Toiling alone in Denmark without vast collections or armies of assistants, Gerhard Heilmann painstakingly compared every feature of theropod dinosaurs, extinct birds, and modern birds, from the tips of their toes to the smallest bones of their skulls. He saw so many similarities, particularly between Archaeopteryx and the delicate coelurosaurs, that he was on the brink of agreeing with British biologist T.H. Huxley that birds were in fact descended from dinosaurs. But because no known specimen of coelurosaur had collarbones, and birds do (in the guise of the wishbone), Heilmann was forced to conclude that birds and dinosaurs merely shared a distant common ancestor.

For want of a collarbone, dinosaur parentage of birds was lost, and in spite of all the other evidence, Heilmann's diagnosis held sway for another fifty years.

In the first decades of the 20th century, Barnum Brown led expeditions for the American Museum of Natural History, with impressive finds in Montana and Wyoming and in Alberta, Canada, including fossilized evidence of dinosaur skin and tendons. But not even his discoveries of the fearsome Tyrannosaurus rex, whose rarity fuels its lasting popularity, could hold the public's imagination for very long.

The Great Depression, bookended by a World War at either side, slowed dinosaur paleontology to a cold-blooded crawl. Henry Fairfield Osborn, who replaced Marsh and Cope as the sponsor of Western American bone hunts in the 1890s, shifted his attention to extinct rhinoceroses and human evolution after 1910. When he gave Roy Chapman Andrews his blessing to explore the Gobi desert, it was to search for the origins of the human race; no one expected to find dinosaurs. It was the era of the Scopes monkey trial, and dinosaurs just weren't interesting anymore.