Combined answer 1 Re: The extinction of small dinosaurs

[David Marjanovic <david.marjanovic@gmx.at>]

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,
> e.g.)

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
> example),

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
> clade.

Certainly not. Remember Neornithes. Ichthyornithidae and Hesperornithiformes 
were almost its sister group(s).

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