[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Nocturnal crocs?

On Sat, 25 Aug 2001, David Marjanovic wrote:

> > I have said there are no data supporting either case.
> Wrong. There is the impact and evidence for all its consequences (forest
> fires, acid rain, tsunami, and whatnot). I'm too lazy at the moment to
> repeat what has been written onlist since 1994.

This is data for a bolide, not extinctions, and definitely not
for _patterns_ of extinctions.  I grant you that it gives a
strong scenario for dino extinction--but not birds.  And you
can't sya that because it killed non avians it also killed
avians!  Remember, the fossil record is totally
mute on this issue--the timing, the means, the death toll--nothing. 

> > In the meantime, my
> > beliefs are, I would argue, as roughly supportable as yours.
> ...whereas your evidence is: *Stagodon* and IIRC *Cimolestes* in the same
> place at the end of the K, as opposed to *Deltatheridium* and the Gurlin
> Tsav skull* in the ?Campanian of Mongolia and *Gobiconodon* in the late EK
> of Mongolia and North America. (I'm leaving out the multituberculate you
> mentioned because it may have been purely herbivorous, and
> *Repenomamus/-nus* because know next to nothing about it.)

You might be able to leave it out on grounds of diet (however, _many_
sp. whose diet consists of plant material are also fab egg eaters), but
you can't as evidence of size increase.
> > My mechanism
> > is not a "holocaust" at all; it is a slow, steady, abiding selecetion,
> ...beginning in the EK with the appearance of cat-sized mammals? Such a
> gradual extinction is a lot too much for me.

I'm not saying that.  I am saying that mammals were tiny for most of the
Cretaceous--with some exceptions--and then there was an increase in mammal
size just before dinos became extinct.  That is either true or
not.  Lillegraven thought it was true.  Even John Alroy's data show an
increase--though more modest, of course than post dino times--in mammals
size immediately pre K/T.  My point is that the threshold for nest and
hatchling predation is fairly low.  This is because dinosaurs had the
smallest offspring relative to parent size.

> The present is not the key to the past. The past is the key to the present,
> and to the future.*

I would say this was not a biologist's statement.  Much of what exists
today, existed since life began.

> It is well
> possible that all birds died out everywhere else and a few managed to
> survive in Antarctica.

Hardly.  For such an unlikely scenario to be true, so must the
following: neos outcompeted enantis nowhere else but
Antarctica.  Conditions were so different in A. that the competitive
advantage gained there did not apply anywhere else.  We are then at the
same old bugaboo: what was the critical adaptation that brought this very
unlikely thing to happen--some magic niche bullet?  What?  We are left
scratching our heads.  OK, if it were some ground-bound beast we could
understand--isolation would be enough.  But birds fly--there is no reason
why such competitive advantages shouldn't be shipped across the
planet.  You need some reason for selection.  Why only neos?

> > Phorusrhacids evolved amid grasses.
> Stop -- wrong. Phorusracoids evolved in the Paleocene, means, before
> grasslands. They are known from IIRC Eocene France (*Ameghinornis*) and
> middle Eocene Messel (an AFAIK unnamed skull).

The biggest Phorusrhacids in grass--this is from Marshall.  What size are
the sp. you're talking about?

> > Diatrymids may have depended upon
> > wetlands,
> How so? Apart from looking more like phorusracoids and tyrannosauroids than
> like, erm, flamingos, they had a pan-Laurasian distribution (*Gastornis* =
> "*Diatryma*" in NA and Europe, *Zhongyuanus* in China).

Gastornithids existed while NA and Europe were covered with
wetlands.  When each continent drained, they disappeared--thopugh the
draining occurred at different times on each continent.

> > a mammalian refuge
> ???

In modern swamps, predator access is increased when water level is
lowered.  Predator density is
greater in uplands surrounding wetlands than in the swamps themselves.
(Can cite both of these).

> > which was in short supply at the terminal Cretaceous.
> when the regression turned the Western Interior Seaway into one huge...
> wetland. Please correct me if I'm wrong.

Archibald (1996) says that they lost alot of their prime habitat--I

> > > But I would argue that their wasn't really any "size relaxation" in the
> > > K/T.  Merely a better represented fossil record.
> >
> > Then we have competing hypotheses.  But I have positive evidence, you,
> > only negative.
> Your positive evidence is, see above, 2:1:1, maybe 3:1:1. I won't call that
> "evidence".

No.  It _is_ evidence.  Not of the quality you like perhaps.  And not
convincing, true.  But it is supportive.

> > > And Ostriches aren't anyware near as predatory as some of the
> > >dinosaurs.  If a modern day jackal tried to drive a dromaeosaur off a
> > >nest, it would be merely dinner.
> >
> > Only if it could _see_ the jackal.
> 1. Any reason to suppose the contrary? 

Yes.  See earlier citation--or ask me for it.

> 2. We're not talking of jackal-sized mammals, but of cat-sized ones.

I would argue it is not the aggressive abilities of the predator, but its
ability to crack the egg.  Gape is irrelevant to this problem as
illustrated by hairy armadillo on rhea eggs--knocks them together to
account for high predation rate.