 The perception that there were two common Allosaurus species in the Morrison Formation was popularized in Gregory S. Paul 's 1988 book Predatory Dinosaurs of the World . Paul proposed that A. fragilis had tall pointed horns and a slender build compared to a postulated second species A. atrox , and was not a different gender due to rarity . Allosaurus atrox was originally named by Marsh in 1878 as the type species of its own genus , Creosaurus , and is based on YPM 1890 , an assortment of bones including a couple of pieces of the skull , portions of nine tail vertebrae , two hip vertebrae , an <unk> , and ankle and foot bones . Although the idea of two common Morrison allosaur species has had support in semi @-@ technical and popular works , it has generally been rejected in the technical literature . 
