{{Taxobox
| name = ''Gastornis''
| fossil_range = 
| image = Gastornis.jpg
| image_width = 220px
|regnum = Állatok ''(Animalia)''
|phylum = Gerinchúrosok ''(Chordata)''
|subphylum = Gerincesek ''(Vertebrata)''
|classis = Madarak ''(Aves)''
| ordo = ''Gastornithiformes''
| familia = ''Gastornithidae''
| genus = '''''Gastornis'''''
| genus_authority = Hébert, 1855
| subdivision_ranks = 
| subdivision =
}}
A '''Gastornis''' a osztályának a rendjéhez, ezen belül a családjához tartozó kihalt nem. A késő paleocén és az eocén idején élt.

== Rendszerezése ==

*''Gastornis parisiensis'' Hébert 1855
*''Gastornis giganteus'' Cope 1876
*''Gastornis ajax''
*''Gastornis russeli''
*''Gastornis sarasini''
*''Gastornis edwardsi''

== Tudnivalók ==
A '''Gastornis''' fajok, nagytestű, röpképtelen madarak voltak. Magasságuk 1.75-2 méter között volt és súlyuk 100 kilogramm is lehetett. Hatalmas csőre arra következtet, hogy a madár vadász és/vagy dögevő volt. De az is lehetséges, hogy a csőre, a párzási időszak alatt mutatványnak szolgált és diókat tört vele, nem húsdarabolásra használta.

{{Taxobox
| name = ''Gastornis''
| fossil_range = Late Paleocene -? middle Eocene
| image = Gastornis.jpg
| image_width = 250px
| image_caption = ''Gastornis'' skeleton
| regnum = Animalia
| phylum = Chordata
| classis = Aves
| subclassis = Neornithes
| infraclassis = Neognathae
| superordo = Galloanserae?
| ordo = †Gastornithiformes
| familia = †Gastornithidae<!-- monotypic - don't link -->
| genus = †'''''Gastornis'''''
| genus_authority = Hébert, 1855<ref>''Fide'' Prévost (1855)</ref>
| subdivision_ranks = Species
| subdivision =
5, see text
| synonyms =
''Barornis'' <small>Marsh, 1894</small><br />
''Diatryma'' <small>Cope, 1876</small><br />
''Omorhamphus'' <small>Sinclair, 1928</small><br />
''Ornithoformipes'' <small>Patterson & Lockley, 2004</small><ref>Ichnotaxon, tentatively placed here. Validity disputed.</ref><br />
''Ornitholithus'' <small>Dughi & Sirugue, 1962<ref>Ichnotaxon, tentatively placed here.</ref></small><br />
and see text
}}

'''''Gastornis''''' (, meaning "Gaston's bird") is an extinct genus of large flightless bird that lived during the late Paleocene and Eocene periods of the Cenozoic. It was named in 1855, after Gaston Planté, who had discovered the first fossils in Argile Plastique formation deposits at Meudon near Paris (France)<ref name = prevost1855>Prévost (1855)</ref>. At that time, Planté (described as a "studious young man full of zeal"<ref name = prevost1855 />) was at the start of his academic career, and his remarkable discovery was soon to be overshadowed by his subsequent achievements in physics.

In the 1870s, the famous American paleontologist Edward Drinker Cope discovered another, more complete set of fossils in North America, and named them ''Diatryma'' (, "DIE-a-TREE-ma"), from Ancient Greek ''διατρέμα''/''diatrêma'' "canoe"<ref>Cope (1976)</ref>.

The fossil remains of these birds have been found in western-central Europe (England, Belgium, France and Germany) as well as North America.

==Taxonomy and systematics==
thumb|left|''Gastornis'' skeleton,  National Museum of Natural History 
The skull of ''Gastornis'' remained unknown except for nondescript fragments, and several bones assigned to it were those of other animals. Thus, the European bird was long reconstructed as a sort of gigantic crane-like ornithuran, very different from the North American species<ref>Lemoine (1881a,b)</ref>. Eventually this was sorted out, and only then it was realized that ''Gastornis'' and ''Diatryma'' were so much alike to make most scientists today consider the latter a junior synonym of the former pending a comprehensive review. Consequently the preferred scientific name is ''Gastornis'', but the genus as a whole is more often called '''diatrymas'''<ref>This should not be capitalized, to indicate it is not the currently valid scientific name.</ref>. In fact, this similarity was recognized as early as 1884 by Elliott Coues, but his reasoning was initially discounted and subsequently ignored until the late 20th century.<ref name = mlikovsky2002>Mlíkovský (2002)</ref>

Diatrymas were variously considered allied with diverse birds, such as waterfowl, ratites or waders; their highly apomorphic anatomy makes reliable assignment to any one group of birds difficult, in particular since no particularly close relatives survive today. In modern times, they were placed with the "Gruiformes" assemblage. But in the 21st century, these birds are most often considered to be Galloanseres. Quite ironically, the original assessment of Hébert - who perceived similarities with the Anseriformes in the original tibia - thus would be far more correct than any later placement. Incidentially, since the Galloanseres are known to originate in the Cretaceous already, it is comfortably explained how such a gigantic bird could be around less than 10 million years after the non-avian dinosaurs became extinct.

