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 long and 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 and a height of at the hip. 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.
Vertebrae
There are ten cervical vertebrae in the neck of Xingxiulong. The proatlas, an atrophied vertebra positioned in front of the atlas, is bounded in front by top sides of the foramen magnum. While it is difficult to say much about the atlas itself due to damage, the immediately following axis has a relatively short centrum, which is slightly compressed on its sides and bottom. Overall, the remainder of the cervical vertebrae are relatively short, being only 2.5 to 3 times as long as they are tall; they become increasingly shorter towards the back of the neck, like Lufengosaurus. Other basal sauropodomorphs, including Jingshanosaurus, have cervical vertebrae that are generally 3 to 4 times as long as they are tall. On the bottom of the centra of cervical vertebrae 4-9, there is a prominent keel as in Lufengosaurus, Jingshanosaurus, and other basal sauropodomorphs. Near the back of the neck, the neural spines become somewhat table-like.