Call it a case of mistaken identity. In the 1870s and 1880s, competing crews of fossil hunters were discovering new fossils in the American West at an astonishing rate. Yale's O.C. Marsh and Philadelphia's E.D. Cope rushed to put out scientific descriptions and name the new animals. In the span of two years, Marsh designated a new skeleton from Wyoming as the type specimen of Apatosaurus and described a partial fossil from Colorado as Brontosaurus. In 1899, his former assistant Samuel Williston reexamined the two specimens and realized they represented the same species. Naming rules do not allow two scientific names for one animal, so the first published name--Apatosaurus--became the only valid one.