If valid, this would mean that there are three groups of giant basal anseriformes, in chronological order of divergence first the gastornids (relatives of Gastornis) then Brontornis and finally the mihirungs of Australia.
In the earlier part of the Cenozoic, the world was dominated by the gastornid birds, terrestrial crocodiles like Pristichampsus, and a handful of primitive large mammal groups like uintatheres, mesonychids, and pantodonts.