| Shuangbaisaurus | |
|---|---|
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Genus: | †Shuangbaisaurus Wanget al., 2017 |
| Type species | |
| †Shuangbaisaurus anlongbaoensis Wanget al., 2017 | |
Shuangbaisaurus (meaning "Shuangbai reptile") isgenus oftheropoddinosaur, possibly a junior synonym ofSinosaurus. It lived in theEarly Jurassic ofYunnan Province,China, and is represented by a single species,S. anlongbaoensis, known from a partial skull. Like the theropodsDilophosaurus andSinosaurus,Shuangbaisaurus bore a pair of thin, midline crests on its skull. Unusually, these crests extended backwards over the level of the eyes, which, along with the unusual orientation of thejugal bone, led the describers to name it as a new genus. However,Shuangbaisaurus also possesses a groove between itspremaxilla andmaxilla, a characteristic which has been used to characterizeSinosaurus as a genus. Among the twomorphotypes present within the genusSinosaurus,Shuangbaisaurus more closely resembles the morphotype that is variably treated as a distinct species,S. sinensis, in its relatively tall skull.
Shuangbaisaurus had a relatively robust (compared toSinosaurus) skull, with a length of 54 centimetres (21 in) and a length/height ratio of 2.84, not including the thin paired crests that were present at least above the eye sockets. These crests are incompletely preserved, but appeared to have been formed mostly from thefrontal bone, as well as possibly thepostorbital andprefrontal bones. The crests may have extended further forward, over thenasal andlacrimal bones. Similar crests are seen inDilophosaurus andSinosaurus; however, the crests ofSinosaurus do not extend over the eyes,[1] thus differentiating it fromShuangbaisaurus.[2]
At the tip of the jaw, thepremaxilla andmaxilla are separated by a groove, which is also a defining characteristic ofSinosaurus.[3] The premaxilla would have been taller than it is long, which is similar to specimen LFGT LDM-L10 ofSinosaurus triassicus but unlike eitherDilophosaurus or specimen KMV 8701 ofSinosaurus sinensis. Also like LFGT LDM-L10, the bottom edge of the premaxilla is much higher than the maxilla. The two bones are at the same level in KMV 8701, and are angled away from each other inDilophosaurus.[1] Similar to both LFGT LDM-L10 and KMV 8701,[3] the maxilla bears a near-vertical front margin; it is lower and more pointed inDilophosaurus. The front portion of the bottom margin of the maxilla is angled such that the first maxillary tooth projects forwards. There is a groove running parallel to the tooth row along the bottom rim of the maxilla.[2]
From behind, the maxilla is connected to thejugal. The lacrimal is located above their joint, enclosing thefossa that surrounds theantorbital fenestra. There is a ridge directed downwards and backwards on the side of the jugal, like inZupaysaurus;[4] this same ridge is horizontal inSinosaurus andDilophosaurus.[1] The same is also true of the joint between the jugal andquadratojugal. The quadratojugal tapers to a point underneath theinfratemporal fenestra, where it is joined at the back to thequadrate. Further above, the lacrimal is joined by the long and slender descending branch of the three-pronged postorbital, which is convex in its upper third. From above, the forward-directed branch of the postorbital is offset from the backward-directed branch, being separated by a small recess. The frontals appeared to have been long and large, forming most of the top of the skull.[2]
The antorbital fenestra ofShuangbaisaurus is large, occupying roughly one third the length of the skull. The back margin of the fenestra slopes backwards, like LFGT LDM-L10 andDilophosaurus but unlike KMV 8701.[1] Further behind, theorbit is keyhole-shaped, with a longer back rim compared to the front rim. The infratemporal fenestra is roughly trapezoidal, with a long axis oriented upwards and backwards towards the top of the rear of the skull. On the top surface of the skull, the roundedsupratemporal fenestra is unusually small, with a diameter equivalent to only about half of the length on the top of the skull behind the level of the orbit.[2]
