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. shows that it is most closely related to Anchisaurus and Jingshanosaurus. The type specimen of Yunnanosaurus youngi was recovered at the Banqing Houshanliangzi locality of the Zhanghe Formation, in Yuanmou County of Yunnan Province, China. The holotype specimen CXMVZA 185 was collected in 2000 in terrestrial sediments deposited during the Aalenian and Bajocianstages of the Middle Jurassic period, approximately 174 to 168 million years ago. This specimen is housed in the collection of the Chuxiong Museum. In 2007, Lü Junchang and colleagues described another species of Yunnanosaurus, Y. youngi. In addition to various skeletal differences, at 13 meters long Y. youngi was significantly larger than Y. huangi. Y. youngi is found later in the fossil record, hailing from the MiddleJurassic. 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". 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.
Description
Dentition
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.
Classification
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. shows that this genus is part of the taxon Massopoda in a clade with Anchisaurus and Jingshanosaurus. Apaldetti et al. also found that Yunnanosaurus belonged in Massopoda, but found that this genus was more primitive than both Jingshanosaurus and Anchisaurus.
A diagnosis is a statement of the anatomical features of an organism that collectively distinguish it from all other organisms. Some, but not all, of the features in a diagnosis are also autapomorphies. An autapomorphy is a distinctive anatomical feature that is unique to a given organism. According to Sekiya et al., Yunnanosaurus can be distinguished from other dinosaurs based on the following characteristics:
the absence of the anteroposterior expansion on the medial end of the astragalus
the medium shaft of the metatarsal IV is dorsoventrally compressed
According to Lu et a., Yunnanosaurus youngi can be distinguished from Yunnanosaurus huangi based on the following characteristics:
The type specimens of Yunnanosaurus huangi and Yunnanosaurus robustus were recovered in the Huangchiatien locality of the Lufeng Formation in Yunnan, China. The Y. huangi holotype specimen IVPP V20 and the Y. robustus holotype specimen IVPP V93, were collected by Chung Chien Young in terrestrial sediments from the upper dark/deep red beds of the Zhangjiawa Member of this formation, that are believed to have been deposited during the Sinemurianstage of the Jurassic period, approximately 199 to 190 million years ago. Several other specimens assigned to Y. huangi, and Y. robustus were also recovered by Young in this locality. These specimens are all housed in the collection of the Institute of Vertebrate Paleontology and Paleoanthropology, in Beijing, China. In the years to come several more specimens assigned to these two species were recovered from Zhangjiawa Member of this formation. Chung Chien Young had also explored the lower dark/dull purple beds of the Shawan Member of the Lufeng Formation and found more specimens that he later assigned to Y. huangi. Specimen IVPP V32 was collected by Young in 1938 in dark red, argillaceous sandstone that is believed to have been deposited during the Hettangian stage of the Jurassic period, approximately 201 to 199 million years ago. Specimens IVPP V57, IVPP V60 and IVPP V272 were collected by Young in blue mudstone from the same formation and were also assigned to Y. huangi. These specimens from the Shawan Member are also housed in the collection of the Institute of Vertebrate Paleontology and Paleoanthropology. Yunnanosaurus huangi and Yunnanosaurus robustus shared their paleoenvironment with the ornithischiansBienosaurus, and Tatisaurus, the sauropodomorphs Gyposaurus, Lufengosaurus, and Jingshanosaurus, and the theropodsSinosaurus triassicus and Eshanosaurus.
