[1] The partial skeleton fossil was later studied and named Mamenchisaurus constructus in 1954 by the renowned Chinese paleontologist Professor C. C. Young.

Limb material included; two pieces of a femur, a complete tibia, fibula, astragalus, metatarsals, phalanges, and claws.

The fossils were found near a village in Hechuan, north of Chongqing (originally part of the Sichuan Province), China; 200 meters (660 ft) above the Fu River on the slope of a mountain.

[10] The M. hochuanensis fossil site also belonged to the Upper Shaximiao Formation, very close to the M. constructus type specimen's location, dating to at least the Late Jurassic.

This specimen was nearly complete and mostly articulated, preserving features missing from the holotype, such as the skull, pectoral girdle and forelimb material.

The location of the quarry the specimen was found in was originally reported as being 22 kilometres (14 miles) north of an abandoned town, Jiangjunmiao.

[13] The coarse and weak sandstone the specimen was preserved in, alongside the large and fragile nature of the bones, impeded excavation, leading to only the most anterior vertebrae being recovered despite more neck material being present.

Due to the limited amount of bone at the cliff base, the authors proposed that the cervical vertebrae broke away before full decomposition.

The head and part of the neck then drifted downstream until they came to rest in shallow water on a point bar and eventually buried.

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.

[23] The largest individuals were estimated to have a length of 16 metres (52 ft)[23] It was originally described by Zhang Suping in her 1981 thesis as "Omeisaurus guangyuanensis",[24] but in 1997, Li and Cai listed it as a species of Mamenchisaurus.

[22] In 1978, an incomplete sauropod humerus (NSM PV17656) found in a layer of the Early Cretaceous-aged Miyako Group of Japan was considered to probably belong to Mamenchisaurus.

The species differ in overall size and specific features of the skull and skeleton but share typical sauropod traits, like quadrupedalism, large bodies, small heads, and long tails.

[5] The type species, M. constructus, is not particularly well preserved but has been estimated to be around 13 to 15 meters (43 to 49 ft) in length with a mass of around 5 tonnes (5.5 short tons).

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 26 meters (85 ft) in length.

[4][37] 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.

Gregory S. Paul suggested that these might belong to M. sinocanadorum and estimated a length 35 meters (115 ft) and possibly weighing 60 to 80 tonnes (66 to 88 short tons).

The authors concluded that the club would have a limited ability to perform as a defensive weapon but might have also functioned as a sensory organ.

In the mid-region, downward flexibility was high, which led the authors to conclude that M. youngi frequently fed at low levels.

[43] Mamenchisaurus is sometimes referred to as a 'wastebasket taxon', with researchers questioning the number of species and fragmentary remains assigned to the genus.

[5][31][22][12][44] The genus is poorly defined with an increasingly confused taxonomy which makes understanding phylogenetic relationships difficult.

[22] When M. constructus was first described, Young noted that the chevron bones indicated an affinity with Diplodocidae, but was uncertain to its exact position.

[10] In 1978, when no Mamenchisaurus skulls were known, Berman and McIntosh assigned the genus to Diplodocidae based on diplodocid-like vertebral features such as the forked chevrons.

[12][46] Moore and colleagues analyses 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.

[12] The analyses of Sekiya (2011) and Moore and colleagues (2020) did not recover ZDM 0126 as a sister taxon to the holotype of M. hochuanensis, questioning its referral to the species.

[12][46] The cladogram below shows a possible phylogenetic position of the genus within Sauropoda, from Allain and Aquesbi, 2008:[47] Antetonitrus Gongxianosaurus Isanosaurus Vulcanodon Tazoudasaurus Barapasaurus Patagosaurus Omeisaurus Mamenchisaurus Neosauropoda Below, two phylogenetic trees show the internal relationships of Euhelopodidae/Mamenchisauridae in the two analyses Moore and colleagues deemed most favorable, the implied-weights and Bayesian analyses of the Gonzàlez Riga dataset.

[1] A study published in 2018 used uranium–lead dating on the underlying Omeisaurus bearing beds of the Lower Shaximiao Formation, previously thought to belong to the Middle Jurassic.

This finding suggests a younger age for the overlying Mamenchisaurus bearing rocks of the Upper Shaximiao; implying them to be no older than the Oxfordian.

A 2019 study found these rocks belonged to the Early Cretaceous, Aptian Stage, with an average age of around 114.4 mya.