The following species are accepted today:
* ''Gastornis parisiensis'' <small>Hébert, 1855</small> - the type species
:Late Paleocene - Early Eocene of WC Europe
:Synonyms: ''Gastornis edwardsii'' <small>Lemoine, 1878</small>; ''G. klaasseni'' <small>Newton, 1885</small>; ''G. pariensis'' <small>(''lapsus'')</small>
* ''Gastornis russeli'' <small>L.Martin, 1992</small>
:Late Paleocene of Berru, France
* ''Gastornis sarasini'' <small>(Schaub, 1929)</small>
:Early Eocene - middle Eocene of WC Europe
:Synonyms: ''Diatryma geiselensis'' <small>Fischer, 1978</small>; ''D. sarasini'' <small>Schaub, 1929</small>
* ''Gastornis ajax'' <small>(Shufeldt, 1913)</small>
:Late Paleocene of Wyoming, USA
:Synonyms: ''Diatryma ajax'' <small>Shufeldt, 1913</small>
* ''Gastornis giganteus'' <small>Cope, 1876</small>
:Early -? middle Eocene of SC North America
:Synonyms: ''Barornis regens'' <small>Marsh, 1894</small>; ''Diatryma gigantea'' <small>Cope, 1876</small>; ''D. steini'' <small>Matthew & Granger, 1917</small>; ''Omorhamphus storchii'' <small>Sinclair, 1928</small>; ''O. storchi'' <small>Wetmore, 1931 (unjustified emendation)</small>

''"Gastornis" minor'' is now in ''below).

==Description==
thumb|''Gastornis'' life restoration 
''Gastornis parisiensis'' measured on average 1.75 metres (5.7 ft) tall, but large individuals grew up to 2 metres (6.6 ft) tall. The diatrymas had a remarkably huge beak with a slighly hooked top, which was taken as evidence suggests that it was carnivorous. ''Gastornis'' had large powerful legs, with large, taloned feet, which also were considered in support of the theory that it was a predator. 

The plumage of ''Gastornis'' is unknown; it is generally depicted with a hair-like covering like in neotenic and their hair-like feathers are actually more like a down and may well have evolved from this rather than from the true pennaceous feathers of adult birds. The assumption that diatrymas had a hair-llike covering is not entirely conjectural however: much smaller flightless birds - such as the Takahē - tend to evolve feathers that are progressively more hair-like, but from pennaceous feathers as these birds are not markedly neotenic. When a bird evolves flightlessness, the advantage of the usual type of plumage is lost, and presumably a covering more like fur is better suited for resisting cold, rain and intense sunlight.

Likewise, the color of ''Gastornis'' plumage is entirely unknown. It is unlikely that these birds would have had particularly gaudy colors, as these would have been a disadvantage, especially if diatrymas were carnivores. Flightless birds of our time generally tend to have more subdued colors than flying relatives - compare the Takahē with the closely related Rifleman. Assuming that the close relationship of the Gastornithiformes to the waterfowl (and a somewhat less close relationship to basal Galliformes such as megapodes) is correct, the general coloration of a diatryma would most probably have been a dull brownish or greyish hue.

==Ecology==
thumb|left|Model of a gaping ''Gastornis sarasini''.
At its time, the environment in which ''Gastornis'' lived in had large portions of dense forest and a moist to semiarid subtropical or even tropical climate. North America and Europe were still rather close, and especially since Greenland probably was covered with lush woodland and grassland then, only narrow straits of a few 100 km at most would have blocked entirely landbound dispersal of the diatrymas' ancestors. While there were large contiguous areas of land in their North American range after the Western Interior Seaway had receded, their European range was an archipelago due to the Alpide orogeny and the high sea levels of the Paleocene-Eocene Thermal Maximum; geographically (but not geologically), it was perhaps roughly similar to today's Indonesia.

Classically, the diatrymas have been depicted as predatory. However, with the size of ''Gastornis''<nowiki>'</nowiki>s legs, the bird would have had to have been more agile to catch fast-moving prey than the fossils suggest it to have been. Consequently, it has been suspected that ''Gastornis'' was an ambush hunter and/or used pack hunting techniques to pursue or ambush prey; if ''Gastornis'' was a predator, it would have certainly needed some other means of hunting prey through the dense forest. 
thumb|''Gastornis'' skull. 
Alternatively, they may have been predominantly scavengers, nuts formed the main food item of diatrymas. Regardless of what these birds ate, the beak may simply have been used for social display - its presence in all known fossils argues against a sexual display role. These contradicting hypotheses, equivocally supported by the material evidence, make the dietary paleobiology of ''Gastornis'' impossible to pinpoint.

Similar gigantic birds of the Cenozoic were the South American terror birds (phorusrhacids) and the Australian mihirungs (Dromornithidae). The former were certainly carnivorous, and the latter are suspected of being predators, too. On the other hand, ratites, the flightless giant birds of our time, feed on plants, small vertebrates, and invertebrates.