RE: The extinction of small dinosaurs

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

**Warning: This is a rather long response**

> Mammals that don't hibernate cannot be induced to.  Most 
> mammals in "equable" climates don't hibernate.  Birds 
> don't hibernate. Dinosaurs
> might have hibernated.

Good call.  Hibernation is actually not that common among 
mammals (since most are tropical).  On top of this, a large 
proportion of those that do are bats, which are not being 
considered here (may have been present pre-K/T, but no 
fossil information at present).

The hibernation-safety hypothesis lacks some backing, 
though it's good food for thought.  A great number of the 
surviving groups were unlikely to hibernate.  (After all, 
among extant birds, only a few Caprimulgiforms hibernate 
for extended periods).

However, I would be careful about the following 
statements:

> Except for...wait for it...mammals, snakes, lizards, crocs, birds,
> turtles, insects, molluscs, amphibians, flowering plants, gymnosperms,
> ferns, fungi, bacteria?

> Look, all animals species _but_ dinosaurs survived! 

This is a bit extreme.  The K/T event likely killed some 
species from all of the above groups (perhaps we'll never 
know for bacteria and fungi).  Mammals suffered 
extinctions, as did squamates, fish, and plants.  

Extinction events can be expected to be stochastic events; 
they seem selective to a large degree, but it is still best 
to think in terms of probabilities.  After all, if even 1% 
of a given clade survives an extinction event it has a 
chance to recover, but this would still represent a 99% 
species kill for that group.  Yes, dinosaur clades appear 
to have been hit especially hard, but to say that only 
non-avian dinosaurs were killed is incorrect; this is 
simply the largest group completely elliminated.

In fact, looking at the K/T from the simple "kill/no kill" 
standpoint does not demonstrate selectivity.  To 
demonstrate that the extinction was selective, you have to 
show that the survival ratios were significantly different 
from random.  Otherwise, we can argue that dinosaurs 
were elliminated by random chance because they had fewer 
species than those groups that survived (ie. as you kill 
species randomly, the smaller clades die first, by 
probability alone).

Now, it turns out that the kill difference is pretty 
substantial for the K/T.  To the best of my knowledge, it 
is highly significant, but I find it interesting how often 
this is assumed simply because some clades were lost 
completely and others only came close.

It is not particularly surprising (though it's still 
interesting) that dinosaurian taxa were hit hard in a mass 
extinction: larger-bodied terrestrial vertebrates are not 
doing very well in the modern extinction event, after all, 
and have been hit hard in other mass extinctions as well.  

I agree that placement factors may have allowed some 
species to survive.  I say this not because of any 
specific speculation, but simply because lucky placement 
has an effect in practically any global extinction 
scenario.  Disease, bolide, volcanism, climate cycles, etc. 
will all have differential effects based on where organsims 
are.

--Mike