Redescription of Gigantspinosaurus sichuanensis (Dinosauria, Stegosauria) from the Late Jurassic of Sichuan, Southwestern China

Gigantspinosaurus sichuanensis is one of the six Stegosauria genera discovered from the Sichuan basin, which preserves the first skin impressions of stegosaurs around the world and a huge pair of ‘comma’ -shaped parascapular spines kept in situ, and being named after the latter feature. The holotype was firstly named and reported in an abstract of a lecture by Ouyang, 1992, since when it has never been detailed studied and the taxonomic position of Gigantspinosaurus is also vague. The morphological redescription shows that G. sichuanensis is a medium-sized stegosaur, with external mandibular foramen developed. The ratio of femur to humerus is large, and the intersacral fenestrae are big. According to the wear degree of teeth, the holotype of G. sichuanensis is regarded as an adult individual. On the basis of the recent data matrix of stegosaurs and the characters revisions of G. sichuanensis, its phylogenetic position has been determined again. By our detailed morphological and phylogenetic analysis, G. sichuanensis is considered to inherit some primitive traits, but it is more derived than Huayangosaurus, and located in a transitional position between Huayangosaurus and Tuojiangosaurus, as a kind of evolved stegosaurs. The ancestors of Stegosauria are small and quadruped, with primitive ornithopod-like skull, and grow leaf-shaped teeth, a large number of bone plates.