Re: attack on dinosaur--horrific video

[don ohmes <d_ohmes@yahoo.com>]

"Crocodiles nest on unprotected riverbanks, with themselves as the only 
protection. And indeed, their nests often are raided in the middle of
 the 
day. How is it possible that they are still with us?" -- David Marjanovic

Not exaactly who is on which side here, and the above may be a rhetorical 
question. But anyhow --

In the case of long-lived but fairly prolific animals it is not necessary to 
have a successful nesting season every year or possibly even every decade. Even 
where the tactical vulnerability of the prey to predation is very high (such as 
when dinos/crocs in egg form are the prey), annual boom/bust cycles in 
short-lived predator populations, perhaps driven by independent factors,  can 
create windows of reproductive opportunity. 

Or there simply may not be enough predators to eat all the prey, even in boom 
years, again due to factors that may be independent of the prey/predator 
relationship (the predator may have pathogen-driven reproductive issues of it's 
own, for instance). This would apply to long-lived predators.

More generally -- tactical vulnerability can also be countered by finding 
refuge, if it lasts. Transgression of nesting refugia is a key component of the 
dino vs mammal case.  

Lastly, a question -- does predation really appear in the record to be a 
primary extinctor of taxa? It sometimes happens of course, transgression of 
island refugia by invasive species being a prime example, but isn't it usually 
more of a 'coup-de-grace' type of thing, in populations reduced by 
environmental change or niche competition?

Don

----- Original Message ----
From: David Marjanovic <david.marjanovic@gmx.at>
To: DML <dinosaur@usc.edu>
Sent: Thursday, November 22, 2007 8:02:53 AM
Subject: Re: attack on dinosaur--horrific video


>>I will just bring up *Repenomamus giganticus* again, which must have 
>>ushered in the Great Barremian-Aptian Boundary Mass Extinction,
 except it 
>>somehow didn't.
>
> Anne Weil discusses this anomally, saying research is going on trying
 to 
> find out if dinosaurs at that place and time were smaller than usual.
 
> Certainly there are selection pressures for being large or small.
  What 
> were they in each case?  Dinosaurs were not smaller in the late K.  
 Also, 
> _N. improvida_ and _C. magnus_ were very different creatures from _R.
 
> giganticus_.  I don't know that much comparison is afforded--other
 than 
> the interesting question of relaxation on size constraints enjoyed by
 both 
> clades, likely for different reasons.

I repeat: By your logic the two species of *Repenomamus*, not to
 mention 
*Gobiconodon*, triggered a mass extinction among egg-layers. How is it 
possible that they didn't? If the answer lies somewhere in "very
 different 
creatures", then please elaborate. Remember the specimen of
 *Repenomamus 
robustus* that has *Psittacosaurus* as stomach contents, so we know
 what it 
did, and we also know that *Psittacosaurus* went on to survive for the
 rest 
of the Early Cretaceous (some 25 million years).

>>Not even the crocodiles died out.
>
> ??

Crocodiles nest on unprotected riverbanks, with themselves as the only 
protection. And indeed, their nests often are raided in the middle of
 the 
day. How is it possible that they are still with us?

>> Are you certain that ground-nesting birds are generally night-blind?
>
> I know an attended ostrich nest is practically immune from predation
 in 
> the
> day--but that, if discovered by black-backed jackals, they are
 invariably
> destroyed at night, i.e., the ostrich can defend effectively in the
 day 
> time
> but not at night.

That is one. I was talking about "generally".

But I don't think this will lead us anywhere. With fossils, all
 evidence we 
have is the relative size of the empty eye socket...