Re: Armadillos at the K/T! (long)

[John Bois <jbois@umd5.umd.edu>]

On Fri, 5 Oct 2001, David Marjanovic wrote:

> > I don't know whose law it is, but it is better to run than to fight.  But
> > if dinosaurs ran every time a large predator approached they would never
> > reproduce.
> 
> So it is not always better to run?

If you are a mammal and face a predator who has a better than even chance
of besting you, it is always better to run.  But baby runs with you
because either he is fast himself (having been gestated for much longer
than similarly-sized amniotic eggs), or he is still inside mummy.  Thse
choices are not as clear cut for a dinosaur who must weigh the destruction
of his brood if he abandons the nest to reproduce another day.

> > This put selection on defense, and, I believe, fueled an arms
> > race for increasing size.
> 
> The problem is that evidence for or against the former is scarce...

Some speculation is better than other speculation.  The speculation that
large dinosaurs were hard to hide and, therefore, had to defend their
nest, is a pretty good one, in my view.

>...and
>the
> latter is destroyed by *Microraptor*, *Archaeopteryx*, birds...

No.  Small things can hide.  They thus avoid the hazard of obligatory
defense.  So, the hypothesis depends upon dinosaurs being big--which,
curse me, is the same assumption for the bolide idea.

> > On the contrary, colonies are wonderful defensive structures--as long as
> > by increasing the numbers of defenders you reduce the probability of
> > predation.
> 
> OK, but this also works when the predators are mammals or neornitheans. Why
> shouldn't it?

Because colonies are only valuable against similarly-sized
predators.  They exist to mount defense against a predator for whom two
defenders are better than one.  I don't believe this would be the case for
small predators.  For example if the David dinosaur was being attacked by
a _T. rex_, a John dinosaur nesting next to him would also be
threatened.  I would join the fight.  If, though, your nest was a seething
mass of furballs, I can't imagine what benefit I would gain by leaving my
nest, especially if mine is similarly targeted!  Indeed, some colonial
species of birds can switch to solitary nesting based on the kind of
predator they are engaging.

> > > I should also mention the
> > > pterosaurs from Chile that nested in the desert (as some seabirds in the
> > > region do today...
> >
> > Not sure what you mean here.
> 
> They fished at sea and nested far away in a desert. Today some seabirds nest
> in the Atacama for the same reason. I'll have to dig up a ref, but AFAIK it
> was reported on an SVP meeting too.

Yes, nesting in remote places is a great idea.  Not to say the dinos
didn't do it, just that they were less effective at distancing themselves
from other walking dinosaurs.

> > late K dinos were larger than
> > ostriches (from memory, now).
> 
> Longer, taller and/or heavier? (I wouldn't trust estimates on the last.)
> Individuals or species? How old is the database?

I looked through Worth's database (3.0) again.  The vast majority
of Late Cretaceous species were over 4 meters in body length.  I'll try
and get a better figure.