Hudiesaurus (meaning "butterfly lizard") is a herbivorous sauropod genus of dinosaur from China. Its fossil remains were found in 1993 by a Chinese-Japanese expedition near Qiketai in Shanshan, Xinjiang province. The genus contains a single species, Hudiesaurus sinojapanorum, was named and described by Dong Zhiming in 1997. The generic name is derived from Mandarin hudie, meaning "butterfly," and refers to a flat butterfly-shaped process on the front base of the vertebral spine. The specific name refers to the members of the Sino-Japan Silk Road Dinosaur Expedition but can also be read as "central part" in Chinese, a pun on the Japanese Chunichi Shimbun (again "central part") press group, which financed the research.

Discovery

Hudiesaurus is known from only two incomplete specimens, uncovered within sediments in the Turpan Basin equivalent to the Kalazha Formation of the Junggar Basin, which perhaps dates to the late Jurassic Period. The type specimen (IVPP V 11120) is represented only by a very large posterior cervical vertebra. A partial skeleton belonging to a smaller individual was found about 1 km away from the holotype. This specimen, IVPP P. 11121, consisting of a nearly complete right front leg and teeth, was referred to Hudiesaurus by Dong. In 2004, Paul Upchurch rejected this proposal because of a lack of overlapping material. Upchurch et al. made the specimen the holotype of a new genus, Rhomaleopakhus, in 2021.

Description

Despite the fragmentary nature of the fossil material, Hudiesaurus is believed to have been very large, even for a sauropod, given the considerable length of the vertebral centrum of 55 cm. Its body length is estimated at 20-30 m. In 2016, Gregory S. Paul estimated its length at 25 m and its weight at 25 tonne. In 2021, Upchurch et al. suggested that the vertebra may be cervical instead of thoracic, suggesting that the animal would be at 32 m and 55 tonne.

Classification

In the original description, Dong thought Hudiesaurus may have been related to Mamenchisaurus and accordingly placed it in the Mamenchisauridae. In 2004, Upchurch limited the precision to a more general Eusauropoda. In the 2021 description of Rhomaleopakhus, the relationships of Hudiesaurus were again revised. In their phylogenetic analysis, Upchurch et al. proposed that Hudiesaurus was a member of the Mamenchisauridae, as a sister taxon to two Mamenchisaurus species (M. youngi and a referred specimen of M. hochuanensis) and Xinjiagtitan, as well as a clade containing the holotype of M. hochuanensis and Klamelisaurus. Their cladogram is shown below: