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 Xintiangou 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.
Highlights
Tetanurae is a special group of theropod dinosaurs that originated by the late Early Jurassic
Yunyangosaurus puanensis is represented by limited material, but it displays numerous informative features for its systematic position and taxonomy
It is highly likely a tetanuran dinosaur based on the general morphology of the presacral vertebrae, and in particular several tetanuran or orinoidan synapomorphies are present[10,38]: the axial neural spine is narrow anteroposteriorly and somewhat rod-like; the prezygapophyses of anterior cervical vertebrae are enlarged and situated entirely lateral to the neural canal; and there is a pronounced ventral keel in the anterior dorsal vertebrae
Summary
Tetanurae is a special group of theropod dinosaurs that originated by the late Early Jurassic. The Middle and Upper Jurassic Shaximiao Formation of southwestern China has produced spectacular fossil remains of dinosaurs and many other vertebrates[1,2] Significant discoveries from this formation in the Chongqing area include the sauropods Mamenchisaurus hochuanensis, Omeisaurus changshouensis, and the theropod Yangchuanosaurus shangyouensis[3] as well as the stegosaur Chungkingosaurus jiangbeiensis[4]. In 2016, we organized a survey in the upper portion of the Xintiangou Formation of Laojun Village, Puan Township, Yunyang City, Chongqing (Fig. 1) This resulted in the discoveries of numerous vertebrate fossils, including a fragmentary theropod specimen recovered from a layer of gray lamellar shell siltstone intercalated with occasional silty mudstone (Fig. 2). We establish a new tetanuran species based on this specimen, describe the specimen, and present a short discussion on its implications for the evolution of anatomical character states among early tetanurans
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