Mamenchisaurus (məˌmʌntʃiˈsɔːrəs ,Dinosauria Translation and Pronunciation Guide M or spelling pronunciation məˌmɛntʃɪˈsɔːrəs) is a genus of sauropod dinosaur known for their remarkably long necks which made up nearly half the total body length. Numerous species have been assigned to the genus; however, many of these might be questionable. Fossils have been found in the Sichuan Basin and Yunnan Province in China. Several species are from the Upper Shaximiao Formation whose geologic age is uncertain. However, evidence suggests that this be no earlier than the Oxfordian stage of the Late Jurassic. M. sinocanadorum dates to the Oxfordian stage (158.7 to 161.2 mya) and M. anyuensis to the Aptian stage of the Early Cretaceous around 114.4 mya. Most species were medium to large size sauropods, around in length and possibly up to based on two undescribed vertebrae.
Mamenchisaurus was first discovered in 1952 on the construction site of the Yitang Highway in Sichuan Province of China. The fossil site belonged to the Upper Shaximiao Formation, dating to at least the Late Jurassic. The partial skeleton fossil was later studied and named Mamenchisaurus constructus in 1954 by the renowned Chinese paleontologist Professor C. C. Young.
The type specimen (IVPP V. 790) was fragmentary, disordered, and not excavated in a technical way. Material included five dorsal vertebrae, 30 caudal vertebrae, rib fragments, dorsal neural spines, and chevrons. Fourteen neck vertebrae were preserved, but none were complete. Young noted that some neck vertebrae might have been missing. Limb material included; two pieces of a femur, a complete tibia, fibula, astragalus, metatarsals, phalanges, and claws. The skull, forelimbs, and pelvic girdle were missing.
Mamenchisaurus means 'Mamenchi lizard', from the Chinese Pinyin mǎ (马 'horse') and mén (门 'gate'), while chi is a 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.