(meaning "western lizard", from Classical Greek, ἕσπερο-/hespero- "western" and σαυρος/saurus "lizard")
a herbivorous dinosaur from the Kimmeridgian to Tithonian epochs of the Jurassic period (approximately 150 million years ago), whose fossils are found in the state of Wyoming in the United States of America. It is from an older part of the Morrison Formation, and so a little older than other Morrison Stegosaurs.
Carpenter K, Miles CA, Cloward K 2001. New Primitive Stegosaur from the Morrison Formation, Wyoming. In Carpenter, Kenneth(ed). The Armored Dinosaurs. Indiana University Press. pp. 55–75.