Neimongosaurus (meaning "Nei Mongol lizard") is a genus of herbivorous therizinosaurian theropod dinosaur that lived in China during the Late Cretaceous period. Its fossils are known from the strata of the Iren Dabasu Formation. It is known from two specimens, discovered in 1999 by researchers from the Ministry of Land and Resources and described two years later. One species, N. yangi, is known, named after Chinese palaeontologist Yang Zhongjian.
Discovery and naming In 1999, a team from the Ministry of Land and Resources, based in Hohhot, Inner Mongolia, was conducting field work at Sanhangobi, 12 mi (20 km) southwest of Erenhot. The strata they were working in belonged to the Iren Dabasu Formation, which has been variably dated to the Turonian, the Santonian, or the Campanian–Maastrichtian. The first specimen, LH V0001, consisted of a partially preserved braincase; the front of the right lower jaw; a nearly complete axial column compromising 15 cervical (including the axis), 4 dorsal and 22 caudal vertebrae; a furcula; both scapulocoracoids; both humeri; left radius; fragmented ilia; both femora; both tibiae; left tarsals and a virtually complete and articulated left pes. The second, LH V0008, consisted of a sacrum composed by 6 sacral vertebrae and both ilia. Both specimens were transported to the Long Hao Institute of Geology and Palaeontology for study. In 2001, Zhang Xiaohong, Xu Xing, Paul Sereno, Kwang Xuewen and Tan Lin assigned them to a new genus and species of therizinosaurid dinosaur, Neimongosaurus yangi, designating LH V0001 as the holotype. The generic name is derived from Nei Mongol, the Chinese name for Inner Mongolia. The specific name honours Yang Zhongjian.
Description
Neimongosaurus was a fairly small therizinosaur. Zhang et al., in 2001, estimated its body length at 2–3 m (6.6–9.8 ft). In 2016, Gregory S. Paul estimated its body length at 3 m (9.8 ft), and its body mass at 350 kg (770 lb).
Skull The skull of Neimongosaurus is represented by only the posterior (rear) part of the braincase, and the anterior (front) half of the right dentary. Similar to ornithomimids, oviraptorosaurs, and most troodontids, the symphyseal region (the area at the very front, where both hemimandibles connected) was U-shaped, rather than V-shaped as in other theropods. At the front of the dentary is an edentulous (toothless) region. Behind that is the alveolar margin which preserves five alveoli, or tooth sockets. Only one, the third alveolus, contains a functioning tooth, though the second bears the crowns of a replacement tooth that had yet to fully erupt. As demonstrated by the unerupted tooth, the crowns were compressed transversely (from side to side), and bore marginal denticles at the front and back. A neurovascular foramen, through which both blood vessels and nerves would have exited the skull, is situated about 15 mm (0.59 in) behind the symphysis. The occipital condyle is very thin, measuring only 12 mm (0.47 in), compared to the condyle of the foramen magnum, which measured roughly 15 mm (0.59 in).
Vertebral column Neimongosaurus' vertebral column is represented by a total of seventeen vertebrae. The first thirteen of these have been tentatively identified as cervical (neck) vertebrae. Zhang et al. suggested, tentatively that a total of fourteen were present. This would mean that Neimongosaurus would have had one of the longest cervical columns of any non-avian theropod, longer than that of taxa such as Nanshiungosaurus, which had twelve or fewer. Some oviraptorosaurs also had an increased number of cervical vertebrae, though the maximum observed is twelve, in Caudipteryx. The axis and the nine vertebrae behind it had long centra, with slightly concave anterior faces and strongly concave posterior ones. Seen from the side, they appeared gently arched, with much of their sides dominated by broad pleurocoels. The zygapophyseal facets of the neural arches were located to the side of the centra. The neural spines were low for most of the vertebral column, around half as tall as the centrum was long. The last cervical vertebra (the fourteenth) was very short. The first four dorsal (back) vertebrae are preserved in articulation with the last cervical vertebra, though four additional vertebrae not found in articulation were tentatively identified as the fifth through eighth. Their centra are spoon-shaped, bearing large pleurocoels. The neural spine of the fourth dorsal vertebra is tall and rectangular. The sacrum consists of six co-ossified (fused) vertebrae, which are known from the paratype. Twenty-two caudal (tail) vertebrae are known, suggesting that Neimongosaurus had a fairly short tail. The first caudal vertebra has long transverse processes, longer than the neural spine. The first few centra overall bore small foramina on each side, apparently reduced pleurocoels.
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