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| Bashanosaurus primitivus Dai, Li, Maidment, Wei, Zhou, Hu, Ma, Wang, Hu & Peng, 2022 |
ABSTRACT
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, Bashanosaurus primitivus, 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. Bashanosaurus 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 Bashanosaurus primitivus 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 Scelidosaurus, indicating that Bashanosaurus possesses a unique mosaic of plesiomorphic thyreophoran and derived stegosaur features. Phylogenetic analysis shows that Bashanosaurus primitivus is the earliest-diverging stegosaur, along with Chungkingosaurus, from the Upper Member of the Shaximiao Formation, consistent with the early age of the taxon.
DINOSAURIA Owen, 1842
ORNITHISCHIA Seeley, 1887
STEGOSAURIA Marsh, 1877
BASHANOSAURUS PRIMITIVUS, gen. et sp. nov.
Diagnosis—Bashanosaurus primitivus differs from all other stegosaurs by possession of the following autapomorphies: (1) the parapophyses of dorsal vertebrae are elevated on stalks at the base of the transverse process and are clearly visible in anterior and posterior views; (2) the scapula blade is slender and flares distally; (3) the acromial process of the scapula is small, triangular and poorly developed; (4) the 4th trochanter of the femur is positioned below the middle of the shaft; (5) the bases of plates are strongly convex and transversely thickened, and are separated from the anterior/posterior margins of the plates by a distinct groove.
Etymology—The generic name “Bashan” refers to the ancient name of Chongqing, and the specific name “primitivus” indicates that this newly named species represents the earliest-diverging stegosaur.
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CONCLUSIONS:
Bashanosaurus primitivus is the earliest record of Stegosauria in Asia and represents one of the earliest records of this clade from anywhere in the world. Geochronological data and analysis support a Middle Jurassic (Bajocian) age for the Shaximiao Formation in the vicinity of Pu’an Township, China. Phylogenetic analysis indicates that Bashanosaurus primitivus is the earliest-diverging stegosaur along with Chungkingosaurus supporting an Asian origin for Stegosauria based on currently available data.
Dai Hui, Li Ning, Susannah C. R. Maidment, Wei Guangbiao, Zhou Yuxuan, Hu Xufeng, Ma Qingyu, Wang Xunqian, Hu Haiqian and Peng Guangzhao. 2022. New Stegosaurs from the Middle Jurassic Lower Member of the Shaximiao Formation of Chongqing, China. Journal of Vertebrate Paleontology. e1995737. DOI: 10.1080/02724634.2021.1995737
