| Gastornithids Temporal range: Paleocene-Eocene, 55–45 Ma |
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| Mounted skeleton of a Gastornis | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Aves |
| Order: | Anseriformes |
| Family: | †Gastornithidae Fürbringer, 1888 |
| Genera | |
| Synonyms | |
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Gastornithiformes are an order of prehistoric birds. The birds from this group lived from the Paleocene to the Eocene and were spread out across Asia, Europe, and North America. All the birds were very large birds that were flightless, similar to an ostrich but more heavily built and with a huge beak. They are generally assumed to be predators, but this is conjectural. It is likely though that they included a considerable amount of meat in their diet, although they may have been scavengers rather than active hunters.
There is no agreement on the relationships of the Gastornithiformes. They were long and still are sometimes placed with the Gruiformes as a family Gastornithidae. The Gruiformes seem paraphyletic though, with some lineages that are exclusively Gondwanan but apparently not closely related to cranes, rails and allies which are common in Eurasia and Africa but far less so in the Americas. Some others, probably unrelated to either group, were very diverse in the Americas but prehistorically also occurred in Europe. Nothing is known of the ancestry of the Gastornithiformes; judging from biogeography, a relationship with either the true Gruiformes or the "Americas" lineage (which might include the seriemas and the phorusrhacids) is possible.
However, the early occurrence of the Gastornithiformes in the fossil record poses problems. These animals were highly apomorphic and thus the lineage must have evolved for significant time after diverging from their closest known relatives. Most purported relatives are not known nor suspected to have been so highly distinct at the time when the Gastornithiformes lived.
More recently, most consider the closest living relatives of the Gastornithiformes to be the Anseriformes (waterfowl and screamers). The present birds would thus be members of the fowl clade, Galloanserae. The clade name Anserimorphae has been proposed for the Gastornithiformes and the Anseriformes, as opposed to birds closer to Galliformes. As Galloanserae are known to have reached some diversity in the Late Cretaceous already, this scenario very plausibly explains the extreme adaptations of the Gastornithiformes: they would have evolved for some 15-25 million years at least after diverging from the ancestors of the Anseriformes.
Moreover, in this case the Neogene mihirungs would have recapitulated the gigantism of the Gastornithiformes, though based on a more advanced anseriform (i.e., more "gooselike" as regards details of their osteology) ancestor. This would be an outstanding example of convergent evolution which was very close to being homologous phylogenetically but still a true convergence of phenes in most aspects.
Apart from the genera listed above, there are some indeterminate gastornithiform fossils: