Re: Armadillos at the K/T!

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

On Sun, 30 Sep 2001, Michael Bruce Habib wrote:
> > I'm only claiming that a niche disappeared: that of terrestrial large
> > oviparous species.  I would like to decouple other extinctions.
> 
> This particular "niche" you have described is so broad, I 
> have a hard time accepting it as such.

How, then, would you describe it?  Assume for a minute that what I am
arguing were true--how would you describe it?  Because of new predators,
the particular reproductive strategy no longer worked.

> Since nearly 
> all large-bodied late Cretaceous vertebrates were 
> oviparious, the extinction of these groups really says that 
> the large Cretaceous vertebrates suffered a high rate of 
> extinction at or near the K-T, it does not represent the 
> ellimination of a particular niche.

No, it depends what killed them!  If an aspect of their normal activity
was made nonviable, and they all shared this aspect, and this aspect has
been non viable ever since, and it's due to predators that arose
before/near the K/T...then?

> Factors that can kill a number of individuals, or even a 
> population, may not have much chance at all of killing a 
> species, if that factor does not act over enough area, 
> populations, etc.

True.  But the re-evolution of large body size may have been _prevented_
by predators on their offspring.  If so, they are able to maintain this
selection globally--except, of course in places like New Zealand where
they don't exist.
 
> Raup (1991) is nice and to the point. He states:
> "Widespread species are hard to kill.  Species extinction 
> can be accomplished only by the elimination of all breeding 
> populations.  Predators must be active over the whole 
> range, not merely most of it.  The same is true for 
> extinction casued by competition.  If the agent of 
> extinction is a physical disturbance, the killing condition 
> must exist everywhere the species lives."

No doubt.  But one thing that makes this easier for predators of nesting
species, especially large, ground bound nesting species, is that the
number of locations they can nest is limited, they probably use the same
sites year after year--thus they are relatively easy to find.  Some
imperatives such as these may account for the release of size constraints
in areas of low predator density (e.g., NZ).

> I would argue that higher taxonomic groups that are 
> widespread are also very hard to kill, for the same 
> reasons.  I dare say that dinosaurs, as a group, were 
> pretty widespread in the late Cretaceous.

And probably after it, as well (and I'm not talking birds).

> An adult nile monitor is indeed larger in mass than a 
> badger, if you compare the maximum weights of each.  The 
> average of the two is quite close though.

Thanks for the data.
John Bois.