Vegaviidae

Vegaviidae is an extinct family of ornithurine dinosaurs of uncertain phylogenetic placement, which existed during the Late Cretaceous and possibly the Paleocene. Definitive fossils attributed to the family have only been found in Antarctica, though other fossils from the Southern Hemisphere including Chile and New Zealand may represent this group. The putative Campanian vegaviid from Canada known as Maaqwi, more likely belongs to the Procellariformes.

Previously the genera Neogaeornis and Polarornis were classified as stem-loons based on the similarities in the anatomy of the leg structure. However, there were some criticism to these assertions as the material are from incomplete specimens from Antarctica lacking several important loon characteristics.

In 2017 Agnolín and colleagues performed a phylogenetic analysis of these genera in addition to the newly discovered Australornis and Vegavis, the latter genus of which a more complete specimen had been found. This allowed the team to do anatomical comparisons between these genera. They found support for them making up a family of birds showing specializations to diving, classified as the sister taxon to crown Anseriformes. This was interpreted as evidence that some families of modern birds crossed the K–Pg boundary unaffected by the extinction event that occurred. The authors also stated this is further evidence of Gondwana having an important role for the evolution of modern birds.

Another 2017 paper by Worthy et al. focusing on the evolution and phylogenetic relationships of giant fowl found weak support for Vegaviidae being the sister taxon to Gastornithiformes (the clade including Gastornithidae and the mihirungs). The description and phylogenetic placement of Maaqwi by McLachlan et al. (2017) found an alternative position for vegaviids as stem-birds in the more inclusive clade Ornithurae.

Mayr et al. (2018) did a review of vegaviid systematics stating that while Vegavis and Polarornis are likely sister genera based on overall similarities in their femur and tibiotarsal bones, the inclusion of other taxa, particularly Australornis, is poorly supported. Furthermore, they argued that comparison of the plesiomorphic traits of the pterygoid and the mandible does not seem to firmly establish anseriform or galloanserine affinities for Vegaviidae, and commented that to try to classify all southern hemisphere birds into a single clade is premature as it may not illustrate the complex relationships and the convergent evolution birds have undergone. He also suggested that it is uncertain whether Neogaeornis belongs to the family since the specimen shows traits of other bird groups including those of podicipediform and gaviiform, but if it were true, then both Neogaeornis and possibly the closely related Antarcticavis might represent vegaviids.

Field et al. (2020) considered Vegavis to be either a sister taxon to the Neornithes outside the crown group birds, or as a taxon at the base of Neognathae with an unresolved position. While other researchers have recovered Vegavis as a neornithine, they did not consider it to be a member of Anseriformes or Galloanserae, since they cannot find any diagnostic traits of those clades.