Definitive fossil evidence for the extant avian radiation in the Cretaceous

Long-standing controversy(1-9) 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(2-5,7-9). 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(11-13)-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 Antarctica(15) 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.