| Xingxiulong | |
|---|---|
 | |
| Photographs of skeletal elements (a-n) and skeletal reconstruction (o) ofXingxiulong chengi | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | †Sauropodomorpha |
| Clade: | †Sauropodiformes |
| Genus: | †Xingxiulong Wang et al., 2017 |
| Type species | |
| †Xingxiulong chengi Wang et al., 2017 | |
| Other species | |
Xingxiulong (meaning "Xingxiu Bridge dragon") is a genus of bipedalmassopodansauropodomorph from theEarly Jurassic ofChina. It contains two species; thetype species isX. 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 this species measured 4–5 metres (13–16 ft) long and 1–1.5 metres (3.3–4.9 ft) tall. A second species,X. yueorum, was described by Chen et al. in 2025 based on a partial articulated postcranial skeleton of an adult individual larger thanX. chengi.
Phylogenetic analysis suggests thatXingxiulong is most closely related to its contemporaryJingshanosaurus, although an alternative position outside of both the Sauropodiformes andMassospondylidae is also plausible. Despite their close relationship,Xingxiulong prominently differs fromJingshanosaurus—and from mostbasalsauropodomorphs—in having a number ofsauropod-like traits. These include asacrum containing four vertebrae; apubis with an exceptionally long top portion; and thefemur, the first and fifthmetatarsals on the foot, and thescapula being wide and robust. These probably represent adaptations to supporting high body weight, in particular a large gut. Unlike sauropods, however,Xingxiulong may have still been bipedal.
LFGT-D0002, the matureholotype specimen ofXingxiulong chengi, consists of a partial skull with jaws; the seventh to ninthcervical vertebrae, the eighth to fourteenthdorsal vertebrae, the entiresacrum, and 35 caudal vertebrae; rib andchevron fragments; the leftilium, and parts of bothpubes andischia; bothfemora andtibiae, parts of thefibula, the left ankle, and both feet (nearly complete).[1]
Two specimens were referred toX. chengi. LFGT-D0003, also mature, consists of a partial skull with jaws; the third to tenth cervical vertebrae, as well as all of the dorsal vertebrae and most of the sacrum; rib and chevron fragments; bothscapulae,humeri,ulnae, andradii, as well as part of the hand; parts of both ilia (the right one is complete) and pubes (the left one is complete); the left femur, parts of both tibiae and fibulae (the right ones are complete), as well as the right ankle and parts of the feet. LFGT-D0001, which is smaller and probably immature, consists of theaxis, the third to tenth cervical vertebrae, all of the dorsal vertebrae and sacrum, and the first nineteenth caudal vertebrae; rib and chevron fragments; the right scapula and ilium, as well as parts of the right pubis and ischium; parts of the femora and left tibia, along with the left ankle.[1]
These specimens were discovered, buried together, near the Sankeshu Village ofLufeng County,Yunnan,China in 2013, and are stored at the Bureau of Land and Resources of Lufeng County. The rocks that these specimens were preserved in, consisting of purplesiltymudstone, belong to theEarly Jurassic Shawan Member of theLufeng Formation, which has been tentatively dated to beingHettangian in age.[1]
Xingxiulong was described in 2017 by Ya-Ming Wang, Hai-Lu You, and Tao Wang. The generic name (literally meaning "constellation") refers to the Xingxiu Bridge (星宿), which was constructed during theMing Dynasty of China. Meanwhile, the specific namechengi honours Professor Zheng-Wu Cheng, who made major contributions to thebiostratigraphy of China, including that of the Lufeng area, and died in 2015.[1]
In 2025, Xiang-Yuan Chen, Ya-Ming Wang, Qian-Nan Zhang, Tao Wang & Hai-Lu You described LF2015-NO01 as the holotype of a second species ofXingxiulong,X. yueorum. The specimen consists of most of a articulated postcranial skeleton, including 13 dorsal vertebrae, 11 paired ribs, four sacral vertebrae, 20 caudal vertebrae and associated chevrons, part of the left scapula, and both pelvic girdles and hindlimbs. It was discovered by the Bureau of Natural Resources of Lufeng City in 2015 in outcrops of the Zhangjiaao Member of the Lufeng Formation. These sediments are located in Erzuanshan, Yunnan Province. The specific name honors the siblings of Shu-Yong Yue for their work in collecting and preparing Lufeng dinosaur fossils, including theX. yueorum.[2]
Xingxiulong was a medium- to large-sizedsauropodomorph. Adults ofX. chengi are estimated at 4–5 metres (13–16 ft) long and 1–1.5 metres (3.3–4.9 ft) tall 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.[1]
