend most of their time deeper in
the sea than the acid may have got, plus the food chains down there start
with dissolved organic carbon and marine snow, so a stop of photosynthesis
takes quite some time to have effects down there. The same applies to
planktonic and benthic foraminifera: the former suffered a catastrophic mass
extinction, I've cited the latest ref not too long ago, while the latter
completely ignored the event and were only decimated when the bottom waters
suddenly warmed up a lot at the Paleocene-Eocene boundary.
Original Message by Rob Gay
Tuesday, 10 December 2002 17:31
> So, what about what did survive, terrestrially, then? Well, mammals (though
> not all of them, of course), crocodiles, turtles, amphibians, snakes,
> lizards, and avians.
Not many mammals, very few terrestrial crocs, a respectable number of turtles
but by no means all, unknown but not very small numbers of amphibians and
presumably snakes (though no Cenozoic relatives of *Dinilysia* or
*Pachyrhachis* are known), 70 % of terrestrial lizard species in North
America, and an unknown but certainly not very big number of birds (but maybe
a very large part of Neornithes).
> Birds and _some_ mammals both have the ability
> to slow down their metabolisms (look at bears, for instance, as a famous
AFAIK few birds, and not so many mammals. Bears are not such a good example,
they must wake up several times a winter. On the other hand, there are
mammals that can both hibernate and aestivate.
> That is what I see as the common denominator for all the surviving
> vertebrate groups: torpor. So why did small non-avian dinosaurs go extinct?
> Well, I would say it was likely they couldn't go into torpor, and that's
> the main reason.
Doesn't this argument go backwards? They died out, and therefore you assume
they didn't know torpor, right?
Original Message by John Bois
Thursday, 12 December 2002 01:03
> That is what I meant to say. Is it possible to say something
> like: on land extinct dinosaurs were separated by more genetic distance
> from their nearest relatives than any other related extinct and surviving
Certainly not. Remember Neornithes. Ichthyornithidae and Hesperornithiformes
were almost its sister group(s).
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