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Shuangbaisaurus

Shuangbaisaurus (meaning "Shuangbai reptile") is genus of theropod dinosaur, possibly a junior synonym of Sinosaurus. It lived in the Early Jurassic of Yunnan Province, China, and is represented by a single species, S. anlongbaoensis, known from a partial skull. Like the theropods Dilophosaurus and Sinosaurus, 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 the jugal bone, led the describers to name it as a new genus. However, Shuangbaisaurus also possesses a groove between its premaxilla and maxilla, a characteristic which has been used to characterize Sinosaurus as a genus. Among the two morphotypes present within the genus Sinosaurus, Shuangbaisaurus more closely resembles the morphotype that is variably treated as a distinct species, S. sinensis, in its relatively tall skull.

Shuangbaisaurus
Temporal range: ?Hettangian
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Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Clade: Theropoda
Genus: †Shuangbaisaurus
Wang et al., 2017
Type species
†Shuangbaisaurus anlongbaoensis
Wang et al., 2017

Description

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Shuangbaisaurus had a relatively robust (compared to Sinosaurus) 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 the frontal bone, as well as possibly the postorbital and prefrontal bones. The crests may have extended further forward, over the nasal and lacrimal bones. Similar crests are seen in Dilophosaurus and Sinosaurus; however, the crests of Sinosaurus do not extend over the eyes,[1] thus differentiating it from Shuangbaisaurus.[2]

At the tip of the jaw, the premaxilla and maxilla are separated by a groove, which is also a defining characteristic of Sinosaurus.[3] The premaxilla would have been taller than it is long, which is similar to specimen LFGT LDM-L10 of Sinosaurus triassicus but unlike either Dilophosaurus or specimen KMV 8701 of Sinosaurus 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 in Dilophosaurus.[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 in Dilophosaurus. 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 the jugal. The lacrimal is located above their joint, enclosing the fossa that surrounds the antorbital fenestra. There is a ridge directed downwards and backwards on the side of the jugal, like in Zupaysaurus;[4] this same ridge is horizontal in Sinosaurus and Dilophosaurus.[1] The same is also true of the joint between the jugal and quadratojugal. The quadratojugal tapers to a point underneath the infratemporal fenestra, where it is joined at the back to the quadrate. 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 of Shuangbaisaurus is large, occupying roughly one third the length of the skull. The back margin of the fenestra slopes backwards, like LFGT LDM-L10 and Dilophosaurus but unlike KMV 8701.[1] Further behind, the orbit 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 rounded supratemporal 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]

Discovery and naming

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Shuangbaisaurus is known from a single specimen, a partial skull missing most of the top of the snout, stored at the Chuxiong Prefectural Museum in Chuxiong 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 muddy siltstone located in Liuna Village, Anlongbao Town, Shuangbai County, Chuxiong Yi, about 100 kilometres (62 mi) south of the well-known Lufeng deposits. These layers have been recognized as belonging to the Early Jurassic Fengjiahe Formation,[2] which has been tentatively dated to the Hettangian.[5]

In 2017, the type and only species of the genus Shuangbaisaurus, S. anlongbaoensis, was formally described and named by Wang Guo-Fu, You Hai-Lu, Pan Shi-Gang, and Wang Tao. The generic name refers to Shuangbai County, and the specific 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 to Sinosaurus triassicus.[6]

Classification

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After Dilophosaurus and Sinosaurus, Shuangbaisaurus is the third large theropod 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 contemporaneous Sinosaurus. However, they are notably differentiated in that the crests clearly extend over the orbit in Shuangbaisaurus, whereas they are restricted to extensions of the nasal and lacrimal that do not reach past the orbit in Sinosaurus. Similarly, the backward and downward orientation of the jugal is a trait of Shuangbaisaurus not seen in Sinosaurus, where it is horizontal. However, the vertical groove separating the premaxilla and maxilla, which seems to be present in Shuangbaisaurus, was denoted as a distinguishing characteristic of Sinosaurus by Matthew Carrano and colleagues in 2012,[3] thus complicating its referral to a new genus by Wang et al.[2]

