 In his 1905 paper naming Albertosaurus , Osborn described two additional tyrannosaur specimens that had been collected from in Montana and Wyoming during a 1902 expedition of the American Museum of Natural History , led by Barnum Brown . Initially , Osborn considered these to be distinct species . The first , he named Dynamosaurus imperiosus ( " emperor power lizard " ) , and the second , Tyrannosaurus rex ( " king tyrant lizard " ) . A year later , Osborn recognized that these two specimens actually came from the same species . Despite the fact that Dynamosaurus had been found first , the name Tyrannosaurus had appeared one page earlier in his original article describing both specimens . Therefore , according to the International Code of Zoological Nomenclature ( ICZN ) , the name Tyrannosaurus was used . 
