Mamenchisaurus

Mamenchisaurus was first discovered in 1952 on the construction site of the Yitang Highway in Sichuan Province, China. The fossil site belonged to the Upper Shaximiao Formation, dating to at least the Late Jurassic. Mamenchisaurus means 'Mamenchi lizard', from the Chinese Pinyin mǎ (马 'horse') and mén (门 'gate'), while chi is an alternative transliteration of xī (溪 'stream' or 'brook'), combined with the suffix -saurus (from Greek sauros meaning 'lizard'). The intention was to name the genus after the place where its fossil was first found. However, due to an accentual mix-up by Young, the location name Mǎmíngxī (马鸣溪 'horse-neighing brook') was mistaken as Mǎménxī (马门溪 'horse-gate brook'). The fact that the first Mamenchisaurus fossil was found during construction work led Young to name the type species as Mamenchisaurus constructus. In 1972, one of these specimens was reassigned to M. hochuanensis. The M. hochuanensis fossil site also belonged to the Upper Shaximiao Formation, very close to the location of the M. constructus type specimen, dating to at least the Late Jurassic. In 1958, Young described a mamenchisaur specimen (IVPP V. 946) from the Haishiwan region of Yongdeng, Gansu Province. This specimen was initially assigned to M. constructus. However, in 1972 it was reassigned to M. hochuanensis as a paratype. This specimen was slightly smaller than the holotype and consisted of less material overall. However, it contained some anatomical details missing in the type specimen. The locality is thought to date to around 162.2 million years ago. Moore and colleagues could not support the referral of these vertebrae to M. sinocanadorum in their redescription of the species because the vertebrae lack anatomical overlap with the known material of the type specimen, and cannot therefore be compared for diagnostic purposes. They also noted an undescribed specimen on display at the China University of the Geosciences in Beijing, labelled as belonging to the species, which has not been evaluated firsthand or mentioned in the scientific literature. Accordingly, they consider the referral to the species premature. M. anyuensis is known from both the top of the Suining Formation and the bottom of the Penglaizhen Formation. Uranium–lead dating places M. anyuensis in the Suining Formation at 114.4 Ma in age; as this would make it roughly 30 million years younger than the other Mamenchisaurus species, it is unlikely that M. anyuensis is actually a member of the genus. The fossils were located in the Sichuan basin, from the Upper Shaximiao Formation. Other referred species and material Other species of Mamenchisaurus have been named over the years. In some cases, species from other genera have been transferred to Mamenchisaurus, but the validity of the referrals is disputed by other researchers. Some of these species are based on fragmentary remains and have been considered undiagnostic. Others are considered invalid or as nomina nuda. In 1976, Hou, Chao, and Chu named a new genus, Zigongosaurus fuxiensis. Known from at least four specimens from the Upper Shaximiao Formation. The type specimen (CV 02501) included skull material: maxilla, dentary, and basioccipital. Additional postcranial material includes dorsal vertebrae, pubis, and ischium. Since the description of Zigongosaurus, other researchers have disagreed over whether the genus is valid. In 1983, Dong, Zhou, and Zhang assigned some of the remains to the similarly named Omeisaurus fuxiensis, and the rest to Omeisaurus junghsiensis. Zhang and Chen assigned the remains to Mamenchisaurus as M. fuxiensis in 1996. Li and Cai considered it a nomen nudum in 1997. The largest individuals were estimated to be long. Wang and colleagues questioned the assignment to Mamenchisaurus in 2019. In 1996, Zhang and Chen considered Omeisaurus changshouensis and Omeisaurus gongjianensis as referable to Mamenchisaurus. However, the humerus was reassessed by Azuma & Tomida in 1998, and Barrett and colleagues in 2002. These authors found no distinguishing features that could place the humerus into a specific sauropod group. They regarded it as an indeterminate sauropod. The remains were given an informal name "Moshi-ryu"; "Moshi" being the local name for the location it was discovered, and "ryu" being Japanese for dragon—also referred to as "Moshisaurus". == Description ==

