Psittacosaurus jugals

["Bruce R. Danz" <brd@worldnet.att.net>]

I appreciate the notes on Psittacosaurus jugal variation among species. 
Any explanation for the lack of a jugal horn on some specimens?  (As
seen in Norman's "The Illustrated Encyclopedia of Dinosaurs," p.132,
where a photo of a Psitticosaurus mongoliensis specimen is shown without
an apparent jugal horn.)

Bruce R. Danz