It's not certain that all the species that have been assigned to "Mamenchisaurus" should belong to the genus. Some "Mamenchisaurus" species are almost completely known, and others are fragmentary. The species vary in size and specific skull and skeleton features but share typical sauropod traits; like large bodies with long tails, and are quadrupedal. However, mamenchisaurids most distinctive feature is their exceptionally long necks, approaching half their body length. Complete necks are known in "M. youngi and M. hochuanensis", consisting of 18 and 19 vertebrae respectively. "Mamenchisaurus" cervical vertebrae are elongated, lightly constructed and highly pneumatic. The neural spines on their posterior cervical and anterior dorsal vertebrae are bifurcated. Their shoulders were somewhat higher than the hips. The different species vary in length, from around to at least , and maybe up to based on two undescribed vertebrae.