 The stegosaurians were widely distributed geographically in the late Jurassic . Palaeontologists believe it would have eaten plants such as mosses , ferns , horsetails , cycads , and conifers or fruits . Grazing on grasses , seen in many modern mammalian herbivores , would not have been possible for Stegosaurus , as grasses did not evolve until late into the Cretaceous Period , long after Stegosaurus had become extinct . 
