 Gilmore was working as a paleontologist for the Carnegie Museum of Natural History in 1901 when he found the skeleton of a young sauropod, which was classified the following year as an "Apatosaurus".In 1903 Gilmore was hired by the United States National Museum (now the National Museum of Natural History), part of the Smithsonian Institution.His first assignment there was to work on the vast O. C. Marsh collection amassed during the Bone Wars; the fossils had been transferred from Yale University's new Peabody Museum of Natural History after the collection outgrew the smaller museum's storage capacity.