Gastornithiformes are an extinct order of giant flightless fowl with fossils found in North America, Eurasia, possibly Australia.

Beginning in the late 1980s and the first phylogenetic analysis of gastornithid relationships, consensus began to grow that they were close relatives of the lineage that includes waterfowl and screamers, the Anseriformes.

[4] While the order is generally considered to be monotypic, a 2017 paper concerning the evolution and phylogeny of giant fowl by Worthy and colleagues have found phylogenetic support in finding the mihirungs (Dromornithidae) to be the sister taxon to the Gastornis.

[3] Worthy et al. (2017) incorporated several new taxa and character traits into existing matrices of Galloanserae resulted in several of their phylogenies to support this grouping.

They also found weak support for the Sylviornithidae (another enigmatic family of giant birds from Oceania that also went extinct during the Quaternary) potentially being a sister group to Gastornithidae.