"Mamenchisaurus" had the longest neck of any known dinosaur, except the newly found "Sauroposeidon".Its neck was about 46 feet (14 m). It had 19 vertebrae in its neck, more than any other known dinosaur had. "Mamenchisaurus" was about 70 to 80 feet (21 to 25 m) long and weighed roughly 12 tons (11 tonnes). 