Gastornis measured on average 1.75m tall, while Diatryma was 2m tall. It had an remarkably huge beak with a hook, which may mean that it was carnivorous (although the beak may simply have been used for sexual display).The closest living relatives of Gastornis are the Rallidae family, which includes crakes and moorhens.Gastornis's name means 'Gaston's bird'; it is named after Gaston Planté, who discovered the first fossils at Geiseltal, Germany. Gastornis had large powerful legs, with large, taloned feet, which also were considered in support of the theory that it was a predator. 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. Alternatively, 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 data, 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 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 Middle 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, out-evolving apex predators in sheer bulk as mammals are often able to do (see Cope's "Rule").