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Long-standing controversy^,,,,,,,, surrounds the question of whether living bird lineages emerged after non-avian dinosaur extinction at the Cretaceous/Tertiary (K/T) boundary^, or whether these lineages coexisted with other dinosaurs and passed through this mass extinction event^,,,,,,. Inferences from biogeography^, and molecular sequence data^,,, (but see ref. ) project major avian lineages deep into the Cretaceous period, implying their ‘mass survival’ at the K/T boundary. By contrast, it has been argued that the fossil record refutes this hypothesis, placing a ‘big bang’ of avian radiation only after the end of the Cretaceous^,. However, other fossil data—fragmentary bones referred to extant bird lineages^,,—have been considered inconclusive^,,. These data have never been subjected to phylogenetic analysis. Here we identify a rare, partial skeleton from the Maastrichtian of Antarctica as the first Cretaceous fossil …