The growing availability of virtual cranial endocasts of extinct and extant vertebrates has fueled the quest for endocranial characters that discriminate between phylogenetic groups and resolve their neural significances. We used geometric morphometrics to compare a phylogenetically and ecologically comprehensive data set of archosaurian endocasts along the deep evolutionary history of modern birds and found that this lineage experienced progressive elevation of encephalisation through several chapters of increased endocranial doming that we demonstrate to result from progenetic developments. Elevated encephalisation associated with progressive size reduction within Maniraptoriformes was secondarily exapted for flight by stem avialans. Within Mesozoic Avialae, endocranial doming increased in at least some Ornithurae, yet remained relatively modest in early Neornithes. During the Paleogene, volant non-neoavian birds retained ancestral levels of endocast doming where a broad neoavian niche diversification experienced heterochronic brain shape radiation, as did non-volant Palaeognathae. We infer comparable developments underlying the establishment of pterosaurian brain shapes.

Department for Earth and Environmental Sciences and GeoBio Center SNSB Bayerische Staatssammlung für Paläontologie und Geologie Ludwig Maximilian University Munich Richard Wagner Str 10 80333 Munich Germany

Department of Biomathematics Institute of Physiology of the Czech Academy of Sciences Vídeňská 1083 142 20 Prague 4 Czech Republic

Department of Zoology and Laboratory of Ornithology Palacký University 17 listopadu 50 77146 Olomouc Czech Republic

European Synchrotron Radiation Facility 71 Avenue des Martyrs CS 40220 38043 Grenoble France

Institute of Biophysics and Informatics 1st Medicine Faculty Charles University Salmovská 1 120 00 Prague 2 Czech Republic

MR Unit Department of Diagnostic and Interventional Radiology Institute for Clinical and Experimental Medicine Vídeňská 1958 9 142 21 Prague 4 Czech Republic

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