Re: Birds as dino-killers

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

On Sun, 2 Sep 2001, David Marjanovic wrote:

> _BACKGROUND_

_Background extinction_ is a concept of limited value.  The idea comes (I
think) from background radiation, the relatively constant amount of
radiation that reaches the planet.  Biological extinction is not at all
like this.  Each extinction has a unique set of circumstances.  There is
no _rate_ for this.  The concept of mass extinctions grading into
"background" extinctions is a topic of discussion among
extinctionologists--actually, paleo-ecologists.  As in everything else in
ecology, there are no black and whites--just clines.

> > If birds could do this to pterosaurs, they
> > must also be able to cause trouble for small to medium-sized non-avian
> > dinosaurs.
> 
> which were able to hide their nests in dense forest undergrowth, something
> the large LK pterosaurs were unable to do.

Again, the ability of these creatures to be able to hide is in
doubt.  Turkeys on the NA continent can do it--but they can also fly to
escape predation--this is a margin of survivability not possessed by
non-avian dinosaurs--and this margin is very small.  This suggests an
upper-limit for this strategy, a limit only exceeded by the cassowary in
Australia/NG.  Also, why couldn't smaller pterosaurs hide?

> > If it is true that there was a reduction in the diversity
> > of these size ranges among dinosaurs,
> 
> Yes, if... any numbers available? :-)

I would love to know the status of this question.  I know that late
Cretaceous species of theropods, ornithopods, ceratopsians, and 
ankylosaurs (?), were the largest known for their clades.  However...

> In which case we need a LK bird capable of opening eggs. What could that be?
> *Ichthyornis*? :-/

Egg-opening plays second fiddle to hatchling snatching as a predatory
tactic in bird on big bird predation.

> > Small-medium dinos were the agents of the
> > "lawn-mower" Cretaceous ecology (large predators probably did not bother
> > with shrew-size mammals).  Once they disappeared, this enabled the
> > evolution of slightly larger mammals.  These mammals then crossed the size
> > threshold for preying on the small offspring of very large parents.
> 
> Which, of course, assumes that there was a size increase among
> end-Maastrichtian mammals of the world...

Right.