The holotype ofX. yueorum is larger than adult specimens ofX. chengi and is estimated to have been 8–10 metres (26–33 ft) long in life; the longest femur ofX. chengi is about 56 centimetres (22 in) long, while the femur ofX. yueorum is 78 centimetres (31 in) long.[2]
Unlike the contemporaryLufengosaurus,[3]Xingxiulong does not have a ridge on the side of itsmaxilla. Eleven tooth sockets are preserved along the length of the maxilla. Further back, in front of the eye socket, thelacrimal bears a prominent projection near the top of its front end, which is also seen inLufengosaurus,[3]Adeopapposaurus,Massospondylus,[4] andRiojasaurus, but notYunnanosaurus,Jingshanosaurus, or subsequent sauropodiforms. The contact surface between thejugal bone andpostorbital bone is fairly long, likeLufengosaurus but notYunnanosaurus.[5] Among the three branches of the jugal, the backwards-directed branch forms an angle of 80° with the upwards-projecting branch, which is similar toPlateosaurus andThecodontosaurus but much larger than othersauropodomorphs.[1][5][6][7]
At the base of the skull, thequadratojugal bears two branches, one pointing forwards and one upwards; they are roughly perpendicular to each other, unlikeLufengosaurus (angle of 45°),Yunnanosaurus (angle of 60°), andJingshanosaurus (angle of 110°). Above the quadratojugal, thequadrate has twoarticulating condyles, a subtriangular one facing outward and a more rounded one facing inwards; the latter condyle is placed closer to the bottom, likeLufengosaurus andYunnanosaurus but notPlateosaurus. At the back of the skull, between theparietals andsupraoccipitals, 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 likePlateosaurus and unlikeLufengosaurus andJingshanosaurus, forming an angle of 80° with each other.[1]
Compared toLufengosaurus,Yunnanosaurus, andJingshanosaurus, theangular andsurangular extend much further in front of the mandibular fenestra inXingxiulong, which is closer toAdeopapposaurus andPlateosaurus.[8] 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 inColoradisaurus,[9]Jingshanosaurus, and an as-of-yet unnamed sauropodomorph.[1]
There are tencervical vertebrae in the neck ofXingxiulong. The proatlas, an atrophied vertebra positioned in front of theatlas, is bounded in front by top sides of theforamen magnum. While it is difficult to say much about the atlas itself due to damage, the immediately followingaxis has a relatively shortcentrum, 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, likeLufengosaurus. Other basal sauropodomorphs,[10][11] includingJingshanosaurus, 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 inLufengosaurus,Jingshanosaurus, and other basal sauropodomorphs. Near the back of the neck, theneural spines become somewhat table-like.[1]
The fourteendorsal vertebrae are weaklyamphicoelous, or concave on both ends, typical of basal sauropodomorphs. Also typical is the presence of blade-like keels on the undersides of the first to third dorsal vertebrae, which are not seen elsewhere; additionally, the dorsal vertebrae near the front have the typical low, short, and somewhat plate-like neural spines. Unusually, however, the same kinds of neural spines are also seen in the last three dorsal vertebrae, which is only otherwise seen in basalsaurischians likeHerrerasaurus andEoraptor. In the middle and back dorsal vertebrae, the top back corner of the neural spines project outward to produce a concave back edge, which is seen in some other basal sauropodomorphs but notLufengosaurus,Jingshanosaurus,Yunnanosaurus, andRiojasaurus.[1]
Unusually among basal sauropodomorphs,Xingxiulong has foursacral vertebrae. This is a trait typically seen in more derived sauropodomorphs, likeMelanorosaurus,Leonerasaurus,[12]Barapasaurus,[13] andShunosaurus. The first of these sacral vertebrae, the dorsosacral, is not very well-fused to the next sacral vertebra; neither is it entirely fused to the corresponding sacral rib on thetransverse processes. Collectively, the dorsosacral and first pair of sacral ribs extend forward to contact theilium. The second and third sacral vertebrae, the two primordial sacrals, are about the same length, and are well-fused to each other. Like other basal sauropodomorphs, the transverse processes of the third sacral vertebra project backwards and outwards. The same is true of the transverse process and sacral ribs for the fourth sacral vertebra, which contact the ilium again at the back. All of the neural spines are plate-like, like the last few dorsal vertebrae.[1]
