The Origin of Birds

Gerhard Heilmann's 1926 monograph arguing that birds descend from 'thecodont' archosaurs rather than dinosaurs — on the strength of the theropods' apparently missing clavicles. The book settled the question for nearly fifty years: from 1926 into the 1970s the governing consensus in ornithology and vertebrate paleontology held that birds were not dinosaur descendants. The baseline against which the first Message's dragons-at-the-bird-stage placement is measured.