A further issue in the relation between Shuangbaisaurus and Sinosaurus involves the taxonomic convolution that has befallen Sinosaurus. 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 either S. triassicus[9][10][11] or a second species S. sinensis.[1] KMV 8701 approaches Shuangbaisaurus more closely in its tall skull and premaxilla than specimens referred to S. triassicus.[1][3] Wang et al. elected to retain S. sinensis as a separate species owing to these differences. No phylogenetic analysis has been conducted to quantitatively determine the relationships of Shuangbaisaurus.[2]

Paleoecology

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Aside from Shuangbaisaurus, several sauropodomorph dinosaurs originate from the Fengjiahe Formation. These include Chinshakiangosaurus chunghoensis (a possible sauropod),[5][12] Lufengosaurus sp.,[13] Yimenosaurus youngi,[12] and Yunnanosaurus huangi (a juvenile specimen, which was also discovered in Shuangbai County).[14] Ostracods of the genus Darwinula have also been found. A diverse ichnofauna is also known from deposits of the Fengjiahe Formation, including the ichnotaxa Eubrontes platypus, Grallator limnosus, Paracoelurosaurichnus monax, Schizograllator xiaohebaensis, Youngichnus xiyangensis, and Zhengichnus jinningensis (with the last four sometimes considered synonymous with Grallator).[13]

Temporally, the Fengjiahe Formation is equivalent to the Lower Lufeng Formation, from where Sinosaurus is known alongside a diverse assemblage consisting of another theropod, Lukousaurus; sauropodmorphs (including Chinshakiangosaurus sp., Lufengosaurus huenei, L. magnus, Xingxiulong chengi and Yunnanosaurus huangi); ornithischians; crocodylomorphs; cynodonts and mammals; turtles; amphibians; and bivalves.[15][16]