It is probable thatXingxiulong would have had more than the 35 preserved caudal vertebrae in the tail. These vertebrae are overall tall and very robust, and have concave sides like other basal sauropodomorphs. They also all appear to be amphicoelous. The transverse processes of the first several caudal vertebrae are wide and flat, and directed upwards and outwards; the transverse processes of caudal vertebrae further back in the tail are more slender and horizontal. As for the neural spines, they are tall and thin, and are directed somewhat backwards.[1]
Both ends of thescapula ofXingxiulong are quite expanded; the bottom end's width is 56% the length of the scapula, and the top end's width is 49% the length of the scapula. Various other basal sauropodomorphs, includingLufengosaurus andJingshanosaurus, have a more expanded bottom end, but their scapulae are generally more slender; conversely,Antetonitrus[14] andLessemsaurus have similarly robust scapulae, but the top end is more expanded in these two taxa. As withJingshanosaurus, the maximum width of the robust shaft of the scapula is 19-20% the length of the overall bone;Antetonitrus[14] andLessemsaurus have even more robust shafts, while the majority of basal sauropodomorphs have narrower shafts.[1]
The tuberosity on the inner surface of the top end of thehumerus is rather poorly-developed inXingxiulong, in contrast to the majority of basal sauropodomorphs (includingLufengosaurus andYunnanosaurus). LikeYunnanosaurus andJingshanosaurus, theulna is about 61% the length of the humerus; this ratio is 68% inLufengosaurus, resulting in a longer ulna. The top end of the ulna is quite expanded, with prominent anteromedial and anterolateral processes; these processes, along with the shallow radial fossa, collectively form the articulation of the ulna with theradius. The latter bone is slender and about 54% the length of the humerus. Parts of the hand, which bore at least four digits, are known but not well-described.[1]
Overall, theilium is similar to other basal sauropodomorphs. The front of the ilium does not exceed the level of the pubic peduncle, or its articulation with thepubis. Unlike other basal sauropodomorphs, the back end is somewhat square instead of being pointed, and the bottom portion is very concave when viewed from the side (in other basal sauropodomorphs, it is mostly straight or even convex). The ischial peduncle, or the portion that articulates with theischium, has a small projecting heel on its bottom end. As for the pubis itself, the top end (known as the pubic plate) is relatively long, at 40% of the length of the bone, and the bottom portion (known as the pubic apron) is conversely relatively short. This is unlike other basal sauropodomorphs but similar to some basal sauropods. The outer face of the pubic apron is somewhat concave, and the bottom end is expanded forwards and backwards to about 16% the length of the entire bone. Finally, the expanded obturator plate of the ischium bears a groove on its side.[1]
On thefemur, thelesser trochanter extends below the level of thefemoral head, unlikeYunnanosaurus andJingshanosaurus. Viewed from the front, the trochanter is close to the midline of the bone, like other basal sauropodomorphs except forAntetonitrus andMelanorosaurus (in which it is closer to the outer edge). Similarly, thefourth trochanter is near the midline, in contrast to many basal sauropodomorphs,[9] includingLufengosaurus. Of the two processes on the bottom of thetibia, the one in the back is thinner, and projects more outwards than downwards than the one in front; this differs from most other sauropodomorphs in which the two are equally thick, and either they project equally outwards (as inLufengosaurus andJingshanosaurus) or the front one projects further outwards (as inYunnanosaurus).[1]
The back of theastragalus bears a bulge close to the bottom end of the bone. A similar bulge is also seen inMussaurus,[7] albeit better developed and placed closer to the midline. Out of themetatarsals, the first one is the widest and most robust. The fifth metatarsal is unusually expanded at its top end, the width of which is 85% the length of the entire bone. In other sauropodomorphs, this figure is usually 50-77%,[15] althoughAntetonitrus may have a similarly expanded metatarsal. Below the metatarsals, the five digits have aphalangeal formula of 2-3-4-5-1.[1]
