The fossil record of Late Cretaceous–Paleogene modern birds in the Southern Hemisphere is fragmentary. It includes Neogaeornis wetzeli from Maastrichtian beds of Chile, Polarornis gregorii and Vegavis iaai from the Maastrichtian of Antarctica, and Australornis lovei from the Paleogene of New Zealand. The phylogenetic relationships of these taxa have been variously interpreted by different authors. In a more recent analysis, Polarornis, Vegavis, Neogaeornis, and Australornis, are including in a new clade: Vegaviidae.
Vegaviids 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.
The recognition of Polarornis, Vegavis, Neogaeornis, Australornis, and a wide array of isolated specimens as belonging to the new clade Vegaviidae reinforces the hypothesis that southern landmasses constituted a center for neornithine diversification, and emphasizes the role of Gondwana for the evolutionary history of Anseriformes and Neornithes.
The most informative source for anatomical comparison among Australornis, Polarornis, Vegavis as well as other southern avian is a recently published Vegavis skeleton (MACN-PV 19.748). Vegavis overlaps with Australornis in the proximal portion of the humerus, proximal part of the coracoid, scapula, and ulna; with Polarornis in the humerus, femur, and proximal end of the tibia; and with Neogaeornis in the tarsometatarsus.
The humerus is probably the most diagnostic element among anseriforms. In Vegavis and Australornis the humerus is notably narrow and medially tilted on its proximal half, and the deltopectoral crest extends for more than one third of the humeral length. The femur is well known both in Vegavis and Polarornis, and share a combination of characters absent in other Mesozoic or Paleogene birds, including strongly anteriorly bowed and anteroposteriorly compressed shaft (especially near its distal end)
Osteohistological analysis of the femur and humerus of V. iaai. shows a highly vascularized fibrolamellar matrix lacking lines of arrested growths, features widespread among modern birds. The femur has some secondary osteons, and shows several porosities, one especially large, posterior to the medullar cavity. The humerus exhibits a predominant fibrolamellar matrix, but in a portion of the anterior and medial sides of the shaft there are a few secondary osteons, some of them connected with Volkman’s canals, and near to these canals, there are a compact coarse cancellous bone (CCCB) with trabeculae. This tissue disposition and morphology suggests that Vegavis had remarkably high growth rates, a physiological adaptation that may be critical for surviving in seasonal climates at high latitudes, and may also constitute the key adaptation that allowed vegaviids to survive the K/T mass extinction event.
Agnolín, F.L., Egli, F.B., Chatterjee, S. et al. Sci Nat (2017) 104: 87. https://doi.org/10.1007/s00114-017-1508-y
Jordi Alexis Garcia Marsà, Federico L. Agnolín & Fernando Novas (2017): Bone microstructure of Vegavis iaai (Aves, Anseriformes) from the Upper Cretaceous of Vega Island, Antarctic Peninsula, Historical Biology, DOI: 10.1080/08912963.2017.1348503