New stegosaurs from the Middle Jurassic Lower Member of the Shaximiao Formation of Chongqing, China

Stegosaurs are a major clade of ornithischian dinosaurs, yet because of their fragmentary fossil record, their interrelationships and early evolution are poorly understood. Here, we describe a new stegosaur, <i>Bashanosaurus primitivus</i>, gen. et sp. nov., and some other indeterminate stegosaur materials. We provide new U-Pb detrital zircon ages for horizons bounding the holotypic quarry in the Lower Member of the Shaximiao Formation, Yunyang, Chongqing Municipality, China, which indicate a Middle Jurassic (Bajocian) age. <i>Bashanosaurus</i> represents the earliest record of stegosaurs in Asia and one of the earliest records of this clade in the world. The dorsal vertebrae, scapula, coracoid, femur, and plates of <i>Bashanosaurus primitivus</i> possess several unique characters among Stegosauria, including the elevation of the parapophyses of dorsal vertebrae on stalks at the base of the transverse processes, a flared distal end of the scapula, and a small acromial process. Among armored dinosaurs (thyreophorans), these features are reminiscent of the basally branching taxon <i>Scelidosaurus</i>, indicating that <i>Bashanosaurus</i> possesses a unique mosaic of plesiomorphic thyreophoran and derived stegosaur features. Phylogenetic analysis shows that <i>Bashanosaurus primitivus</i> is the earliest-diverging stegosaur, along with <i>Chungkingosaurus</i>, from the Upper Member of the Shaximiao Formation, consistent with the early age of the taxon.