Thomas Horne Preparing Diplodocus Specimen, 1924, Smithsonian Institution Archives, United States National Museum preparator Thomas Horne working on the Diplodocus specimen collected in 1923 from Dinosaur National Monument Quarry. He is working to free some of the specimen's vertebrae from the surrounding sandstone. Under the direction of curator Charles Gilmore, preparators Norman Boss, Thomas Horne, and John Barrett worked for more than seven years to prepare and mount the dinosaur for display. The project was finally completed in 1931..