 The origin of Stegosaurus is uncertain , as few remains of basal stegosaurs and their ancestors are known . Recently , stegosaurids have been shown to be present in the lower Morrison Formation , existing several million years before the occurrence of Stegosaurus itself , with the discovery of the related Hesperosaurus from the early Kimmeridgian . The earliest stegosaurid ( the genus Lexovisaurus ) is known from the Oxford Clay Formation of England and France , giving it an age of early to middle Callovian . 