A 2017phylogenetic analysis conducted based on the dataset of McPhee et al., published in 2015 with the description ofPulanesaura,[16] found thatXingxiulong was a basal member of the groupSauropodiformes onceBlikanasaurus was removed from the dataset. Within this group, it is closest to the contemporaryJingshanosaurus. Traits that are shared byXingxiulong andJingshanosaurus include theinfratemporal fenestra being placed entirely behind the eye socket; thescapula being at least 20% as wide as it is long; the pubic apron, or the bottom of thepubis, having a concave outer face; the expansion at the bottom of the apron being at least 15% the length of the entire bone; and the angle between thefemoral head and the cross-sectional axis of the femoral shaft being about 30°. The topology recovered by this analysis is reproduced below.[1]
Various alternative phylogenetic placements ofXingxiulong were tested; out of these, the most plausible alternative involves it being placed as abasal member of theMassopoda, outside of eitherMassospondylidae or Sauropodiformes, which only requires two additional evolutionary steps. This is probably due to the curved top margin of thepostorbital and the presence of a process behind the jaw joint on thearticular, which are traits that, compared to either Massospondylidae or Sauropodiformes, can be regarded as relatively primitive. Such an alternative scenario would involve significantmosaic evolution in this evolutionary grade. Placements that required three additional evolutionary steps placedXingxiulong betweenJingshanosaurus andAnchisaurus, or betweenYunnanosaurus andJingshanosaurus.[1]
Curiously,Xingxiulong has many characteristics that are normally otherwise seen among theSauropoda. These include the four-vertebrasacrum (whichMussaurus alsoconvergently acquired);[7][12][17] the long pubic plate, or top portion of the pubis, which occupies 40% of the length of the bone (this figure is 25% in most other basalsauropodomorphs,[9] 33% in most sauropods,[18] and 45-50% in theCamarasauromorpha[19]); and the relative robustness of the femoral shaft,[12] firstmetatarsal,[20] top end of the fifth metatarsal, and scapula. These unique characteristics are probably the product of convergent evolution rather than common ancestry.[1]
In their 2025 description ofX. yueorum, Chen et al. tested the phylogenetic position of this second species. They recoveredXingxiulong as amonophyletic genus outside of the Sauropodiformes. Both species possess a sacrum with four sacral vertebrae, a robust scapula with greatly expanded proximal and distal ends, an ilium with a strongly concave ventral margin of postacetabular process, and the absence of an astragalar pyramidal dorsal process. The results of their reduced consensus analyses are displayed in the cladogram below:[2]
| Massopoda |
| |||||||||||||||||||||||||||||||||||||||||||||
The overall robustness of the skeleton ofXingxiulong, especially in the hip,femur, and foot, areconvergent uponsauropods, and collectively suggest that it had a relatively large gut and overall high body mass. However, unlike sauropods,Xingxiulong would have beenbipedal; it lacks sauropodan adaptations toquadrupedalism including relatively longer forelimbs, theulna bearing a prominent process on the front of its side, and thefemur having a relatively straight shaft.[20] Instead, its ulna and femur are overall more reminiscent of the typical basalsauropodomorph. The large and robustscapulae ofXingxiulong,Jingshanosaurus, andYunnanosaurus may have increased the mobility of the forelimb during bipedal browsing, but this trait was later adopted as an adaptation to quadrupedality in sauropods.[1][16]
TheLufeng Formation containsmudstones andsiltstones from lakes, rivers, andoverbank deposits.[21]
Manysauropodomorphs aside fromXingxiulong are known from the Lufeng Formation, includingLufengosaurus huenei,L. magnus,Yunnanosaurus huangi,"Gyposaurus" sinensis,Jingshanosaurus xinwaensis,Chuxiongosaurus lufengensis,[1]Xixiposaurus suni,Yizhousaurus sunae,[22] andPachysuchus imperfectus. Also present are thetheropodsSinosaurus triassicus,Lukousaurus yini,[21]Shidaisaurus jinae,[22][23] andEshanosaurus deguchiianus;[24] theornithischiansTatisaurus oehleri[21] andBienosaurus lufengensis; thecrocodylomorphsDibothrosuchus elaphros,Platyognathus hsui,Microchampsa scutata, andDianosuchus changchiawaensis, andDianchungosaurus lufengensis; the indeterminatearchosaurStrigosuchus licinus; thesphenodontiansClevosaurus petilus,C. wangi, andC. mcgilli; thetritylodontidcynodontsBienotherium yunnanense,B. minor,B. magnum,Lufengia deltcata,Yunnanodon brevirostre, andDianzhongia longirostrata; the mammalsSinoconodon rigneyi,Morganucodon oehleri,M. heikoupengensis, andKunminia minima;proganochelyid turtles; and a"labyrinthodont" amphibian.[21]
