Re: large body size vs little dinos

[John Alroy <jack@homebrew.uchicago.edu>]

1) Even a "small" dinosaur like Troodon would have been a giant  
compared to most other tetrapods living at the time. In the Recent,  
most land-dwelling tetrapods are either birds (uniformly small),  
frogs (ditto), squamates (including only a few giant snakes and  
varanids), or mammals (which are mostly rodents, bats, insectivores,  
and other small beasties).

2) Crocodylians SURVIVED the K-T, but they also suffered a mass  
extinction, and a pretty serious one.

3) It may or may not be true that specialists outcompete generalists  
in normal times. The paleontological data on this haven't been  
analyzed fully, as far as I know (maybe I've overlooked a key  
reference). However, if you look at large mammals (the only good  
living analog of dinosaurs we have), the carnivores are typically  
generalists (just look at hyaenas, bears, whatever), and the large  
herbivores tend to be indiscriminate apart from a preference for  
browse or graze, as I mentioned before. More importantly, what you  
are getting at is that terrestrial dinosaurs were RELATIVELY  
specialized compared to all the other tetrapods that had been  
diversifying since the Permo-Triassic, e.g., birds, mammals,  
squamates, amphibians, crocodylians, pterosaurs, turtles. But if you  
look at the RATE of speciation in those groups, as measured directly  
from the fossil record, it's clear that mammals speciate like crazy  
compared to dinosaurs - so shouldn't the mammals have evolved  
themselves into an over-specialized grave? And if you look at the  
total diversity of those groups (another way to get at rates of  
diversification), three of those groups (mammals, squamates,  
amphibians) must have been an order of magnitude more diverse than  
terrestrial dinosaurs, and mammals were probably twice or three times  
as diverse.

4) I think we're not going to have much success trying to figure what  
kind birds survived the K-T by looking at Recent groups that appear  
to be old enough to have been around (e.g., ratites), just because we  
know next to nothing about the groups that made it into the Tertiary  
but not into the Recent. This is exactly the kind of reason I am  
always obsessing on mammals as a test-case for everything; as  
tetrapods go they have an excellent fossil record and you can look at  
these questions more directly. That said, there were a few giant  
predatory terrestrial birds in the Paleocene and Early Eocene of  
Europe, North America, and South America (they lasted until much  
later in South America), although I think their ancestry is obscure.