1 Theropod Diversity and the Refinement of Avian Characteristics PETER J. MAKOVICKY AND LINDSAY E. ZANNO Field Museum, Chicago, USA Bird origins have been debated ever since Darwin published his “Origin of Species,” and was subse- quently challenged on the topic by Sir Richard Owen, who pointed out the lack of transitional fossil forms in the evolution of the highly derived avian body plan. Indeed, Owen likely carefully selected birds to make his point due to their many unique traits and physiological features such as flight, feathers, bipedality, and a remark- able respiratory system in which the lungs are connected to and ventilated by a complex system of air sacs that pneumatize the skeleton. Within two years of this debate, the discovery of the first specimens of Archaeopteryx provided conclusive evidence of avian evolution in the fossil record and became the focal point for research and delibera- tion on the topic for more than a century. While the fossils of Archaeopteryx provided incontro- vertible evidence for a reptilian origin for birds, opinions varied as to which group of reptiles birds may have originated from. Following the discovery of the small nonavian theropod Compsognathus in the same limestone deposits as Archaeopteryx, Huxley (1868) pre- sciently proposed an evolutionary relationships between birds and nonavian theropods based on shared traits such as three principal, weight- bearing toes in the foot (confirmed by foot-prints), a tall ascending process of the astragalus, and hollow bones. Other contemporary evolutionary biologists such as Cope favored an evolutionary relationship between birds and ornithopod dino- saurs such as hadrosaurs, based again on a three- toed foot and a retroverted pubic shaft. While a variety of ancestors or sister taxa were proposed for birds, a broad consensus that they were related to dinosaurs prevailed until the publication of the English edition of Heilmann’s (1926) “Origin of Birds.” Heilmann’s book presented a detailed study of neontological, embryological, and fossil evidence, all of which pointed to a theropod ancestry for birds. Nevertheless, based on the prevailing assumption of the time that dinosaurs ancestors had lost their clavicles, their reappear- ance in birds would violate Dollo’s (1893) law of irreversibility. Heilmann therefore concluded that the similarities between birds and theropods were due to convergence, and that birds were derived from more basal archosaurs that still retain clavicles. Due to the thoroughness of his book, Heilmann’s (1926) hypothesis held sway for the next four decades until the discovery and descrip- tion of the mid-sized dromaeosaurid theropod Deinonychus by Ostrom (1969). Ostrom’s detailed study of the skeletal anatomy of Deinonychus led him to recognize derived characters shared between it and the basal bird Archaeopteryx (Ostrom, 1976), and to the discovery of a misiden- tified specimen of Archaeopteryx (Ostrom, 1970). Living Dinosaurs: The Evolutionary History of Modern Birds, First Edition. Edited by Gareth Dyke and Gary Kaiser. Ó 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. COPYRIGHTED MATERIAL