Photographs by Stephanie Abramowicz for Birds of Stone: Chinese Avian Fossils from the Age of Dinosaurs by Luis M. Chiappe and Meng Qingjin By Jeff Hecht JUAN CARLOS LEAL and José Bonaparte were lost in a thick, thorny acacia forest in north-west Argentina when Leal stumbled on the bones. The big ones turned out to be from a new long-necked dinosaur species. But the 60 small bones they found alongside were puzzling. They were far too small to be from dinosaurs. Intrigued, Bonaparte flew to London to show them to a fossil expert at the Natural History Museum. Cyril Walker immediately recognised them as a rare find – the remains of ancient birds. By this point in the 1970s, some palaeontologists suspected that all birds had evolved from flying dinosaurs, but the idea was not yet mainstream. Looking closely, Walker discovered that the fossil shoulders and feet had grown quite differently to those of modern birds. A key ball-and-socket joint in the shoulder was reversed. This was a whole new avian category, not just a new species. In a short paper published in 1981, he named the fossils Enantiornis leali: Leal’s opposite bird. Now we know that Enantiornis wasn’t an evolutionary oddity. Millions of years ago, the skies were full of such creatures. Then, some 65 million years ago, an asteroid hit Earth and opposite birds were relegated to the history books along with the vast majority of dinosaurs. The only survivors gave rise to every bird alive today. Therein lies the mystery. How did modern birds escape total annihilation?