Gastornis

Gastornis is an extinct genus of large, flightless birds that lived during the mid-Paleocene to mid-Eocene epochs of the Paleogene period. Most fossils have been found in Europe, and some species typically referred to the genus are known from North America and Asia. Several genera, including the well-studied genus Diatryma, have historically been considered junior synonyms of Gastornis. However, this interpretation has been challenged recently, and some researchers currently consider Diatryma to be a valid genus.

Gastornis species were very large birds that were traditionally thought to have been predators of various smaller mammals, such as ancient, diminutive equids. However, several lines of evidence, including the lack of hooked claws (in known Gastornis footprints), studies of their beak structure and isotopic signatures of their bones, have caused scientists to now consider that these birds were probably herbivorous, feeding on tough plant material and seeds. Gastornis is, generally, agreed to be related to the Galloanserae, the group containing waterfowl and gamebirds.

Gastornis was first described in 1855 from a fragmentary skeleton. It was named after Gaston Planté, described as a "studious young man full of zeal", who had discovered the first fossils in clay (Argile Plastique [fr]) formation deposits at Meudon, near Paris. The discovery was notable, due to the large size of the specimens, and because, at the time, Gastornis represented one of the oldest known birds. Additional bones of the first known species, G. parisiensis, were found in the mid-1860s. Somewhat more-complete specimens, then referred to the new species G. edwardsii (now considered a synonym of G. parisiensis), were found a decade later. These specimens, found in the 1870s, formed the basis for a widely- circulated and reproduced skeletal restoration by Lemoine. The skulls of these original Gastornis fossils were unknown, other than nondescript fragments and several bones used in Lemoine's illustration, which turned out to be those of other animals. Thus, this European specimen was long reconstructed as a sort of gigantic "crane-like" bird.

In 1874, the American paleontologist Edward Drinker Cope discovered another fragmentary set of fossils at the Wasatch Formation, New Mexico. Cope considered the fossils to be of a distinct genus and species of giant ground bird; in 1876, he named the remains Diatryma gigantea (/ˌdaɪ.əˈtraɪmə/ DY-ə-TRY-mə), from the Ancient Greek διάτρημα (diatrema), meaning "through a hole", in reference to the large foramina (perforations) that penetrated some of the foot bones. In 1894, a single gastornithid toe bone from New Jersey was described by Cope's "rival" Othniel Charles Marsh, and classified as a new genus and species: Barornis regens. In 1911, it was recognized that this, too, could be considered a junior synonym of Diatryma (and therefore, later, Gastornis). Additional, fragmentary specimens were found in Wyoming in 1911, and assigned (in 1913) to the new species Diatryma ajax (also now considered a synonym of G. giganteus). In 1916, an American Museum of Natural History expedition to the Bighorn Basin (Willwood Formation, Wyoming) found the first nearly-complete skull and skeleton, which was described in 1917 and gave scientists their first clear picture of the bird. Matthew, Granger, and Stein (1917) classified this specimen as yet another new species, Diatryma steini.

After the description of Diatryma, most new European specimens were referred to this genus, instead of Gastornis; however, after the initial discovery of Diatryma, researchers recognized the similarity between the two genera as early as 1884 when Elliott Coues placed Diatryma gigantea under the genus Gastornis as G. giganteus, a synonymy agreed upon by the American Ornithologists' Union in 1886. Further meaningful comparisons between Gastornis and Diatryma were made more difficult by Lemoine's incorrect skeletal illustration, the composite nature of which was not discovered until the early 1980s. Following this, several authors began to recognize a greater degree of similarity between the European and North American birds, often placing both in the same order (†Gastornithiformes) or even family (†Gastornithidae). This newly-realized degree of similarity caused many scientists to, tentatively, accept the animals' synonymy pending a comprehensive review of the anatomy of both genera, in which Gastornis has the taxonomic priority. Some subsequent studies either continued to use the genus Diatryma or argued against the synonymy, since a detailed comparison of type specimens has not been done yet and notable differences can be found in the species originally assigned to Diatryma from the type species of Gastornis.

Gastornis is known from a large amount of fossil remains, but the clearest picture of the bird comes from a few nearly complete specimens of the species G. giganteus. These were generally very large birds, with huge beaks and massive skulls superficially similar to the carnivorous South American "terror birds" (phorusrhacids). The largest known species, G. giganteus would have reached about 2.15 m (7 ft 1 in) in maximum height, and weighed up to 175–225 kg (386–496 lb) in mass.

The skull of G. giganteus was huge compared to the body and powerfully built. The beak was extremely tall and compressed (flattened from side to side). Unlike other species of Gastornis, G. giganteus lacked characteristic grooves and pits on the underlying bone. The 'lip' of the beak was straight, without a raptorial hook as found in the predatory phorusrhacids. The nostrils were small and positioned close to the front of the eyes about midway up the skull. The vertebrae were short and massive, even in the neck. The neck was relatively short, consisting of at least 13 massive vertebrae. The torso was relatively short. The wings were vestigial, with the upper wing-bones small and highly reduced, similar in proportion to the wings of the cassowary. A largely complete skull specimen (GMH XVIII-1178-1958) of G. geiselensis was also described in 2024 after its discovery in 1958. The upper beaks of G. geiselensis show possible sexual dimorphism and are wider than those of G. giganteus and proportionally longer than those of G. laurenti.

Gastornis and its close relatives are classified together in the family Gastornithidae, and were long considered to be members of the order Gruiformes. However, the traditional concept of Gruiformes has since been shown to be an unnatural grouping. Beginning in the late 1980s with 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. A 2007 study showed that gastornithids were a very early-branching group of anseriformes, and formed the sister group to all other members of that lineage.