Clarke, J. A., C. P. Tambussi, J. I. Noriega, G. M. Erickson, and R. A. Ketcham. 2005. Definitive fossil evidence for the extant avian radiation in the Cretaceous. Nature 433:305-308.

Abstract

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 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 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.

Slice Movies:

Primary bone-bearing slab - scan 1 (4.0mb)	Primary bone-bearing slab - scan 2 (3.8mb)	Counter slab (6.2mb)
Color animation through reconstruction of realigned datasets (3.7mb)

Additional imagery on the UTCT website

The Vegavis iaai holotype specimen from Vega Island, is a disarticulated partial postcranial skeleton preserved in two halves of a concretion. This specimen from western Antarctica, was discovered in 1992 and received rudimentary preparation that, in fact, degraded delicate bones that were originally exposed. Significant new preparation, X-ray computed tomography (CT) and recovery of latex peels of the specimen before its original preparation reveal numerous, previously unknown bones and anatomical details.

The principal bone-bearing half of the specimen was scanned twice using the high-energy subsystem at the UT CT Facility, using X-ray settings of 420 kV and 4.7 mA. First the specimen was surrounded by garnet sand and scanned horizontally with a slice thickness of 1.0 mm and spacing of 0.5 mm in the principal bone-bearing layer and 1.0 mm below. X-ray settings were 420 kV and 4.7 mA. This protocol provided data relatively free of beam-hardening artifacts but with relatively low resolution. The second scan was done at 0.25 mm slice thickness and spacing with the specimen oriented so that its short axis was parallel to the scan plane. Beam-hardening artifacts, including uneven attenuation values, rings, and streaks, were ameliorated through pre-processing of the raw scan data and post-processing of the reconstructed images. The counter-slab was also scanned with the second protocol.

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Additional imagery on the UTCT website