One of the most unusual of the giant dinosaurs was Mamenchisaurus: it had more cervical or neck vertebrae than any known dinosaur. It was about 26 m (85 ft.) long and weighed about 16 t (18 tn.). The disproportionately long neck most likely allowed this relatively light sauropod to crop vegetation over a wide area without moving its body.

The genus Mamenchisaurus was named in 1954 by the famous Chinese paleontologist, C.C. Young, in honour of the locality where the first specimen was found: the Mamenchi Ferry on the upper Yangtze River in China.

There are numerous species within this genus. All have been found in China. One species, M. sinocanadorum, was named in 1993 in honor of the Chinese and Canadian expedition members of Sino-Canadian Dinosaur Project (1987–1990) who jointly discovered this particular species. Palaeontologists at the Canadian Museum of Nature participated in this expedition.

Mamenchisaurus lived during the Late Jurassic Period, about 160 to 145 million years ago.