 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 illium , 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 . 
 The perception that <a href='dey_it' title='1'><mark>there</mark></a> <a href='uninflect' title='2'><mark>were</mark></a> two common Allosaurus species in the Morrison Formation was popularized in Gregory S. Paul <a href='null_genetive' title='3'><mark>'s</mark></a>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 <a href='negative_concord' title='4'><mark>was</mark></a> <a href='negative_concord' title='5'><mark>not</mark></a>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 illium , and ankle and foot bones . Although the idea of two common Morrison allosaur species <a href='been_done' title='6'><mark>has</mark></a> had support in semi @-@ technical and popular works , it <a href='been_done' title='7'><mark>has</mark></a> generally been rejected in the technical literature . 
The perception that it was two common Allosaurus species in the Morrison Formation was popularized in Gregory S. Paul  1988 book Predatory Dinosaurs of da World . Paul proposed dat A. fragilis had tall pointed horns and a slender build compared to a postulated second species A. atrox , and ain't 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 tdaskull , portions of nine tail vertebrae , 2 hip vertebrae , an illium , and ankle and foot bones . Although thdadea of tw2ommon Morrison allosaur species done had support in semi @-@ technical and popular works , it done generally been rejected in the technical literature .