Here's an interesting question: After the Fall, did carnivory and animal violence originate gradually or suddenly? It seems to me that going from a world with no animal death to one where animals and people died would require a huge change in the entire creation. The old scenario of carnivores getting a taste of meat by nibbling on carcasses really doesn't cut it. Think about it: if you were a happy vegetarian, why would you ever take a bite out of some dead body? Ick. It's far more likely that carnivory originated by an intentional redesign of creation at the Fall.
A new paper in PLoS ONE is relevant to this question. In "Evidence of combat in Triceratops," Farke et al. looked at damage to the skulls of two ceratopsian species: the aforementioned trikes and Centrosaurus. Why two ceratopsians? It's rather clever, actually: Centrosaurus lacks the two large horns present above the eyes of Triceratops. Centrosaurus has much smaller horns above their eyes, nothing they could do much damage with. Now if trikes used their horns to fight with each other (for territory or mates), they should have lesions on their skulls just behind the eyes on the squamosal bone. Centrosaurus, with their stubby horns, should have less damage behind the eyes than trikes.
Sure enough, of the 58 trike squamosals examined, ten showed signs of damage. In contrast, only one Centrosaurus squamosal out of 62 showed signs of damage. Other bones of the skull had similar frequencies of damage in both species. These results imply that trikes used their elaborate frills and horns for combat with each other.
Now I could ramble on about pre-Flood animal aggression and blah blah blah, but the real point of this post is to talk about fighting dinosaurs!!! Come on, with a title like "Evidence of combat in Triceratops," how could I resist? I'm actually watching Willis O'Brien's stop-motion dinos in 1925's The Lost World as I write this.
And so in conclusion, go fighting dinosaurs!
Farke, A.A., E.D.S. Wolff, and D.H. Tanke. 2009. Evidence of combat in Triceratops. PLoS ONE 4(1): e4252 doi:10.1371/journal.pone.0004252.