Gastornithiformes

Gastornithiformes; Temporal range: Middle Paleocene - middle Eocene PreꞒ Ꞓ O S D C P T J K Pg N Possible Neogene and Pleistocene records
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Aves
Superorder:	Galloanserae
Order:	†Gastornithiformes; Stejneger, 1885
Subgroups
	†Gastornithidae; ?†Brontornis; ?†Dromornithidae; ?†Sylviornithidae;

Gastornithiformes were an extinct order of giant flightless fowl with fossils found in North America, Eurasia, possibly Australia.^[1] Members of Gastornithidae were long considered to be a part of the order Gruiformes. However, the traditional concept of Gruiformes has since been shown to be polyphyletic.

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.^[2] Recognizing the apparent close relationship between Gastornis and waterfowl, some researchers even classify them within the anseriform group itself.^[3] Others restrict the name Anseriformes only to the crown group formed by all modern species, and label the larger group including extinct relatives of anseriformes in the clade Anserimorphae (which this article and related pages have adopted).^[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.^[1]

The mihirungs are also another family of giant flightless birds that have been classified as anserimorphs either as crown anseriforms closely related to the screamers (Anhimidae)^[5] or the sister taxon to Anseriformes.^[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.^[1] The authors did note the bootstrap support is weakly supported and one of their phylogenies even found gastornithiforms to be stem-galliforms instead.^[1] These were also weakly supported.^[1] Below is a simplified phylogeny showing their one phylogeny supporting gastornithiforms as anserimorphs.^[1]

Anserimorphae

Anseriformes (waterfowl)

†Vegaviiformes

Gastornithiformes

	†Gastornithidae

	†Dromornithidae (mihirungs)

However, later studies such as McInerny et al (2024) have instead recovered mihirungs as crown-group anseriformes related to modern screamers. However, McInerny et al instead 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. The lineage containing these two families was found to alternatively represent a group of stem-Galliformes, or a group of crown-group Galliformes more derived than the megapodes but basal to all other members of the group.^[6]

In a 2021 paper by Agnolin found the Argentinian genus Brontornis from the Miocene deposits, normally considered to be a terror bird, found to a gastornithiform sister to the mihirungs.^[7]