Sinosaurus

The composite term Sinosaurus comes from Sinae, the Latin word for the Chinese, and the Greek word sauros (σαυρος) meaning "lizard"; thus "Chinese lizard". The specific name, triassicus, refers to the Triassic, the period that the fossils were originally thought to date from. Sinosaurus was described and named by Chung Chien Young, who is known as the 'Father of Chinese Vertebrate Paleontology', in 1940.[3]

Maxilla of specimen ZLJT01

The holotype, IVPP V34, was found in the Lower Lufeng Formation, and consists of two maxillary (upper jaw) fragments, four maxillary teeth, and a lower jaw fragment with three teeth. The teeth are laterally compressed, and feature fine serrations both at their anterior and posterior edges. The teeth are also variable in size and are curved backwards. This material is too fragmentary to determine the length and weight of this dinosaur. Over the years, other fossils were referred to Sinosaurus, some of which were material that was shown to belong to two sauropodomorphs.[4][5] The fossils include postcrania,[6] with a sacrum with three preserved sacral vertebrae. The material assigned to "Sinosaurus postcrania" includes a mix of plateosaurid and melanorosaurid elements. All the material from the Red Beds block has now been reassigned to Jingshanosaurus.[7]

Frontal view of mounted skeleton cast.

KMV 8701 was originally discovered in 1987. The specimen was identified as a new species, and was named Dilophosaurus sinensis.[8] Then in 1994, during a field expedition, a more complete specimen was found, and was assigned to the same species. In 2003, Dong Zhiming studied the material of Sinosaurus triassicus, finding it to be quite similar to Dilophosaurus sinensis.[1] As Sinosaurus was named earlier, "Dilophosaurus" sinensis became its junior synonym. In 2013, a study by Currie et al., confirmed that D. sinensis was the same animal as S. triassicus[9] On the other hand, Wang et al. (2017) stated that it needs to be further investigated whether D. sinensis is indeed a junior synonym of S. triassicus, and noted that the two species are different at least in the anatomy of the premaxilla. The authors tentatively assigned D. sinensis to the genus Sinosaurus, but retained it as a species distinct from Sinosaurus triassicus.[10] Specimen KMV 8701 consists of a skull (measuring 525 mm), and is nearly complete. Dong claimed that animal was about 5.6 m (18 ft) long.[1] It has been assigned now to Sinosaurus, but the specimen still lacks sufficient description and preparation.[11]

Over the years, paleontologists referred additional specimens to D. sinensis which are now assigned to Sinosaurus. Dong (2003) referred specimen LDM-LCA10 which consists of a skull and an incomplete skeleton.[1] In 2012, Xing referred two individuals, ZLJ0003 which consists of a partial skull and an incomplete skeleton, and ZLJT01 which is a juvenile individual that consists of a premaxillary fragment, an incomplete maxilla, a maxillary fragment, a lacrimal, both frontals, both parietals, an incomplete braincase, an incomplete dentary, an atlantal intercentrum, two dorsal rib fragments, and a partial proximal caudal neural arch, to Sinosaurus.[11]

In 2012, a new specimen of Sinosaurus was described, and was found to represent a new species.[11]

The species Shuangbaisaurus anlongbaoensis, discovered and named in 2017, has also been considered a synonym of Sinosaurus triassicus.[12]

Sinosaurus was a relatively large theropod during the Early Jurassic, reaching 5.5 m (18 ft) in length and 300 kg (660 lb) in body mass.[13] According to Carrano et al. (2012) D. sinensis, now considered to be at least congeneric with Sinosaurus triassicus, can be distinguished based on the fact that a vertical groove is present on the lateral premaxilla adjacent to contact with the maxilla.

Sinosaurus is the only "dilophosaurid" known from a complete braincase. Cryolophosaurus, Dilophosaurus, Zupaysaurus and Coelophysis kayentakatae are all known from partial braincases. Two partial braincases were found before 2012, and are probably mostly complete, except that large sections are obscured by sediments. In 2011, an exceptionally well-preserved braincase was found, only missing the frontal bones and orbitosphenoid.[11] A complete skull with a preserved mandible and 11 cervical vertebrae was described for Sinosaurus in 2023, after it was discovered near the locality where the holotype was found. The specimen also suggests three autapomorphies are unique to this theropod, all regarding crest development and the various fenestrae of the skull.[14]

Originally thought to be a coelophysoid related to Dilophosaurus and Cryolophosaurus, Oliver Rauhut in 2003 showed Sinosaurus to be a more advanced theropod, related to Cryolophosaurus and "Dilophosaurus" sinensis.[15] In 2013, in an unpublished work, Carano agreed that Sinosaurus is a theropod.[16] Sinosaurus has been considered a nomen dubium in a few works,[8][17][18] although now that "Dilophosaurus" sinensis is referred to it, it is considered valid.[1][2]

Dilophosaurus sinensis was shown to be a junior synonym of Sinosaurus in 2003.[1] It is possibly closer to the Antarctic theropod Cryolophosaurus, based on the fact that the anterior end of the jugal does not participate in the internal antorbital fenestra and that the maxillary tooth row is completely in front of the eye socket. D. sinensis was exhibited in 1998 at Dinofest in Philadelphia.[19] Although the skull of D. sinensis sports large nasolacrimal crests superficially like those reconstructed in D. wetherilli, features elsewhere in the skeleton suggest it is closer to tetanuran theropods.[7] Rauhut (2003) regarded D. sinensis as a basal tetanuran most closely related to Sinosaurus and Cryolophosaurus.[15] Lamanna et al. (1998b) examined the material ascribed to D. sinensis and found it to be synonymous with Sinosaurus triassicus.[20] This cladistic finding was confirmed in 2003 by Dong.[1][2][9]

Skull showing notch.

The Lufeng Dinosaurian Museum discovered a new specimen of Sinosaurus (ZLJT01) in 2007 from the Lufeng Basin. It consists of an incomplete skull and other postcranial fragments. Phylogenetic analysis of this specimen, demonstrates that Sinosaurus is a more derived theropod, and is not the most basal dilophosaurid, as held by Smith et al.[2] A cladogram was identified by Christophe Hendrickx and Octávio Mateus. It placed Sinosaurus and Cryolophosaurus in a polytomy at the base of Tetanurae.[21]

Recent studies placed Sinosaurus outside the Ceratosauria+Tetanurae clade,[22][23][24] while Wang et al. (2016) considered it the basalmost ceratosaur.[25][26]