Psittacosaurus is a bipedal/quadrupedal ceratopsid sister to Yinlong, recently added to the large reptile tree (now at 689 taxa). Scoring involved a good look at the skull. Note a recent interpretation (You et al. 2008) deletes the vomer, palatine and ectopterygoid (Fig. 2).
The ceratopsian palate
rises, with essentially vertical maxillary walls lining the central choanae. This obviates the need for the vomers, palatine and ectopterygoid, which become incorporated with the maxilla. Evidently You et al. thought they were gone. I see vestiges here, at least on one side, and note them above.
The ceratopsian ancestral taxon
is currently Hexinlusaurus, a gracile, long-necked, small headed ornithischian close to Dryosaurus. Unfortunately, the premaxilla and naris are missing in the Hexinlusaurus holotype.
Osborn HF 1923. Two Lower Cretaceous dinosaurs from Mongolia. American Museum Novitates 95: 1–10.
Xu X, Forster CA, Clark J M and Mo J 2006. A basal ceratopsian with transitional features from the Late Jurassic of northwestern China. Proceedings of the Royal Society B: Biological Sciences. First Cite Early Online Publishing. online pdf
You H−L, Tanou K and Dodson P 2008. New data on cranial anatomy of the ceratopsian dinosaur Psittacosaurus major. Acta Palaeontologica Polonica 53 (2): 183–196.