The genus Mamenchisaurus contains several named species, but there is disagreement about whether all species assigned to it should be included. Mamenchisaurus cervical vertebrae are elongated, lightly constructed, and highly pneumatic. Young and Zhao estimated the mass of M. hochuanensis at . In 1972, Young and Chao described M. hochuanensis as having 19 cervical and 12 dorsal vertebrae. Paul Upchurch and colleagues suggested this vertebral count may be incorrect. The authors noted that the vertebra usually referred to as the second dorsal possessed a hyposphene, a feature not typically seen until the third or fourth dorsal in sauropods. They provisionally proposed that the actual vertebral count might be 18 cervicals and 13 dorsals in M. hochuanensis. M. sinocanadorum is known from fragmentary remains, but these suggest that it was a large species. The cervical vertebrae are on average 1.19 times longer than those of M. hochuanensis; based on this, Russel and Zheng estimated the type specimen at in length. However, the 2023 redescription gave a longer estimate of between and based on comparison to Xinjiangtitan, though the authors stressed the level of uncertainty. Based on CT imaging, Moore and colleagues estimated the cervical vertebrae of M. sinocandorum to be 69–77% air by volume, assuming complete removal of bone marrow. The mass of M. youngi was estimated at using volumetric techniques. Paul has argued that these types of chevrons are adaptations linked to rearing behaviour. In a tripodal stance, the tail acts as a prop, and the forked chevrons would help distribute the weight evenly. Paul also notes that the pelvises of mamenchisaurs are retroverted (tilted), which may have allowed slow bipedal walking. The neck posture and feeding strategies of sauropod dinosaurs are controversial. Andreas Christian and colleagues analysed the neck of M. youngi and found that when articulated in a neutral posture, the neck was almost straight, with a slight upward bend at the base and a slight downward bend towards the head. The base of the neck has comparatively high upward flexibility but limited downward flexibility. The region near the head had better downward flexibility and low upward flexibility. In the mid-region, downward flexibility was high, which led the authors to conclude that M. youngi frequently fed at low levels. Long overlapping cervical ribs may have limited flexibility. The authors also estimated the stress on the intervertebral joint cartilage. Their results suggest the neck was kept mostly straight, with the possible exception of the neck base and near the head. A large ulna (GPIT SGP 2006/10) measuring less than in length, referred to Mamenchisaurus sp. from the Shishugou Formation, was used in a bone histology analysis. By sectioning the bone and counting the growth rings, the age at death was estimated at 43 years. == Classification ==

Mamenchisaurus is sometimes referred to as a 'wastebasket taxon', with researchers questioning both the number of referred species and the number of fragmentary specimens assigned to the genus. The genus is poorly defined, and its taxonomy is increasingly confused, making it challenging to understand the phylogenetic relationships. Several analyses have failed to show Mamenchisaurus as monophyletic, suggesting the need to revise the genus. Several later analyses found Euhelopus to be a more distantly related macronarian, with Mamenchisaurus in Mamenchisauridae just outside of Neosauropoda. Moore and colleagues found the position of M. constructus to be unstable, probably due to the limited character information in its description. Depending on the dataset used, Euhelopus would either be within Macronaria, as other studies have found, or outside Neosauropoda in a more traditional position, grouped with other Mamenchisaurus-like taxa. The latter scenario would make mamenchisaurids members of Euhelopodidae. }} }} }} }} }} }} }} }} Below, two phylogenetic trees show the internal relationships of Euhelopodidae/Mamenchisauridae in the two analyses Moore and colleagues deemed most favourable, the implied-weights and Bayesian analyses of the Gonzàlez Riga dataset. Topology A: Implied-weights analysis, Gonzàlez Riga dataset {{clade| == Paleochronology ==

Mamenchisaurus was originally thought to have ranged from the Middle to Late Jurassic. However, there is no reliable dating for the Upper Shaximiao Formation, where many of the Mamenchisaurus species are found. M. sinocanadorum was found from the Middle to Upper Shishugou Formation. Radiometric dating suggests this formation dates to the Oxfordian, ranging from 158.7 to 161.2 mya. M. anyuensis, was found in the Suining Formation, thought initially to be Middle to Late Jurassic. A 2019 study found these rocks belonged to the Early Cretaceous, Aptian Stage, with an average age of around 114.4 mya. This indicates that mamenchisaurids might have existed around 30 million years longer than previously thought. == Notes ==