This is a rather better photo of the skull of this theropod than I’ve managed before, and with my recent paper on dinosaur crests now out, it seemed a relevant time to post this up. Monolophosaurus is famous for having quite a large crest of bone along the midline of the skull that runs from the front to the orbit – it’s rather hidden here as the shot is taken from a low angle, but you can see the enlarged naris and parts of it above the antorbital fenestra.
For a theropod, this is really quite a large crest which might sound odd, but in fact makes a lot of sense. We sort of discuss this within the sexual selection paper but in hindsight we could have made clearer so I thought I’d mutter about it a bit here. The short version is that herbivores spend a great deal of their time foraging for, and consuming, food. That puts them in the firing line of predators of course, but that’s going to be a major issue no matter what they do, and of course adults tend to be at a rather lower risk than do juveniles. As such, for something like a Triceratops or your average hadrosaur, having a large crest on the head is not likely to quite have such a major effect on their survival.
A predator however has a rather bigger issue. It’ll feed less often and unlike the average plant chewer, it’s prey can get out of the way and could see it coming. No matter your hunting style, the sooner potential prey sees you coming, the harder it’s going to be to grab it. As such, even a relatively small crest on your head might well be the end of your dinner if it’s seen, and of course the head is the one part any hunter would have to poke out of cover to see, hear or smell it’s targets.
In short, you’d expect theropods to have smaller crests than ornithischians becuase they have fundamentally different ecologies and are under rather different degrees of evolutionary pressure, even if the mechanism and origins (obviously here I’d suggesting sexual selection) are the same. And what is what you see, in proportion to the size of the animal, theropods crests tend to be quite small (Guanlong has the biggest one I can think of) but for the ornithischians, a crest that’s close to the size of the head it’s attached to is pretty normal. I’m not aware of anyone having commented on this size discrepancy before, but regardless of the drive for a crest, the ecological differences really would explain it quite simply.