Re: Armadillos at the K/T!

[Michael Bruce Habib <mbh3q@cms.mail.virginia.edu>]

A couple of rather generalized questions and comments...

> 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.  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.

> First comes mortality, then extinction.  Right?  If extant organisms
> suffer much mortality due to a certain factor, that factor should be
> suspect if extinction occurs.

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.

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."

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.

> > > Badger-size is a threshold under (or around) which many offspring
> > > predators of today operate: hairy armadillo, caracara hawk, coatis,
> > > monitor lizards, skunks, squirrels, cats, rats, weasels, foxes...I could
> > > go on.

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.  Caracaras are 
significantly smaller than a badger, as are rats, weasels, 
squirrels.  There are numerous nest-raiding snake species 
as well, some of these are very small indeed. There may be 
a threshold, but the threshold mass (if it exists) is 
somewhere below "badger" mass.

Michael Habib
mbh3q@virginia.edu
University of Virginia