Re: Nocturnal crocs?

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

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.

> > 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.

> > 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?

> 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.

> > 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.