Yunnanosaurus Youngi

''Yunnanosaurus'' ( ) is an extinct genus of sauropodomorph dinosaur that lived approximately 199 to 183 million years ago in what is now the Yunnan Province, in China, for which it was named. ''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.

Discovery

Yang Zhongjian (also known as C. C. Young) discovered the first ''Yunnanosaurus'' skeletons in the upper Zhangjiawa Member of the Lufeng Formation of Yunnan, China, dating to the Sinemurian stage of the Early Jurassic. The fossil find was composed of over twenty incomplete skeletons, including two skulls, it was excavated by Tsun Yi Wang. These remains were the basis for the species ''Y. huangi'' (the type species) and ''Y. robustus.''

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 3 ft. 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 ilium. The skull of this species is not known.Lu, J., Li, T., Zhong, S., Azuma, Y., Fujita, M., Dong, Z., and Ji, Q. (2007). "New yunnanosaurid dinosaur (Dinosauria, Prosauropoda) from the Middle Jurassic Zhanghe Formation of Yuanmou, Yunnan Province of China." ''Memoir of the Fukui Prefectural Dinosaur Museum'', 6: 1-15.

The type specimen of ''Yunnanosaurus youngi'' was recovered at the Banqing Houshanliangzi locality of the Fengjiahe Formation, previously thought to be from 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 Pliensbachian stage of the Early Jurassic period, approximately 191 to 183 million years ago. This specimen is housed in the collection of the Chuxiong Museum.

In 2013, Sekiya et al. described the discovery of a juvenile individual which was assigned to ''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 " oreagainst each other as the animal fed.""Yunnanosaurus." In: Dodson, Peter & Britt, Brooks & Carpenter, Kenneth & Forster, Catherine A. & Gillette, David D. & Norell, Mark A. & Olshevsky, George & Parrish, J. Michael & Weishampel, David B. ''The Age of Dinosaurs''. Publications International, LTD. p. 47. . 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''. The specific name honours Huang Chiching ("T.K. Huang"), the director of the National Geological Survey of China.Young, C. C. (1942).
''Yunnanosaurus huangi'' Young (gen. et sp. nov.), a new Prosauropoda from the red beds at Lufeng, Yunnan.
''Bulletin of the Geological Society of China'', 22 (1-2): 63-104. Young also named a second species, ''Y. robustus'', in 1951,Young, C. C. (1951). "The Lufeng saurischian fauna in China."
''Paleontologica Sinica'', ser. C, 13: 1-96. 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 juvenile individuals while the ''Y. robustus'' specimens represented fully grown adults. However, Sekiya ''et al.'' (2013) described a juvenile of ''Y. robustus'' and were able to differentiate it from ''Y. huangi''. ''Yunnanosaurus'' had been assigned to several taxa over the years, including Thecodontosauridae and Plateosauridae, but a 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''.Fernando E. Novas, Martin D. Ezcurra, Sankar Chatterjee and T. S. Kutty (2011). "New dinosaur species from the Upper Triassic Upper Maleri and Lower Dharmaram formations of central India". Earth and Environmental Science Transactions of the Royal Society of Edinburgh 101 (3-4): 333–349. doi:10.1017/S1755691011020093 Apaldetti et al. (2011) also found that ''Yunnanosaurus'' belonged in Massopoda, but found that this genus was more primitive than both ''Jingshanosaurus'' and ''Anchisaurus''.

Distinguishing anatomical features

A diagnosis is a statement of the anatomical features of an organism (or group) 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.

''Y. huangi''

According to Barrett et al. (2007), the skull of ''Y. huangi'' can be distinguished from other sauropodomorphs based on the following characteristics:

* a small external naris
* a robust expanded nasal process of the premaxilla
* a downward projection from the rear of the maxillary ascending process
* no nutritive foramina on the lateral surface of maxilla
* a shallow circular depression on the lateral surface of the ventral lacrimal process
* a midline boss near the front of the frontals
* a prominent midline boss on the parietals
* the anterolateral process of the parietal is expanded relative to the width of the posterolateral process
* the maxillary teeth are narrow and do not have denticles

Additionally, Sekiya ''et al.'' (2013) add one more character distinguishing ''Y. huangi'': a hemispherical neural spine in the posterior cervical vertebrae.

''Y. youngi''

According to Lu et al. (2007), ''Y. youngi'' can be distinguished from ''Y. huangi'' based on the following characteristics:
* the sixth cervical vertebra is the longest among the vertebral column
* the neural spines of the posterior cervical vertebrae are short with an expanded distal end, which is wider than its anteroposterior length
* three sacral vertebrae are tightly fused with a stout sacrocostal yoke
* the ventral margin of the postacetabular process of the ilium is slightly concave
* the ischium is longer than the pubis
* the distal end of the pubis is round

''Y. robustus''

According to Sekiya et al. (2013), ''Yunnanosaurus robustus'' can be distinguished from other sauropodomorphs based on the following characteristics:
* the absence of the anteroposterior expansion on the medial end of the astragalus
* the shaft of the metatarsal IV is dorsoventrally compressed

Paleoecology

The type specimens of ''Yunnanosaurus huangi'' and ''Yunnanosaurus robustus'' were recovered in the Huangchiatien (Dahungtien) 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 Sinemurian stage of the Jurassic period, approximately 199 to 190 million years ago.Luo, Z., and X.-C. Wu. 1994. The small tetrapods of the Lower Lufeng Formation, Yunnan, China; pp. 251–270 in N. C. Fraser and H.-D.Sues (eds.), In the Shadow of the Dinosaurs. Cambridge University Press, New York Several other specimens assigned to ''Y. huangi'' (IVPP V54, IVPP V47, IVPP V61, IVPP V62, IVPP V63, IVPP V96, IVPP V264), and ''Y. robustus'' (IVPP V39, IVPP V94) 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 ornithischians '' Bienosaurus'', and '' Tatisaurus'', the sauropodomorphs '' Gyposaurus'', '' Lufengosaurus'', and '' Jingshanosaurus'', and the theropods '' Sinosaurus triassicus'' and '' Eshanosaurus''.

Notes

References

{{Taxonbar, from=Q427650

Early Jurassic dinosaurs of Asia
Massopoda
Paleontology in Yunnan
Fossil taxa described in 1942
Taxa named by Yang Zhongjian
Early Jurassic first appearances
Middle Jurassic extinctions