Why did small dinos become extinct?

[Philip Chalmers <philipchalmers@blueyonder.co.uk>]

The standard type of explanation for the extinction of the dinosaurs is 
"Vegetation was devastated by catastrophe(s) which shut down 
photosynthesis, so vegetarian dinos starved to death, and carnivorous 
dinos quickly followed them." To explain why birds and mammals survived 
the end-Cretaceous extinction, the same type of theory says something 
like, "They ate seeds and invertebrates, i.e. their food chains depended 
on seeds and detritus rather than vegetation. In addition many of them 
were burrowers and / or divers, so had some protection again the effects 
of the  catastrophe(s)."

I have always been puzzled about why no burrowing or small predatory 
dinos survived the K-T extinction. Small small predatory dinos should 
have been able to feed on birds, mammals, lizards and small amphibians, 
which survived rather well - in other words, small predatory dinos 
should have survived at the top of the detritus-based food chain.

We now know of burrowing dinos (Oryctodromeus and possibly Orodromeus). 
Admittedly Oryctodromeus was vegetarian and Orodromeus was a close 
relative, so both were doomed under the standard explanation. But why 
were there no burrowing small predatory dinos in the latest Cretaceous 
or, if there were, why did they perish?

The only answer I can come up with is that there were no small predatory 
dinos in the latest Cretaceous, burrowing or otherwise. 
http://www.dinosauria.com/jdp/misc/hellcreek.html says Dromaeosauridae 
represent only about 2% of the paleopopulation at Hell Creek and 
Troodontidae  only 1% - and both groups are represented only by teeth. 
The fact that these groups are represented only by teeth is a little 
suspicious and may point to re-working. And Troodon's teeth have led 
some researchers to suggest that it was an omnivore or even vegeratian 
(Holtz, T.R., Jr., Brinkman, D.L. and Chandler, C.L. (1998). "Denticle 
morphometrics and a possibly omnivorous feeding habit for the theropod 
dinosaur /Troodon/." /Gaia/, *15*: 159-166)

The best explanation I can think of for the absence of latest Cretaceous 
small predatory dinos is that they died out earlier as a result of 
competition from birds, mammals (ecological successors of Repenomamus) 
and juveniles of larger carnivorous species - and juveniles of larger 
carnivorous theropods were doomed because they grew and in a few years 
had to depend on the photosynthesis-based food chain. But I've been 
unable to find anything that might support this idea.

Can anyone provide or point to a good explanation for the extinction of 
the small predatory dinos? If so, could you please provide names and 
authors of relevant books / articles.




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