Gastornis is an extinct genus of large, flightless birds that lived from the middle Paleocene to the middle Eocene epochs of the Paleogene. Most known fossils have been recovered from Europe, although species historically assigned to the genus have also been reported from North America and Asia. Several genera, including the well-studied Diatryma, have traditionally been regarded as junior synonyms of Gastornis. This classification has been challenged by more recent studies, however, and some researchers regard Diatryma as a distinct and valid genus.
Gastornis species were very large birds historically considered to be predators of small mammals, including early, diminutive equids. This interpretation has since been questioned by several lines of evidence. The absence of hooked claws in known Gastornis footprints, together with analyses of its beak morphology and the isotopic signatures of its bones, suggests that it was more likely herbivorous, feeding primarily on tough plant material and seeds. Gastornis is generally considered to be related to the Galloanserae, the clade comprising modern waterfowl and gamebirds.
History
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 () 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 ( ),The biologist's handbook of pronunciations (1960) 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.
Description
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 in maximum height, and weighed up to 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.
Classification
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.Agnolin, F. (2007). "Brontornis burmeisteri Moreno & Mercerat, un Anseriformes (Aves) gigante del Mioceno Medio de Patagonia, Argentina." Revista del Museo Argentino de Ciencias Naturales, n.s. 9, 15–25
Recognizing the apparent close relationship between gastornithids and waterfowl, some researchers classify gastornithids within the anseriform group itself. Others restrict the name Anseriformes only to the crown group formed by all modern species, and label the larger group, including extinct relatives of anseriformes like the gastornithids, with the name Anserimorphae. Gastornithids are therefore sometimes placed in their own order, Gastornithiformes.Buffetaut, E. (2002). "Giant ground birds at the Cretaceous-Tertiary boundary: Extinction or survival?" Special papers – Geological Society of America, 303–306. A 2024 study, however, found little support for Gastornithiformes and instead places Gastornis as a member of the Galliformes crown group, as more closely related to Phasianoidea than to megapodes, being sister to the extinct Sylviornithidae, a recently extinct group of medium-sized flightless birds known from subfossil deposits in the Western Pacific.
A simplified version of the family tree found by Agnolin et al. in 2007 is reproduced below.
As of 2024, at least three species are confidently placed within the genus Gastornis: G. parisiensis (type species), G. russelli and G. laurenti. The type species, Gastornis parisiensis, was named and described by Hébert in two 1855 papers. It is known from fossils found in western and central Europe, dating from the late Paleocene to the early Eocene. Other species previously considered distinct, but which are now considered synonymous with G. parisiensis, include G. edwardsii (Lemoine, 1878) and G. klaasseni (Newton, 1885). Additional European species of Gastornis are G. russelli (Martin, 1992) from the late Paleocene of Berru, France, and G. sarasini (Schaub, 1929) from the early-middle Eocene. The supposed small species G. minor is considered to be a nomen dubium.
Eggshells and a tibiotarsus of Gastornis recovered in southwestern France near Saint-Papoul were originally classified as part of the type species G. parisiensis. In 2020, however, additional specimens from this location were identified as a distinct species and named G. laurenti, making them the most recently described species of Gastornis. The holotype (MHNT.PAL.2018.20.1) is a nearly complete mandible which differs from other species within the genus, and the paratypes consist of the maxilla, right quadrate, femur shaft, tibiotarsus (two left and one right) and six cervical vertebrae. A 2024 study attributed more postcranial remains from the same locality to G. laurenti.
Possible species and synonyms
Gastornis giganteus (Cope, 1876), formerly Diatryma gigantea, dates from the middle Eocene of western North America. Its junior synonyms include Barornis regens (Marsh, 1894) and possibly Omorhamphus storchii (Sinclair, 1928). O. storchii was described based on fossils from lower Eocene rocks of Wyoming. The species was named in honor of T. C. von Storch, who found the fossils remains in Princeton 1927 Expedition. The fossil bones originally described as Omorhamphus storchii are considered to be the remains of a juvenile Gastornis giganteus by Brodkorb (1967), but Louchart et al. (2021) argued that no definitive juvenile specimens of G. giganteus are known and that the two taxa have no known association, so there is no unambiguous evidence to support this synonymy. Specimen YPM PU 13258 from lower Eocene Willwood Formation rocks of Park County, Wyoming also seems to be a juvenile—perhaps also of G. giganteus, in which case it would be an even younger individual.
G. geiselensis, from the middle Eocene of Messel, Germany, has been considered a synonym of G. sarasini; however, other researchers have stated that there is currently insufficient evidence to synonymize the two, and that they should be kept separate at least pending a more detailed comparison of all gastornithids.
In 2024, Gerald Mayr and colleagues argued against the synonymy of Diatryma with Gastornis based on the distinct features of the coracoid and tarsometatarsus of G. giganteus and G. geiselensis, referred to as D. gigantea and D. geiselensis in the paper, when compared to those of G. parisiensis. They further suggested that these two features support the placement of G. sarasini within Diatryma as D. sarasini, and that assigning all species of gastornithiforms to the genus Gastornis would not properly reflect the interrelationships of this taxonomic group. A simplified version of their phylogenetic analysis is reproduced below: