Yangchuanosaurus Zigongensis

''Yangchuanosaurus'' is an extinct genus of metriacanthosaurid theropod dinosaur that lived in China from the Middle to Late Jurassic periods (Bathonian to Oxfordian stages), and was similar in size and appearance to its North American and European relative, ''Allosaurus''. ''Yangchuanosaurus'' hails from the Upper Shaximiao Formation and was the largest predator in a landscape that included the sauropods ''Mamenchisaurus'' and ''Omeisaurus'' and the stegosaurs ''Chialingosaurus'', ''Tuojiangosaurus'' and ''Chungkingosaurus''. This theropod was named after the area in which was discovered, Yongchuan, in China.

Discovery and species

Dong ''et al.'' (1978) named ''Yangchuanosaurus shangyouensis'' on the basis of CV 00215, a complete skull and skeleton which was collected from the Shangshaximiao Formation, near Yongchuan, Yongchuan District, Sichuan. It dates to the Oxfordian stage of the Late Jurassic period, about 161.2 to 157.3 million years ago. It was discovered in June 1977 by a construction worker during the construction of the Shangyou Reservoir Dam. A second species from the same locality, ''Y. magnus'', was named by Dong ''et al.'' (1983) on the basis of CV 00216, another complete skull and skeleton. A detailed revision of tetanuran phylogeny by Carrano, Benson & Sampson (2012) revealed that both species are conspecific. Dong ''et al.'' (1978) and Dong ''et al.'' (1983) differentiated these species primarily on the basis of size. In addition, Dong ''et al.'' (1983) noted that the maxilla of ''Y. magnus'' has an additional fenestra within the antorbital fossa, whereas ''Y. shangyouensis'' possessed only a fossa in this location. However, it is considered to be an intraspecific, possibly ontogenetic, variation. Furthermore, the apparent difference in cervical vertebral morphology can be explained by comparing different positions within the column. Hence, the holotypes of the two species of ''Yangchuanosaurus'' are effectively identical, and their codings are identical in Carrano ''et al.'' (2012) matrix. Gregory S. Paul (1988) regarded this genus as a synonym of ''Metriacanthosaurus'', but this has not been supported.

Carrano ''et al.'' (2012) assigned a third specimen to ''Y. shangyouensis''. CV 00214 is represented by a partial postcranial skeleton lacking the skull. It was collected in the Wujiaba Quarry, near Zigong city, Sichuan, from the lower part of the Shangshaximiao Formation. CV 00214 was initially listed by Dong ''et al.'' (1978) in a faunal list as a new species of ''Szechuanosaurus'', ''Szechuanosaurus'' "yandonensis". There is no description or illustration of it, making ''S.'' "yandonensis" a ''nomen nudum''. Later, Dong ''et al.'' (1983) described it, and assigned it to ''Szechuanosaurus campi'', a dubious species which is known only from four teeth. Carrano ''et al.'' (2012) noted that CV 00214 can't be assigned to ''S. campi'' because the holotype materials of ''S. campi'' (IVPP V.235, V.236, V.238, V.239; teeth) are non-diagnostic and no teeth are preserved in CV 00214. A recent restudy of CV 00214 by Daniel Chure (2001) concluded that it represented a new taxon, informally named "Szechuanoraptor dongi", into which ''Szechuanosaurus zigongensis'' should also be subsumed. However, the most recent revision (by Carrano ''et al.'' (2012)) suggested that CV 00214 and "S." ''zigongensis'' cannot be cospecific as there are no autapomorphies shared between them, and the latter derives from the underlying Xiashaximiao Formation. A phylogenetic analysis found CV 00214 to be most closely related to ''Y. shangyouensis'', and thus the former is assignable to it. Furthermore, ''Szechuanosaurus zigongensis'' was found to be closely related to ''Y. shangyouensis'' and therefore was designated as the second species of ''Yangchuanosaurus''.

''Yangchuanosaurus zigongensis'' is known from four specimens including ZDM 9011 (holotype), a partial postcranial skeleton; ZDM 9012, a left maxilla; ZDM 9013, two teeth and ZDM 9014, a right hind limb. It was first described by Gao (1993), and all specimens were collected from the Middle Jurassic Xiashaximiao Formation in the Dashanpu Dinosaur Quarry of Zigong, Sichuan.

A third, informal species, ''Yangchuanosaurus'' "longqiaoensis", was briefly mentioned in a faunal listing of the Penglaizhen Formation (initially believed to date to the Late Jurassic but is probably Early Cretaceous (Berriasian – Valanginian) in age) by Li, Zhang and Cai (1999). However, since it was published solely in a faunal list and not described in detail, it is a ''nomen nudum'' and is questionably referable to ''Yangchuanosaurus''.

Description

The type specimen of ''Y. shangyouensis'' had a skull long, and its total body length was estimated at . Another specimen, assigned to the new species ''Y. magnus'', was even larger, with a skull length of . It may have been up to long, and weighed as much as . Gregory S. Paul suggested that these are the same species and gave a length of 11 meters (36 feet) and a weight of 3 metric tons (3.3 short tons). There was a bony ridge on its nose and multiple hornlets and ridges, similar to ''Ceratosaurus''.

''Yangchuanosaurus'' was a large, powerful meat-eater. It walked on two large, muscular legs, had short arms, a strong, short neck, a big head with powerful jaws, and large, serrated teeth. It had a long, massive tail that was about half of its length. Its arms were short. The first digit of its foot was a small dewclaw. The three outer toes were used to bear weight and each was equipped with a large claw.

Classification

A phylogenetic analysis by Carrano ''et al.'' (2012) found ''Yangchuanosaurus'' to be the basalmost known metriacanthosaurid and the only non-metriacanthosaurine metriacanthosaurid. The cladogram presented below follows their study.

The cladogram presented below follows Zanno & Makovicky (2013).

References

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*''Fantastic Facts About Dinosaurs'' ()

{{Portalbar, Dinosaurs, Paleontology

Late Jurassic dinosaurs of Asia
Metriacanthosaurids
Taxa named by Dong Zhiming
Fossil taxa described in 1978
Paleontology in Sichuan