Birds are one of the most successful groups of vertebrates. The origin of birds from their reptilian ancestors is traditionally rooted near the Jurassic “Urvogel” Archaeopteryx, an approach that has contributed in defi ning the dichotomy between the “reptilian” (pre- Archaeopteryx) and “avian” (post-Archaeopteryx) phases of what is instead a single evolutionary continuum. A great and still ever increasing amount of evidence from the fossil record has fi lled the gaps between extinct dinosaurs, Mesozoic birds and modern avians, and led to the revision of the misleading dichotomy between pre- and post-Archaeopteryx stages in the evolution of bird biology. Herein, the progressive assembly of the modern avian body plan from the archosaurian ancestral condition is reviewed using a combination of phylogenetic methods. The stem lineage leading to modern birds is described using 38 internodes, which identity a series of progressively less inclusive ancestors of modern birds and their Mesozoic sister taxa. The 160-million-year long assembly of the avian bauplan is subdivided into three main stages on the basis of analyses of skeletal modularity, cladogenetic event timing, divergence rate inference and morphospace occupation. During the first phase (“Huxleyian stage”: Early Triassic to Middle Jurassic), the earliest ancestors of birds acquired postcranial pneumatisation, an obligate bipedal and digitigrade posture, the tridactyl hand and feather-like integument. The second phase (“Ostromian stage”: second half of Jurassic) is characterised by a higher evolutionary rate, the loss of hypercarnivory, the enlargement of the braincase, the dramatic reduction of the caudofemoral module, and the development of true pennaceous feathers. The transition to powered fl ight was achieved only in the third phase (“Marshian stage”: Cretaceous), with the re-organisation of both forelimb and tail as fl ight-adapted organs and the full acquisition of the modern bauplan. Restricting the investigation of the avian evolution to some Jurassic paravians or to the lineages crown-ward from Archaeopteryx ignores the evolutionary causes of over 60% of the features that defi ne the avian body. The majority of the key elements forming the third phase are exaptations of novelties that took place under the diff erent ecological and functional regimes of the Huxleyian and Ostromian stages, and cannot be properly interpreted without making reference to their original historical context.

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