Yunnanosaurus

Yunnanosaurus (/ˌjuːnænɵˈsɔrəs/ EW-nan-o-SAWR-əs) is an extinct genus of prosauropod dinosaur that lived approximately 201 to 168 million years ago in what is now the Yunnan Province, in China, which would make it one of the last "prosauropods" and the only one to live into the middle Jurassic Period. Yunnanosaurus was a large sized, moderately-built, ground-dwelling, quadrupedal herbivore, that could also walk bipedally, and ranged in size from 7 meters (23 feet) long and 2 m (6.5 ft) high to 13 m (42 ft) long in the largest species.

Discovery
Yang Zhongjian (aka C. C. Young) discovered the first Yunnanosaurus skeletons in the Lufeng Formation of Yunnan, China. The fossil find was composed over twenty incomplete skeletons, including two skulls, and were excavated by Tsun Yi Wang. When first discovered, Yunnanosaurus was considered to be closely related to Lufengosaurus, but more recent research by Novas et al. (2011) shows that it is most closely related to Anchisaurus and Jingshanosaurus.

Description
Yunnanosaurus was one of the first known "prosauropod" dinosaurs, which were primitive herbivores with long necks, long tails, and high, narrow heads with short snouts. It was a facultative quadruped, meaning that it was primarily adapted for walking on all four legs, but was capable of also walking on its two hind legs.

Dentition
There were more than sixty spoon shaped teeth in the jaws of Yunnanosaurus, and were unique among prosauropods in that its teeth were self-sharpening because they "[wore] against each other as the animal fed."[2] Scientists consider these teeth to be advanced compared to other prosauropods, as they share features with the sauropods.[2] However, scientists do not consider Yunnanosaurus to be especially close to the sauropods in phylogeny because the remaining portions of the animals body are distinctly prosauropod in design.[2] This critical difference implies that the similarity in dentition between Yunnanosaurus and sauropods might be an example of convergent evolution.

Yunnanosaurus youngi
In 2007, Lu and colleagues described another species of Yunnanosaurus, Y. youngi (named in honor of C. C. Young). In addition to various skeletal differences, at 13 meters (42 ft) long Y. youngi was significantly larger than Y. huangi (which reached only 7 meters [23 ft]). Y. youngi is found later in the fossil record, hailing from the Middle Jurassic, which would make it latest surviving "prosauropod". The holotype specimen CXMVZA 185 consists of ten cervical vertebrae, fourteen dorsal vertebrae, three fused sacral vertebrae, seventeen caudal vertebrae, both pubic bones, both ischia, and the right illium. The skull of this species is not known.

Juvenile Specimen ZMNH-M8739
In 2013, Sekiya et al. described the discovery of a juvenile individual which was assigned to what the authors termed "Yunnanosaurus robustus" (i.e. Y. huangi).[Note 1] Specimen ZMNH-M8739 consists of partial cranial material and an almost complete post-cranial skeleton. This individual possesses characteristic dentition that suggests a potentially unique feeding mechanism as evidenced a tooth–tooth wear facet on its mesial maxillary and dentary teeth, and maxillary teeth that have coarse serrations. Comparison of this juvenile specimen with adult specimens of Yunnanosaurus huangi reveals very distinctive growth changes.