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However, I still think that six species of Psittacosaurus was an awful large amount, especially if they were the same general size and had the same general diet. P. neimongoliensis is very similar to P. mongoliensis, judging by what I have read. <<<

Although the skulls are fairly similar, the postcrania are realy quite diferent. In the process of doing a skeletal reconstruction of P. neimongoliensis a few years ago I had the opertunity to look over a cast fairly for quite a while, and I can assure you that there is a strong downcurve in the dorsal column, and no visible ligamental ossfication. For a while I assumed that the P. mongoliensis specimens were probably restored with too straight a dorsal column, but after further review I'm fairly certain that Osborn (and so far everyone after him) was correct in the articulation of the dorsal column.
As a byproduct of a more downturned dorsal column, P. neimongoliensis' forlimbs nearly drag on the ground if not moving quadrepedaly, and so may have spent a lot more time on all fours than P. mongoliensis. In this way, P. neimongoliensis is much more similar to what I would exect for the ancestor of Certaposians; of course, all Psittacosaurs are too derived to be literal ancestors (without lots of nasty reversals and other non-parsimonious hogwash...).
Psttacosaursus may be over split, but then look at Felis, which often has multiple species' ranges overlapping. Also remember that many of the finds that are in the "same" strata can still be separated by thousands of years, so temporal variation in a population, as well as range expansion and contraction by related species could both result in a geological column that at our measly level of resolution appears to contain many congeneric species.
I dare say that it _may_ be only the paucity of the fossil record that keeps us from seeing this kind of speciation pattern in all dinosaur bearing rocks.

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