This is a nice one. I never liked when people put those strange long feathers on just one small part of its back and color it green. Really, it seems so silly for me. Like some kind of bush or grass sticking out of a poor dinosaur :/ So good job not adding them to it!
Where's the quills?
The quills aren't found on all specimens, even some well-preserved Liaoning ones, it's possible that some Psittacosaurus didn't have them.
I'm guessing gender dimorphism. I.e. perhaps the males had quills and the females didn't.
Maybe they were just not preserved
That's definitely possible, but there are quite a few good specimens of Psittacosaurus from the Yixian that don't show quills. I imagine that probably some species had them, and others might not have - or maybe preservational bias IS to blame completely.
Thanks CMIPaleo for jumping in on that question Couldn't of said it better myself. And Thanks XStreamChaosOfficial for your starting the discussion. I'll do other Psittacosaurs in the future I'm sure. I do like the look of the quills and it would be fun to paint or sculpt a specimen that shows them preserved.