See also

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References

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  1. ^ a b c d e f Xing, L. (2012). Sinosaurus from southwestern China (M.Sc.). University of Alberta.
  2. ^ a b c d e f g h Wang, G.-F.; You, H.-L.; Pan, S.-G.; Wang, T. (2017). "A new crested theropod dinosaur from the Early Jurassic of Yunnan Province, China" (PDF). Vertebrata PalAsiatica. 55 (2): 177–186. Archived from the original (PDF) on 2017-07-29.
  3. ^ a b c d Carrano, M. T.; Benson, R. B. J.; Sampson, S. D. (2012). "The phylogeny of Tetanurae (Dinosauria: Theropoda)". Journal of Systematic Palaeontology. 10 (2): 211–300. doi:10.1080/14772019.2011.630927. S2CID 85354215.
  4. ^ Ezcurra, M.D.; Novas, F.E. (2007). "Phylogenetic relationships of the Triassic theropod Zupaysaurus rougieri from NW Argentina". Historical Biology. 19 (1): 35–72. doi:10.1080/08912960600845791. S2CID 84277003.
  5. ^ a b McPhee, B.W.; Upchurch, P.; Mannion, P.D.; Sullivan, C.; Butler, R.J.; Barrett, P.M. (2016). "A revision of Sanpasaurus yaoi Young, 1944 from the Early Jurassic of China, and its relevance to the early evolution of Sauropoda (Dinosauria)". PeerJ. 4: e2578. doi:10.7717/peerj.2578. PMC 5075712. PMID 27781168.
  6. ^ Currie, P.J.; Xing, L.; Wu, X.; Dong, Z. (2019). Murray, A.M.; LeBlanc, A.; Holmes, R.B. (eds.). Anatomy and relationships of Sinosaurus triassicus (Theropoda, Coelophysoidea) from the Lufeng Formation (Lower Jurassic) of Yunnan, China. 7th Annual Meeting of the Canadian Society of Vertebrate Palaeontology. Vertebrate Anatomy Morphology Palaeontology. Vol. 7. p. 17. ISSN 2292-1389.
  7. ^ Gates, T.A.; Organ, C.; Zanno, L.E. (2016). "Bony cranial ornamentation linked to rapid evolution of gigantic theropod dinosaurs". Nature Communications. 7: 12931. doi:10.1038/ncomms12931. PMC 5052652. PMID 27676310.
  8. ^ Rowe, T. (1989). "A New Species of the Theropod Dinosaur Syntarsus from the Early Jurassic Kayenta Formation of Arizona". Journal of Vertebrate Paleontology. 9 (2): 125–136. doi:10.1080/02724634.1989.10011748. JSTOR 4523249.
  9. ^ Xing, L.D.; Bell, P.R.; Rothschild, B.M.; Ran, H.; Zhang, J.P.; Dong, Z.M.; Zhang, W.; Currie, P.J. (2013). "Tooth loss and alveolar remodeling in Sinosaurus triassicus (Dinosauria: Theropoda) from the lower Jurassic strata of the Lufeng Basin, China". Chinese Science Bulletin. 58 (16): 1931–1935. doi:10.1007/s11434-013-5765-7.
  10. ^ Xing, L.; Paulina-Carabajal, A.; Currie, P.J.; Xu, X.; Zhang, J.; Wang, T.; Burns, M.E.; Dong, Z. (2014). "Braincase Anatomy of the Basal Theropod Sinosaurus from the Early Jurassic of China". Acta Geologica Sinica. 88 (6): 1653–1664. doi:10.1111/1755-6724.12335. S2CID 131595322.
  11. ^ Xing, L.; Wang, Y.; Snively, E.; Zhang, J.; Dong, Z.; Burns, M.E.; Currie, P.J. (2015). "Model-Based Identification of Mechanical Characteristics of Sinosaurus (Theropoda) Crests". Acta Geologica Sinica. 89 (1): 1–11. doi:10.1111/1755-6724.12390. S2CID 128688200.
  12. ^ a b Upchurch, P.; Barrett, P.M.; Zhao, X.; Xu, X. (2007). "A re-evaluation of Chinshakiangosaurus chunghoensis Ye vide Dong 1992 (Dinosauria, Sauropodomorpha): implications for cranial evolution in basal sauropod dinosaurs". Geological Magazine. 144 (2): 247–262. doi:10.1017/S0016756806003062. S2CID 85775483.
  13. ^ a b Chen, P.-J.; Li, J.; Matsukawa, M.; Zhang, H.; Wang, Q.; Lockley, M.G. (2006). "Geological ages of dinosaur-track-bearing formations in China" (PDF). Cretaceous Research. 27 (1): 22–32. doi:10.1016/j.cretres.2005.10.008.
  14. ^ Sekiya, T.; Jin, X.; Zheng, W.; Shibata, M.; Azuma, Y. (2013). "A new juvenile specimen of Yunnanosaurus robustus (Dinosauria: Sauropodomorpha) from Early to Middle Jurassic of Chuxiong Autonomous Prefecture, Yunnan Province, China". Historical Biology. 26 (2): 252–277. doi:10.1080/08912963.2013.821702. S2CID 85132483.
  15. ^ Wang, Y.-M.; You, H.-L.; Wang, T. (2017). "A new basal sauropodiform dinosaur from the Lower Jurassic of Yunnan Province, China". Scientific Reports. 73: 41881. doi:10.1038/srep41881. PMC 5312170. PMID 28205592.
  16. ^ Luo, Z.; Wu, X.-C. (1997). "The small tetrapods of the Lower Lufeng Formation, Yunnan, China". In Fraser, N.C.; Sues, H.-D. (eds.). In the Shadow of the Dinosaurs: Early Mesozoic Tetrapods. Cambridge: Cambridge University Press. pp. 251–270. ISBN 9780521458993.
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