Abstract
Tetanurae is a special group of theropod dinosaurs that originated by the late Early Jurassic. It includes several early-diverging groups of generally large-bodied predators (megalosauroids, allosauroids, tyrannosauroid coelurosaurs) as well as morphologically disparate small-bodied coelurosaurs, including birds. Aspects of the evolutionary history of tetanurans remain contested, including the topology of their deep phylogenetic divergences (among Megalosauroidea, Allosauroidea and Coelurosauria). We report a new theropod, Yunyangosaurus puanensis gen. et sp. nov., based on a fragmentary specimen recovered from the Middle Jurassic **ntiangou Formation of Chongqing, southwestern China. It shares several features uniquely with some megalosauroids (the clade of megalosaurids + spinosaurids + piatnitzkysaurids), such as prominent rims around the anterior articular surfaces of cervical centra and bifurcated anterior dorsal neural spines (present in piatnitzkysaurids). Nevertheless, it also shows several features that are rare or absent among megalosauroids and more crownward tetanurans, including prominent spinopostyzgopophyseal laminae (also present in non-tetanurans and metriacanthosaurid allosauroids), flat anterior articular surfaces of the cervical centra (also present in piatnitzkysaurids and some earlier-diverging tetanurans), and the presence of a posterior pneumatic foramen or fossa (absent in most tetanurans, but sporadically present in some cervical vertebrae of piatnitzkysaurids). Yunyangosaurus therefore presents a combination of derived and apparently primitive character states that are not seen in other theropods. This suggests that patterns of morphological evolution associated with deep tetanuran divergences were more complex than currently recognized, with implications for understanding the character evolution in theropods.
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Introduction
The Middle and Upper Jurassic Shaximiao Formation of southwestern China has produced spectacular fossil remains of dinosaurs and many other vertebrates1,2. Significant discoveries from this formation in the Chongqing area include the sauropods Mamenchisaurus hochuanensis, Omeisaurus changshouensis, and the theropod Yangchuanosaurus shangyouensis3 as well as the stegosaur Chungkingosaurus jiangbeiensis4. Compared to the fossiliferous Shaximiao Formation, the underlying lower Middle Jurassic **ensis - a new species of carnosaur from Zigong, Sichuan. Vertebrata PalAsiatica 30, 313–324 (1992)." href="/article/10.1038/s41598-019-56959-x#ref-CR3" id="ref-link-section-d142529658e1166">3,12,16. Of these, vertebrate of Leshansaurus, ‘Szechuanosaurus’ zigongensis are relatively complete and well-preserved17 whereas Xuanhanosaurus includes relatively few, incomplete vertebrate18 and Gasosaurus includes most presacral vertebrae, but they are incomplete and highly reconstructed by plaster, making their original morphology difficult to observe (IVPP)4. All four are apparently different from the new taxon, as indicated in our diagnosis and the description below.
Eleven presacral vertebrae of Yunyangosaurus are preserved, seven of them are identified as cervical vertebrae, and four as dorsal vertebrae, based on their anatomy (e.g., the position of parapophysis, the orientation of diapophysis, and the presence of a hypapophysis). The cervical series is represented by two anterior cervicals, three middle cervicals, and two posterior cervicals (probably the posteriormost two cervicals). The dorsal series is represented by three anterior dorsal vertebrae (probably the anteriormost ones) and an anterior middle dorsal vertebra.
The axis is mostly preserved, missing the odontoid and part of left spinopostzygapophyseal lamina. The axial intercentrum is attached to the axial centrum, but the line of fusion is obviously visible (Fig. 3A,B). The axial intercentrum is sub-triangular in outline in lateral view. This is similar to the condition in many theropods, including Leshansaurus16, Marshosaurus (CMNH 21704), Monolophosaurus20, Sinraptor20, and Sinraptor12).
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Acknowledgements
We thank Wei Guangbiao and He Jianjun for coordinating the fieldwork in Yunyang, Han **angsong for collecting the specimen, Zhang Yuqing for preparing the specimen. This work is supported by Natural Science Foundation of Chongqing, China (No. cstc2018jcyjAX0435), Postdoctoral Project of Chongqing, China (No. Xm2017069) and Project of Dinosaur Fossil Protection and Research of Chongqing Planning and Natural Resources Bureau (No. kj-2018035). X.X is supported by the National Natural Science Foundation of China (Grant No. 41688103).
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Hui Dai, **ng Xu and Roger Benson wrote the main manuscript text. Qingyu Ma, Shan Jiang, Deliang Li and Guangzhao Peng revised and improved the manuscript. Hui Dai, Qingyu Ma, Haiqian Hu and Yuxuan Zhou prepared all figures. Xufeng Hu, Chao Tan, Ming **ao, Feng Zhang and Yilun Yu oversaw the project. Ning Li and Zhaoying Wei assisted with the measurement of the specimens and the writing of the manuscript. Yilun Yu helped to revise the phylogenetic tree. All authors reviewed the manuscript.
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Dai, H., Benson, R., Hu, X. et al. A new possible megalosauroid theropod from the Middle Jurassic **ntiangou Formation of Chongqing, People’s Republic of China and its implication for early tetanuran evolution. Sci Rep 10, 139 (2020). https://doi.org/10.1038/s41598-019-56959-x
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DOI: https://doi.org/10.1038/s41598-019-56959-x
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