Re: Nesting with Dinosaurs.

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

On Fri, 21 Apr 2000, Matt Wedel wrote:
> > This question assumes as fact something which is clearly not: that mammals
> > and birds changed little from the Triassic.
> 
> Actually, I hadn't assumed that.  What I had assumed was that:

> 1) banging two eggs together requires neither special structures nor a great
> deal of intelligence, and I see no reason why therians should be so much
> better at it than "lower" groups of mammals as to drive entire clades to
> extinction.

I disagree.  It is a behavior which takes some level of intention.  I
don't believe it is amenable to fixed action patterns since the parameters
must be very complex.  I mean, you have to find the nest, consider where
the parent/if the parent is present, dig, set up the eggs so that one
connects with the other, and so on.  I loved the scene in WWD where the
_Didelphodon_ plucks the egg out of the nest and rolls it onto a
rock."Smart little bugger," I thought.  The Egyptian vulture picks up a
rock and hurles it at the egg.  But this probably is a fixed action
pattern: see round thing; pick up rock with beak; throw in direction of
egg; repeat.  Didelpodon's behavior (I realize it's fiction) reminded me
more of crow-style smarts.
        This, then, leaves dinosaurs themselves as nest predators.  While
I believe they were probably very effective at this, they had deficits
relative to mammals.  First, they tended to be large; it's possible that
the very size of dinosaurs was adaptive for defending against large
size--and _maladaptive_ at defending against small creatures. Second,
predator prey oscilations that may have affected dinosaurs
may not have affected mammals if they were omnivorous (were not dependent
on dinosaurs but took egg delicacies where they could get them). Third, 
arguments can be made that dinosaurs were diurnal, mammals nocturnal--this
advantage is manifested in attacks on ostrich nests by much smaller
jackals (only at night).

> 2) AFAIK Cretaceous neornithines do not appear to be any more specialized for
> whacking baby dinosaurs than enantiornithines.

Neornithines are very well-adapted for such whacking!
Enantiornithines aren't here--we may never know their talents
in this regard.  However, it may be
possible to discover that the neornithines gradually outcompeted
enantiornithines.  If this happened, it's reasonable to hypothesize they
were better at some things than enantis.  For example, if neornithines
could alight closer to dinosaur nests because they could escape more
quickly, this gives them a wider window of opportunity for stealing
hatchlings.  This behavior is seen against rhea chicks, and ostrich.

 > 3) if dinosaur extinction was caused by egg-predation by metatherian or
> eutherian mammals, then I would expect the extinctions on each continent to be
> temporally correlated with the arrival of those mammals.

I believe the exact timing of extinctions is known in very few places.
 
> with little hint or prevue.  Evolution takes time--why didn't the dinos evolve
> some defenses to increasing pressure?

You could ask this question of any extinct organism.  I don't see what
defences dinosaurs could have.  Indeed, the paucity of post K/T big egg
layers argues strongly that this is the case regardless of the cause of
non-avian dinosaur extinction.  And, in the apparent disappearance of
small dinosaurs towards the K/T we may have a preview.

> If placentals and neornithines are so good at taking out dinosaur nests and
> nestlings, why do ratites survive at all?  You are arguing that placentals and
> neornithines could wipe out ALL non-avian dinos in the Cretaceous, but can't
> take out a handful of flightless birds that don't cover their nests.

There are refugia.  Grasslands--the biome of most ratites is an important
one; it didn't exist in dino days!

> > Irony usually functions to ridicule ridiculous ideas.  The above idea
> > seems rather grounded.  If not, why not?
> 
> Whoa, hoss.  I meant no offense--just having some fun with the concepts and
> introducing some humor into this all-too-serious list.

Thank you for pulling me back from the precipice!  I found your piece
to be very funny.  See archives for reasons for overquick reaction time on
my part.