Hesperosaurus (Greek for "western lizard"); pronounced HESS-per-oh-SORE-us
Woodlands of North America
Late Jurassic (150 million years ago)
Size and Weight:
About 20 feet long and one ton
Short, wide head; oval-shaped plates
Stegosaurs--the spiked, plated dinosaurs--first appeared in Asia during the middle to late Jurassic period, then crossed over to North America a few million years later. That would explain the relatively evolved appearance of one of the first North American stegosaurs, Hesperosaurus, with its wide, round, mushroom-shaped dorsal plates and unusually short and blunt head (earlier stegosaurs had narrower skulls and less ornate plates). Ironically, the near-complete skeleton of Hesperosaurus was discovered in 1985 during the excavation of a fossil of its much more famous cousin, Stegosaurus.