Mamenchisaurus was an immense sauropod. This giant was up to 75-85 feet long and 15-25 tons depending on the species. Mamenchisaurus had the longest neck of any dinosaur. It's neck alone was up to 46 feet long in the largest individuals. Long even for a sauropod. This dinosaur was so big, it may not have had any natural predators. Mamenchisaurus lived about 150 million years ago in Asia.