New data on cranial anatomy of the ceratopsian dinosaur Psittacosaurus major. 2008. H.−L. You et al. Acta Palaeontologica Polonica 53: 183-196
Abstract : An exceptionally preserved skull and mandible of the ceratopsian dinosaur Psittacosaurus major revealed the existence of an elliptical median interpremaxillary foramen, a prominent neurovascular canal on the internal wall of the beak, long, slightly divergent basipterygoid processes developed as vertical blades with a deep cleft between them, and horizontally oriented vomer.
The new specimen shows two autapomorphies of P. major, the transversely narrow dorsal skull roof and very prominent dentary flanges, confirming the presence of two large−skulled psittacosaur species in the Lujiatun Bed of the Lower Cretaceous Yixian Formation in Beipiao City, western Liaoning Province, China, the long− and narrow−skulled P. major, and broad−skulled P. lujiatunensis.