Last night I had one of those ideas that made my eyes go wide, wake my wife up, and try to explain what was bouncing around in my brain before it skittered away into the recesses of my thoughts. I think I was able to grab a fragment of the idea, but the more I turned it over in my head the more I realized that it represents a much bigger problem than I originally anticipated. I was hoping to have a blog post up about it today, but it's going to require a bit more work than I expected.
In the meantime, I suppose I can tell you this. The idea occurred to me while thinking about the diversity of large carnivores in the African grassland vs. the diversity of large herbivores. In thinking about the evolutionary pressures put on these animals, the herbivores especially, I started to think about ornaments, sexual selection, natural selection (catching prey, or not being caught), and noticed a difference in the push and pull of selection in carnivores vs. herbivores. Translating this to the Mesozoic, herbivorous dinosaurs were much more disparate in form and diverse than large carnivores, which (with some exceptions) were much more conservative in body form. The recent work involving the strange correlation between abelisaurids, carcharodontosaurids, and spinosaurids in Africa then came to mind, especially when carnivores that live in the same area start to specialize and exploit different niches (if they were all after the same prey and had the same manner of catching it, they'd often come into direct confrontations with each other).
Anyway, the "big idea" I'm trying to get at is how predator-prey relationships, competition for food, and competition within a species for mates are all acting on large animals, and what happens when a certain pressure (like a predator) is removed. It's probably too big and complicated for me to effectively address here, but I'm going to keep at it for a bit.
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Sounds great! I look forward to reading the results.
...What?
One wonders if food pyramid has something to do with it. The herbivores have a much larger and more diverse resource base than the predators. Thus the possibility of both larger populations of individuals and larger numbers of kinds of herbivores. Perhaps the reduced, both in size and compexity, resource base of the carnivores limits both population size and number of supportable kinds of carnivores.
I see what Jim means. It seems to me that if predation pressure were removed, the prey organisms would deviate further, based on not being pushed to compete in an evolutionary arms race in the same direction. In any case, I'm intrigued.
I have commented on the surprising conservativeness of the theropod bauplan compared to ornithischians and sauropods in a couple of grant applcaitions, thopugh not yet formally in print, and it has been ahppily ignored by all and sundry. It is an interesting point and something I am working on (as is sexual selection in dinosaurs). I would say more, but ideas / papers are in the proto-stage. I don't want to go shouting about unpublished ideas, so I'll leave it there.
Just to let yo know it is out there. I am not being cocky, just pointing out that you are not the only one puzzled by this :-)