1. Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Campus Box 8208, Raleigh, North Carolina 27695, USA
2. North Carolina Museum of Natural Sciences, 11 West Jones Street, Raleigh, North Carolina 27601-1029, USA
3. Museo de La Plata-CONICET, Paseo del Bosque s/n. La Plata (1900), Argentina
4. Centro de Investigaciones Científicas y TTP- CONICET, Matteri y España, 3105 Diamante, Entre Ríos, Argentina
5. Department of Biological Science, Florida State University, Conradi Building, Dewey Street and Palmetto Drive, Tallahassee, Florida 32306-1100, USA
6. Division of Paleontology, American Museum of Natural History, Central Park West at 79 Street, New York, New York 10024-5192, USA
7. Department of Geology, The Field Museum, 1400 South Lake Shore Drive, Chicago, Illinois 60605, USA
8. High-Resolution X-Ray Computed Tomography Facility, Jackson School of Geosciences, University of Texas at Austin, 1 University Station, C-1100, Austin, Texas 78712-0254, USA

Correspondence to: Julia A. Clarke^1,2 Correspondence and requests for materials should be addressed to J.A.C. (Email: Julia_Clarke@NCSU.edu).

Long-standing controversy^{1, 2, 3, 4, 5, 6, 7, 8, 9} surrounds the question of whether living bird lineages emerged after non-avian dinosaur extinction at the Cretaceous/Tertiary (K/T) boundary^{1, 6} or whether these lineages coexisted with other dinosaurs and passed through this mass extinction event^{2, 3, 4, 5, 7, 8, 9}. Inferences from biogeography^{4, 8} and molecular sequence data^{2, 3, 5, 9} (but see ref. 10) project major avian lineages deep into the Cretaceous period, implying their 'mass survival'³ 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^{1, 6}. However, other fossil data—fragmentary bones referred to extant bird lineages^{11, 12, 13}—have been considered inconclusive^{1, 6, 14}. These data have never been subjected to phylogenetic analysis. Here we identify a rare, partial skeleton from the Maastrichtian of Antarctica¹⁵ as the first Cretaceous fossil definitively placed within the extant bird radiation. Several phylogenetic analyses supported by independent histological data indicate that a new species, Vegavis iaai, is a part of Anseriformes (waterfowl) and is most closely related to Anatidae, which includes true ducks. A minimum of five divergences within Aves before the K/T boundary are inferred from the placement of Vegavis; at least duck, chicken and ratite bird relatives were coextant with non-avian dinosaurs.

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