Without lips, that is. So since dinosaurs could not drink from straws or even whistle to call a cab, there's no doubt that they could not have evolved into more intelligent species. What kind of a civilization could they have without swirly straws?! Okay back to topic.
Now cheeks. So I drew this wounded Triceratops for paleontologist Andy Farke:
It was a fun learning experience. I started with the bones (which were later modified). The wounded fossil lacked much of the skull, so I reconstructed the fossil cranium according to Andy's direction (i.e. which skull to base this skull off of) so you can see the chunk missing from the frill:
Among the many things I found out is that ceratopsians (horned dinosaurs like Triceratops) probably had ridges on their horns, like you see today on rams and such. These ridges grow on the sheath that covers the bony core of the horn, so they wouldn't be present in the fossil specimens. Here's an example from a modern ram:
The drawing itself illustrated the wound on the frill. A big chunk was missing and the edge of the bone showed regrowth and healing. Which is awesome.
There were other decisions about what the dinosaur should look like. These horned dinsoaurs have a long row of plant-chopping teeth that is a little inset from the edge of the jaw. If I can find an illustration of that, or the time to illustrate it, you'll see it soon. I wanted to know how Andy wanted the cheeks to look, so I sent him some sketches.
Sketch one is the classic cheeked look. Scientists long assumed that the tooth row was inset to make room for a cheek, which would keep the food from falling out of its mouth when eating:
This is a half-cheek; same great flesh, less cheeky:
The next sketch is a lipless, cheekless face. The lower jaw has an upward-projecting muscle attachment site (like the coronoid process on human jaws), which means a lipless, cheekless jaw would still end at that muscl-ey area:
This is a lizard-like lipped mouth closed:
And open, like a smile:
Now, the mouth would have occluded (I learned), so there would be no space between the upper and lower beaks.
The final choice was cheeks not because they were the most scientifically accurate, but because they are conventional and the focus is the frill, which is very rational. Here's the drawing in color:
So that was a little bit more about the choices that went into a scientific illustration...