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). At that time, Planté (described as a "studious young man full of zeal") 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 (/ˌdaɪəˈtriːmə/, "DIE-a-TREE-ma"), from Ancient Greek διατρέμα/diatrêma "canoe.
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" minor is now usually in Remiornis again, at least tentatively. "Diatryma" cotei is a large bird of unclear relationships, possibly a gastornithiform, although it does not seem to belong in Gastornis.. "Diatryma filifera" is the name given to fibers erroneously believed to be Gastornis feathers (see below). Finally, some indeterminable gastornithiform remains from the Paleogene of Walbeck (Germany) and the Early Eocene of Park County (USA) may well belong in this genus; see below for more on the latter.
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 Purple Swamphen, the Flightless Cormorant with other cormorants, or the Stephens Island Wren with the 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.
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'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. Alternatively, they may have been predominantly scavengers, omnivores or even herbivores. Indeed, Gastornis's large beak would have been as well suited for crushing seeds and tearing off vegetation. But it seems excessively strong for a purely vegetarian diet, except in the rather improbable case that huge hard-shelled seeds and 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.
The diatrymas were among the largest, if not the largest birds alive during the Paleogene. They had few natural enemies and serious competitors apart from other Gastornis or then-rare large mammals, such as the predatory bear-like Arctocyon of Europe. If these huge birds were active hunters, they must have been important apex predators that dominated the forest ecosystems of North America and Europe until the middle Eocene. The mid-Eocene saw the rise of large creodont and mesonychid predators to ecological prominence in Eurasia and North America; the appearance of these new predators coincides with the decline of Gastornis and its relatives. This was possibly due to an increased tendency of mammalian predators to hunt together in packs (prevalent especially in hyaenodont creodonts). The fact that no birds appear to have ever weighed much more than half a metric ton suggests that they were restricted in their ability to evolve to larger and larger sizes, and thus in their ability to out-evolve apex predators by sheer bulk as mammals are often able to do (see Cope's Rule).
The fossil bones originally described as Omorhamphus storchii are the remains of a juvenile Gastornis giganteus. Specimen YPM PU 13258 from Early Eocene Willwood Formation rocks of Park County, Wyoming also seems to be a juvenile - perhaps of G. giganteus too, in which case it would be an even younger individual.
In Late Paleocene deposits of Spain and Early Eocene deposits of France, shell fragments of huge eggs have turned up, namely in the Provence. These were described as the ichnotaxon Ornitholithus and are presumably from diatrymas. While no direct association exists between Ornitholithus and Gastornis fossils, no other birds of sufficient size are known from that time and place: while the large Diogenornis and Eremopezus are known from the Eocene, the former lived in South America (still separated from North America by the Tethys Ocean then) and the latter is only known from the Late Eocene of North Africa, which also was separated by an (albeit less wide) stretch of the Tethys Ocean from Europe. The mid-Eocene ostrich relative Palaeotis from Europe, on the other hand, was a bustard-sized bird, and the mysterious large Remiornis from France is only known from Paleocene remains.
Some of these fragments were complete enough to reconstruct a size of 24 by 10 cm (about 9.5 by 4 inches) with shells 2.3-2.5 mm (0.09-0.1 in) thick, roughly half again as large as an ostrich egg and very different in shape from the more rounded ratite eggs. If Remiornis is indeed correctly identified as a ratite (which is quite doubtful however), Gastornis remains as the only known animal that could have laid these eggs. It is also notable that at least one species of Remiornis is known to have been smaller than the Gastornis. This would nicely fit the fact that remains of eggs a bit smaller than those of the living Ostrich have also been found in Paleogene deposits of Provence, were it not for the fact that these eggshell fossils also date from the Eocene but no Remiornis bones are known from that time yet.
There is no footprint record which is unequivocally known to be of diatrymas. This may be somewhat surprising, as there is an abundand record of fossilized bird and non-avian dinosaur tracks from the Cretaceous and Paleogene. But most of these are from shorelines or swamps, and thus the lack of Gastornis footprints might indicate that these birds stayed off soft ground, preferring terrain where they had better footing and consequently did not leave many trackways.
However, two mysterious footprint fossilizations are known that might be of diatrymas. One set of footprints was reported from Late Eocene gypsum at Montmorency and other locations of the Paris Basin in the 19th century, from 1859 onwards. Described initially by Jules Desnoyers and later on by Alphonse Milne-Edwards, these ichnofossils were quite celebrated, and most French geologists of the late 19th century knew about them. They were discussed by Charles Lyell in his Elements of Geology as an example of the incompleteness of the fossil record - no bones had been found associated with the footprints. Unfortunately, these fine specimens which sometimes preserved even details of the skin structure are now lost. They were brought to the Muséum national d'histoire naturelle when Desnoyers started to work there, and the last documented record of them deals with their presence in the geology exhibition of the MNHN in 1912. The largest of these footprints, although only consisting of a single toe's impression, was a stunning 40 cm (16 in) long. Interestingly, the large footprints from the Paris Basin could also be divided into huge and "merely" large examples, much like the eggshells from southern France which are 20 million years older.
The other footprint record consists of a single imprint which still exists, though it has proven to be even more controversial. It was found in Late Eocene Puget Group rocks in the Green River valley near Black Diamond, Washington. After its discovery, it raised considerable interest in the Seattle area in May-July 1992, being subject of at least two longer articles in the Seattle Times. Variously declared a hoax or genuine, this apparent impression of a single bird foot measures about 27 cm wide by 32 cm long (11 by 13 in) and lacks a hallux (hind toe); it was described as the ichnotaxon Ornithoformipes controversus. 14 years after the initial discovery, the debate about the find's authenticity was still unresolved. The specimen is now at Western Washington University.
The problem with these trace fossils is that no fossil of Gastornis has been found to be younger than about 45 million years. The enigmatic "Diatryma" cotei is known from remains almost as old as the Paris basin footprints (whose date never could be accurately determined), but in North America the fossil record of unequivocal diatrymas seems to end even earlier than in Europe.
Sometimes, diatrymas have been placed in an Ice Age setting. As noted above, this is quite wrong. It may be that authors confused Gastronis with the phorusrhacids, superficially similar giant predatory birds. Some of these indeed lived during the early Quaternary glaciation.