Re: Nocturnal crocs?

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

----- Original Message -----
From: "John Bois" <jbois@umd5.umd.edu>
To: "David Marjanovic" <david.marjanovic@gmx.at>
Cc: "The Dinosaur Mailing List" <dinosaur@usc.edu>
[no problem, but that way I always get mails twice]
Sent: Saturday, August 25, 2001 1:53 PM
Subject: Re: Nocturnal crocs?


> On Sat, 25 Aug 2001, David Marjanovic wrote:
>
> > > Dinosaur diversity was probably not affected much at the K/T
> > > (inasmuch as bird diversity making it through may well have
> > > been greater than non-avian esxtinction).
> >
> > Dinosaur diversity was reduced to a few species of Neornithes... I don't
> > understand something here.
>
> We have no idea how many species of birds were alive at the time.  We
> have no evidence to suggest _any_ became extinct.  I suggest the burden of
> proof rests with those who would claim that what you say is true--I mean,
> there is no evidence (other than negative, and birds are famously
> underrepresented in the fossil record) of bird extinctions.

Hesperornithiformes, Ambiortimorphae incl. *Ichthyornis*, Enantiornithes...
at least some of those were present late in the Maastrichtian.

> > > Instead, the K/T represents the
> > > death of a niche--that filled by large oviparous species in
terrestrial
> > > locations.  This niche has been practically empty ever since!
> >
> > Why should there be extra niches for _oviparous_ animals? How does this
> > affect their ecology?
>
> Let me count the ways.  Just one example.  A nesting dino must stay in one
> place for 3 months and live off local resources.

Then why didn't they die out much earlier -- and evolve in the first place?
3 months?

> > > Today,
> > > mammals and birds (and, to a lesser extent, reps) appear to be the
agents
> > > keeping that niche empty.  The question then becomes: at what time did
> > > this first start being the case?
> >
> > IMHO modern mammals occupy quite closely the same niches as dinosaurs
> > earlier (some niches for large animals being empty since we emptied them
in
> > the Pleistocene).
>
> Then why no competition?

I repeat: _Modern_ mammals, which evolved to occupy these niches _after_ the
dinos were gone (and _after_ a short empty-world period). I mean,
*Uintatherium* is Eocene, not Cretaceous.

> > Well, *Gobiconodon* also had such a size, and *Repenomamus* (*-nus*?)
was
> > even larger.
> > Such animals (along with LK varanoids) would have produced a selection
> > pressure for better nest defense/concealment, and obviously dinosaurs
> > survived this.
>
> I'm trying to build a case for increasing pressure, summation, etc.  Yes,
> there were likely always predators, just as there are for today's
> oviparous survivors. This doesn't mean that pressure cannot become too
> strong for a specific reproductive strategy to withstand.

But it does mean that you have to show the pressure increased... :-)

> > > Whichever sex it is, it is driven off the nest by jackals taking
advantage
> > > of their superior night vision.
> >
> > Jackals have better night vision than ostriches? How come?
>
> Different eyes.

That's what I mean -- don't ostriches have much better eyes? Their eyes are
not only much larger but are _bird_ eyes, means, they have a lot more
light-sensitive cells per area than mammals. (Apart from having 4
color-sensitive opsins instead of 2, which is most likely irrelevant at
night.)

BTW, apparently ostrich males seem to be capable of quite some defense nest
defense -- they are still alive and have lived on canid-infested continents
as long as the canids themselves AFAIK...

Wait a minute. Do you think any mammal would dare to attack a brooding
troodontid or dromaeosaur?