Xingxiulong in Yunnan

Xingxiulong (meaning “Xingxiu Bridge dragon”) is a genus of bipedal sauropodiform from the Early Jurassic of China. It contains a single species, X. chengi, described by Wang et al. in 2017 from three specimens, two adults and an immature individual, that collectively constitute a mostly complete skeleton. Adults of the genus measured 4–5 metres (13–16 ft) long and 1–1.5 metres (3 ft 3 in–4 ft 11 in) tall. Phylogenetic analysis suggests that Xingxiulong is most closely related to its contemporary Jingshanosaurus, although an alternative position outside of both the Sauropodiformes and Massospondylidae is also plausible.

Despite their close relationship, Xingxiulong prominently differs from Jingshanosaurus – and from most basal sauropodomorphs – in having a number of sauropod-like traits. These include a sacrum containing four vertebrae; a pubis with an exceptionally long top portion; and the femur, the first and fifth metatarsals on the foot, and the scapula being wide and robust. These probably represent adaptations to supporting high body weight, in particular a large gut. Unlike sauropods, however, Xingxiulong would still have been bipedal.

Description

Xingxiulong was a medium-sized sauropodiform, with an adult length of 4–5 metres (13–16 ft) and a height of 1–1.5 metres (3 ft 3 in–4 ft 11 in) at the tip. The two larger specimens, LFGT-D0002 and LFGT-D0003, are adults judging by the complete fusion in their skull; the third specimen, LFGT-D0001, is 14% smaller and probably represents a subadult judging by the incomplete fusion in the vertebrae.

Skull and jaw

Unlike the contemporary Lufengosaurus, Xingxiulong does not have a ridge on the side of its maxilla. Eleven tooth sockets are preserved along the length of the maxilla. Further back, in front of the eye socket, the lacrimal bears a prominent projection near the top of its front end, which is also seen in Lufengosaurus, Adeopapposaurus, Massospondylus, and Riojasaurus, but not Yunnanosaurus, Jingshanosaurus, or subsequent sauropodiforms. The contact surface between the jugal bone and postorbital bone is fairly long, like Lufengosaurus but not Yunnanosaurus. Among the three branches of the jugal, the backwards-directed branch forms an angle of 80° with the upwards-projecting branch, which is similar to Plateosaurus and Thecodontosaurus but much larger than other sauropodomorphs.

At the base of the skull, the quadratojugal bears two branches, one pointing forwards and one upwards; they are roughly perpendicular to each other, unlike Lufengosaurus (angle of 45°), Yunnanosaurus (angle of 60°), and Jingshanosaurus (angle of 110°). Above the quadratojugal, the quadrate has two articulating condyles, a subtriangular one facing outward and a more rounded one facing inwards; the latter condyle is placed closer to the bottom, like Lufengosaurus and Yunnanosaurus but not Plateosaurus. At the back of the skull, between the parietals and supraoccipitals, there is a prominently developed postparietal fenestra; the supraoccipital itself slopes forwards at its bottom end so as to round off the base of the skull. The basipterygoid processes are long, slender, and project downwards and outwards like Plateosaurus and unlike Lufengosaurus and Jingshanosaurus, forming an angle of 80° with each other.

Compared to Lufengosaurus, Yunnanosaurus, and Jingshanosaurus, the angular and surangular extend much further in front of the mandibular fenestra in Xingxiulong, which is closer to Adeopapposaurus and Plateosaurus. The articular bears an inward-projecting and pyramidal process as an extension of the jaw joint; at its back end, it also possesses an upward-directed and tab-like process, which is also seen in Coloradisaurus, Jingshanosaurus, and an as-of-yet unnamed sauropodomorph.