Shuangbaisaurus is known from a single specimen, a partial skull missing most of the top of the snout, stored at the Chuxiong Prefectural Museum inChuxiong Yi Autonomous Prefecture,Yunnan,China under the specimen number CPM C2140ZA245. Additionally, the front half and back portion of the jaw are also associated with the skull. The specimen's snout has also become bent to the left due to deformation during fossilization. It was discovered in layers of purple muddysiltstone located in Liuna Village, Anlongbao Town,Shuangbai County, Chuxiong Yi, about 100 kilometres (62 mi) south of the well-knownLufeng deposits. These layers have been recognized as belonging to theEarly JurassicFengjiahe Formation,[2] which has been tentatively dated to theHettangian.[5]
In 2017, the type and only species of the genusShuangbaisaurus,S. anlongbaoensis, was formally described and named by Wang Guo-Fu, You Hai-Lu, Pan Shi-Gang, and Wang Tao. Thegeneric name refers to Shuangbai County, and thespecific name refers to Anlongbao Town (with Anlongbao literally meaning "dragon-placing fort"), and thus both elements of the binomial name reference its provenance.[2]
In a 2019 conference abstract,Philip Currie and colleagues suggested that CPM C2140ZA245 fell within the range of variation of specimens assigned toSinosaurus triassicus.[6]
AfterDilophosaurus andSinosaurus,Shuangbaisaurus is the third largetheropod known definitely to have had paired crests on its skull (though the smaller[7]Coelophysis kayentakatae[8] also has similar crests).Shuangbaisaurus seems to generally be similar to the roughly contemporaneousSinosaurus. However, they are notably differentiated in that the crests clearly extend over the orbit inShuangbaisaurus, whereas they are restricted to extensions of the nasal and lacrimal that do not reach past the orbit inSinosaurus. Similarly, the backward and downward orientation of the jugal is a trait ofShuangbaisaurus not seen inSinosaurus, where it is horizontal. However, the vertical groove separating the premaxilla and maxilla, which seems to be present inShuangbaisaurus, was denoted as a distinguishing characteristic ofSinosaurus by Matthew Carrano and colleagues in 2012,[3] thus complicating its referral to a new genus by Wanget al.[2]
A further issue in the relation betweenShuangbaisaurus andSinosaurus involves the taxonomic convolution that has befallenSinosaurus. At least one species is recognized in the genus,Sinosaurus triassicus, consisting of specimens LFGT LDM-L10 and LFGT ZLJT01; another specimen, KMV 8701, originally named "Dilophosaurus"sinensis, has been referred to eitherS. triassicus[9][10][11] or a second speciesS. sinensis.[1] KMV 8701 approachesShuangbaisaurus more closely in its tall skull and premaxilla than specimens referred toS. triassicus.[1][3] Wanget al. elected to retainS. sinensis as a separate species owing to these differences. No phylogenetic analysis has been conducted to quantitatively determine the relationships ofShuangbaisaurus.[2]
Aside fromShuangbaisaurus, severalsauropodomorph dinosaurs originate from the Fengjiahe Formation. These includeChinshakiangosaurus chunghoensis (a possiblesauropod),[5][12]Lufengosaurus sp.,[13]Yimenosaurus youngi,[12] andYunnanosaurus huangi (a juvenile specimen, which was also discovered in Shuangbai County).[14] Ostracods of the genusDarwinula have also been found. A diverseichnofauna is also known from deposits of the Fengjiahe Formation, including theichnotaxaEubrontes platypus,Grallator limnosus,Paracoelurosaurichnus monax,Schizograllator xiaohebaensis,Youngichnus xiyangensis, andZhengichnus jinningensis (with the last four sometimes considered synonymous withGrallator).[13]
Temporally, the Fengjiahe Formation is equivalent to theLower Lufeng Formation, from whereSinosaurus is known alongside a diverse assemblage consisting of another theropod,Lukousaurus; sauropodmorphs (includingChinshakiangosaurus sp.,Lufengosaurus huenei,L. magnus,Xingxiulong chengi andYunnanosaurus huangi);ornithischians;crocodylomorphs;cynodonts and mammals; turtles; amphibians; and bivalves.[15][16]
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