Perspectives in Ornithology Why Ornithologists Should Care about the Theropod Origin of Birds

OVER THE LAST 25 years, few topics in systematics and paleontology have been as divisively debated as the origin of birds (Padian and Chiappe 1998; Feduccia 1999a, b; Dalton 2000). For most of the middle of the twentieth century, G. Heilmann’s (1926) monumental analysis of the topic reigned unchallenged. Heilmann established definitively that birds are members of the Archosauria (the large reptilian clade that also includes crocodiles, pterosaurs, and dinosaurs). But Heilmann was unable to accept the voluminous evidence he gathered that placed birds within the carnivorous theropod dinosaurs because theropods apparently lacked a furcula, and Heilmann believed that lost features cannot re-evolve (Dollo’s law). Instead, Heilmann (1926) concluded that birds had evolved from ‘‘thecodonts’’—a polyphyletic garbage bag assemblage of early archosaurs. It is a tribute to the careful detail of Heilmann’s treatise that his hypothesis was so influential given his own ambivalence about his conclusions. Heilmann described his thecodont hypothesis as ‘‘wholly without shortcomings’’ (i.e. character conflict) while essentially admitting that there was no evidence other than avian membership in the archosaurs to support it. Heilmann’s hypothesis became unquestioned orthodoxy and was the basis of many scenarios