 There have been a number of potential species assigned to the <unk> dinosaur genus Allosaurus since its description in 1877 by Othniel Charles Marsh , but only a handful are still regarded as valid . Allosaurus was originally described from material from the Upper Jurassic Morrison Formation of the western United States of America ; the type species A. fragilis became one of the best @-@ known species of dinosaur . 
