A new clade of diving modern birds from the Cretaceous and Paleogente of the Southern Hemisphere

Abstract

The fossil record of Late Cretaceous-Paleogene modern birds in the Southern Hemisphere is patchy. It includes the Maastrichtian Neogaeornis wetzeli, represented by an isolated tarsometatarsus from Chile, and Polarornis gregorii and Vegavis iaai, from Antarctica, as well as Australornis lovei, from the Paleogene of New Zealand. Phylogenetic relationships of these taxa, as well as diverse isolated materials from Paleogene and Cretaceous beds of Antarctica have been variously interpreted by different authors. The recent finding of a new and nearly complete Vegavis skeleton constitutes the most informative source for anatomical comparisons between Australornis, Polarornis, Vegavis, and other specimens. Present contribution includes for the first time Vegavis, Polarornis, and Australornis in an abarcative phylogenetic analysis. This analysis resulted in the recognition of these taxa as a monophyletic clade at the base of Anseriformes. They share a combination of characters related to diving adaptations, including compact and thickened cortex of hindlimb bones, femur with anteroposteriorly compressed and bowed shaft, deep and wide popliteal fossa delimited by a medial ridge, tibiotarsus showing notably proximally expanded cnemial crests, expanded fibular crest, anteroposterior compression of the tibial shaft, and a tarsometatarsus with a strong transverse compression of the shaft. This evidence, plus isolated bones coming from Cretaceous and Paleogene of South America, Antarctica, and New Zealand, support the view that they constitute a new clade of basal anseriforms. Moreover, they represent the first avian lineage to have confidently crossed the K-Pg boundary, supporting that some avian clades were not affected by the end Mesozoic mass extinction event, countering previous interpretations.