Intra-Species Weapons
Some of the most fearsome weapons sported by herbivorous dinosaurs probably had little (or nothing) to do with fending off predators. You can see this by analogy with modern grass-eaters like bighorn sheep or deer, the males of which have evolved huge horns and antlers--not to scare away mountain lions, but to challenge other males for dominance in the herd or for the right to mate with females.
From a paleontologists' point of view, the frustrating thing about sexually selected characteristics is that it can be hard to tell between features that have evolved to impress males and females of the species and features that have evolved to impress predators. Many times, it's a combination of both--so while the main function of a Triceratops' horns may have been to grapple with other males, they may also have helped to deter smaller predators.
Here's a list of the intra-species weapons of the Mesozoic Era:
Horns. As mentioned above, the fearsome-looking horns of ceratopsians like Triceratops may only have been secondarily used to warn away hungry tyrannosaurs. Based on the position and orientation of ceratopsian horns, scientists have concluded that their main purpose was to duel other males for dominance or breeding rights. Of course, some unlucky males might be wounded, or even killed, in the process--paleontologists have discovered bones bearing these marks of intra-species combat.
Frills. The giant head ornaments of ceratopsians and other herbivorous dinosaurs served two purposes. First, oversized frills made herbivores look bigger in the eyes of hungry carnivores, which might opt to chase after smaller fare. And second, if these frills were brightly colored (as paleontologists believe they were), they could have been used to signal to and attract females. (Frills may also have had a third purpose, since their large surface area would have helped cool the animal down.)
Crests. Not quite a "weapon" in the classic sense, crests were protrusions of bone most often found on hadrosaurs (duck-billed dinosaurs). These backward-pointing growths would have been useless in a fight, but they may well have been employed to attract females (there's evidence that the crests of some Parasaurolophus males were larger than those of the females). It's also likely that hadrosaurs funneled air through these crests as a way of signaling to others of their kind.
Skulls. This peculiar weapon was unique to a family of dinosaurs known as pachycephalosaurs ("thick-headed lizards"). Pachycephalosaurs like Stegoceras and Sphaerotholus sported up to a foot of bone on their skulls, which they presumably used to head-butt one another for dominance in the herd and the right to mate. (There's some speculation that pachycephalosaurs may also have butted the flanks of approaching predators with their thickened domes.)