Paleobiology

The overall robustness of the skeleton of Xingxiulong, especially in the hip, femur, and foot, are convergent upon sauropods, and collectively suggest that it had a relatively large gut and overall high body mass. However, unlike sauropods, Xingxiulong would have been bipedal; it lacks sauropodan adaptations to quadrupedalismincluding relatively longer forelimbs, the ulna bearing a prominent process on the front of its side, and the femur having a relatively straight shaft.^[19] Instead, its ulna and femur are overall more reminiscent of the typical basal sauropodomorph. The large and robust scapulae of Xingxiulong, Jingshanosaurus, and Yunnanosaurus may have increased the mobility of the forelimb during bipedal browsing, but this trait was later adopted as an adaptation to quadrupedality in sauropods.

Paleoecology

The Lufeng Formation contains mudstones and siltstones from lakes, rivers, and overbank deposits. Many sauropodomorphs asides from Xingxiulong are known from the Lufeng Formation, including Lufengosaurus huenei, L. magnus, Yunnanosaurus huangi, “Gyposaurus” sinensis, Jingshanosaurus xinwaensis, Chuxiongosaurus lufengensis, Xixiposaurus suni, “Yizhousaurus sunae”, and Pachysuchus imperfectus. Also present are the theropods Sinosaurus triassicus, Lukousaurus yini,Shidaisaurus jinae, and Eshanosaurus deguchiianus; the ornithischians Tatisaurus oehleri and Bienosaurus lufengensis; the crocodylomorphsDibothrosuchus elaphros, Platyognathus hsui, Microchampsa scutata, and Dianosuchus changchiawaensis, and Dianchungosaurus lufengensis; the indeterminate archosaur Strigosuchus licinus; the sphenodontians Clevosaurus petilus, C. wangi, and C. mcgilli; the tritylodontid cynodonts Bienotherium yunnanense, B. minor, B. magnum, Lufengia deltcata, Yunnanodon brevirostre, and Dianzhongia longirostrata; the mammals Sinoconodon rigneyi, Morganucodon oehleri, M. heikoupengensis, and Kunminia minima; proganochelyid turtles; and a “labyrinthodont” amphibian.

Discovery and Naming

• Discovery: Fossils of Xingxiulong were discovered in the Lower Jurassic-aged rock formations of the Lufeng Basin in Yunnan Province, China.
• Name Origin: The genus name “Xingxiulong” combines “Xingxiu,” referring to a renowned geologist in China, and “long,” meaning dragon in Mandarin Chinese.

Physical Characteristics

• Anatomy: Xingxiulong is known from a partial skeleton, including elements of the skull, vertebrae, ribs, and limbs. It represents a relatively small ceratopsian dinosaur.
• Size: The size of Xingxiulong is estimated to have been comparable to other small ceratopsians, likely reaching a length of several meters.

Classification

• Ceratopsian Dinosaur: Xingxiulong belongs to the Ceratopsia, a group of herbivorous dinosaurs characterized by their beaked skulls and often elaborate frills and horns.
• Basal Position: It is considered a basal member within Ceratopsia, indicating it represents an early stage in the evolution of this dinosaur group.

Paleoecology

• Habitat: During the Early Jurassic period, Yunnan was characterized by a warm, humid climate with lush vegetation. Xingxiulong would have inhabited this terrestrial environment.
• Contemporary Fauna: Xingxiulong coexisted with other dinosaurs such as prosauropods like Lufengosaurus, early theropods, and various other herbivorous and carnivorous dinosaurs.

Significance

• Early Ceratopsian Diversity: Xingxiulong contributes to our understanding of the early diversification of ceratopsian dinosaurs. Its discovery in Yunnan expands the geographic range and temporal distribution of early ceratopsians.
• Paleontological Importance of Yunnan: The discovery of Xingxiulong in Yunnan underscores the region’s significance in preserving Early Jurassic dinosaurs. It highlights the diversity of dinosaurs during this period and provides insights into their evolution and ecology.

Xingxiulong represents an important discovery in the study of Early Jurassic dinosaurs, particularly within the Ceratopsia. Its fossils contribute valuable information about the early stages of ceratopsian evolution and the ancient ecosystems of Yunnan Province, China.