Yunnanosaurus

Yunnanosaurus (/ˌjuːnænoʊˈsɔːrəs/ YOO-nan-o-SAWR-əs) is an extinct genus of sauropodomorph dinosaur that lived approximately 201 to 168 million years ago in what is now the Yunnan Province, in China. 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 4 m (13 ft) high in the largest species. Yang Zhongjian (also known as C. C. Young) discovered the first Yunnanosaurus skeletons in the Lufeng Formation of Yunnan, China. The fossil find was composed of over twenty incomplete skeletons, including two skulls, it was 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. There were more than sixty spoon shaped teeth in the jaws of Yunnanosaurus, and were unique among early sauropodomorphs in that its teeth were self-sharpening because they ' against each other as the animal fed.' Scientists consider these teeth to be advanced compared to other early sauropodomorphs, as they share features with the sauropods. 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. This critical difference implies that the similarity in dentition between Yunnanosaurus and sauropods might be an example of convergent evolution. In 2007, Lü Junchang 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 ). Y. youngi is found later in the fossil record, hailing from the Middle Jurassic. 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. 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). 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. The type species, Y. huangi, was named by C. C. Young in 1942, who erected the family Yunnanosauridae to contain it, though the family currently comprises only this genus and sometimes Jingshanosaurus. Young also named a second species, Y. robustus, in 1951, but this has since been included in the type species. The confusion in classification arose due to that the earliest specimens of Y. huangi were of juveniles individuals while the 'Y. robustus' specimens represented fully grown adults. Yunnanosaurus had been assigned to several taxa over the years, including Thecodontosauridae and Plateosauridae, but more recent phylogenetic analysis conducted by Novas et al. (2011) shows that this genus is part of the taxon Massopoda in a clade with Anchisaurus and Jingshanosaurus. Apaldetti et al. (2011) also found that Yunnanosaurus belonged in Massopoda, but found that this genus was more primitive than both Jingshanosaurus and Anchisaurus.