Mamenchisaurus (pronounce /mɑːˈmʌntʃiːˈsɔrəs/ mah-MUN-chee-SAW-rus, or spelling pronunciation /məˌmɛntʃiːˈsɔrəs/) was a plant-eating four-legged dinosaur, known for its remarkably long neck. Most species lived 145 to 150 million years ago, in the Tithonian age of the late Jurassic Period.