Re: Armadillos at the K/T!

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

> > 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.

I would describe it as such: that most of the large-bodied 
vertebrates were killed near or at the K-T, and that the 
large-bodied vertebrates of the time happened to be 
oviparous.  The subsequent large-bodied species radiation 
included mostly mammals, which happen to be live-bearers.  
Thus the reproductive mode of large-bodied species changed 
between the Mesozoic and Cenozoic faunas, but not due to 
selective pressures working on reproductive modes 
themselves.  The extinction event may have killed 
large-bodied species, for example, without regards to 
reproductive mode. 

I would also tend not to decouple the other extinctions of 
the same time (marine extinctions, avian extinctions, 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.

I (like several others thus far) place an argument for 
competitive exclusion, instead of predation, as the 
mechanism that has maintained a small number of large 
oviparous species.  As evidence, I too point to New 
Zealand.  There are plenty of predators in New Zealand, and 
there have been for some time.  There are few mammalian 
predators, to be sure, but the islands have a number of 
predatory bird species.  These most certainly eat young and 
eggs, as well as adults.  Some fossil raptors from the 
islands were extremely large, and probably fed on moas.  
In New Zealand, nest predators are certainly present, but 
mammalian predators are not (well, they were not until 
recently), and so some avian lineages responded to 
selection for larger size, as they were no longer excluded 
from this niche competitively.  

Michael Habib
mbh3q@virginia.edu
University of Virginia