Gallimimus

Gallimimus Temporal range: Late Cretaceous,
Reconstructed skeleton (based on the adult holotype and a juvenile specimen), Natural History Museum, London
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Clade:	†Ornithomimosauria
Family:	†Ornithomimidae
Genus:	†Gallimimus Osmólska et al., 1972
Species:	†G. bullatus
Binomial name
†Gallimimus bullatus Osmólska et al., 1972

Gallimimus (/ˌɡælɪˈmaɪməs/ GAL-i-MY-məs) is a genus of theropod dinosaur that lived in modern day Mongolia during the Late Cretaceous period, about 70 million years ago. Gallimimus was the largest known ornithomimid; typical adults were about 6 metres (20 ft) long, 1.90 metres (6.2 ft) tall at the hip, and weighing about 440 kilograms (970 lb). Like its relative Ornithomimus, it would have had feathers, with adults having wing-like structures on the forelimbs. The neck was proportionally long relative to the trunk, each vertebra featuring hollow depressions for air. The hands had three digits, with curved claws, connected to weak forelimbs that were proportionally the shortest of any ornithomimosaur. In contrast, the hindlimbs were long but lacking a declaw, which was common among ornithomimids. The head was small and light with large, side mounted eyes. The snout was long compared to other ornithomimids, although it was broader and more rounded at the tip than other species. Gallimimus was toothless and had a keratinous beak, with a delicate shovel-like lower jaw, instead.

Several ornithomimid fossils in various stages of growth were discovered by Polish-Mongolian expeditions in the Gobi Desert of Mongolia during the 1960s; a large skeleton discovered in this region was made the holotype specimen of the new genus and species Gallimimus bullatus in 1972. The generic name means "chicken mimic" referring to the similarities between their neck vertebrae and those of the Galliformes. The specific name is derived from bulla, a gold capsule worn by Roman youth, in reference to a bulbous structure on the back of the skull of Gallimimus. At the time it was named, the fossils of Gallimimus represented the most complete and best preserved ornithomimid material yet discovered, and the genus remains one of the best known members of the group. The family Ornithomimidae is part of the group Ornithomimosauria, the "ostrich dinosaurs". Anserimimus, also from Mongolia, is thought to have been the closest relative of Gallimimus.

As an ornithomimid, Gallimimus would have been a fleet animal, using its speed to escape predators. It may have had good vision and intelligence comparable to ratite birds. Gallimimus may have lived in groups, a conclusion based on the discovery of several specimens preserved in bone beds. Various theories have been proposed regarding the diet of Gallimimus and other ornithomimids. The highly mobile neck may have helped locate small prey on the ground, but it may also have been an opportunistic omnivore. It has also been suggested that it used small columnar structures in its beak for filter feeding in water, although it has also been suggested that these structures were used for feeding on tough plant material indicative of a herbivorous diet. Gallimimus is the most commonly found species in the Nemegt Formation, where it lived alongside dinosaurs including other ornithomimosaurs, such as Anserimimus and Deinocheirus.

Description

Size compared to a human

Gallimimus was the largest known member of the family Ornithomimidae; the adult holotype specimen was about 6 metres (20 ft) long, 1.90 metres (6.2 ft) tall at the hip, its skull was 330 millimetres (1.08 ft) long, the femur (thigh bone) was 660 millimetres (2.17 ft) long, and it weighed about 440 kilograms (970 lb). In comparison, a juvenile specimen would have been about 2.15 metres (7.1 ft) long, 0.79 metres (2.6 ft) tall at the hip, and weighed about 26 kilograms (57 lb). Based on fossils of the related Ornithomimus, it is known that ornithomimosaurs were feathered, and that the adults bore wing-like structures on their arms as evidenced by the presence of quill-knobs, bumps that indicate where feathers would have attached, on the lower arm.

Gallimimus had 64-66 vertebrae in its vertebral column, less than other ornithomimids. The centra (or bodies) of the vertebrae were platycoelous, with a flat front surface and a concave hind surface, except for the first six caudal (tail) vertebrae, where the hind surface was also flat, and those at the end of the tail, which were amphiplatyan, with both surfaces flat. The neural arches and centra were separated by sutures, except for barely visible ones within the cervical (neck) vertebrae and caudal vertebrae. Most of the centra had pleurocoels or foramina (hollow depressions), and were therefore air filled), although only the neural arches of the two first vertebrae of the tail appear to have had foramina. The neck consisted of 10 cervical vertebrae, which were all long and wide, except for the atlas bone, the first vertebra that connects with the occipital condyle at the back of the skull. The atlas differed from that of other ornithomimids in that the surface of its intercentrum was slanted downwards towards the back, instead of being concave and facing upwards. The neck appears to have been proportionally longer in relation to the trunk than in other ornithomimids. The cervical vertebrae were divided into two distinct sections; those at the front had centra which were nearly triangular in side view, tapered towards the back, with low neural arches and short, broad zygapophyses (the processes of the vertebrae that articulated between the vertebrae), while the cervical vertebrae at the back had spool-like centra, with neural arches that became gradually higher, and long, thin zygapophyses. The pleurocoels here were small and oval with the neural spines forming long, low, and sharp ridges, except for in the hindmost cervical vertebrae.

Pneumatic structures in the caudal vertebrae of Shenzhousaurus (A), and the cervical (B, C, D), dorsal (E), sacral (F, G), and caudal (H) vertebrae of Gallimimus

The back of Gallimimus had 13 dorsal vertebrae, with spool-like centra that were short, but tended to become deeper and longer towards the back. Their transverse processes (side projecting processes) slightly increased in length towards the back, directed upwards, except for in those at the front. The pleurocoels here were extensive and shallow, though deep in the first two dorsal vertebrae, and the neural spines were prominent being somewhat triangular or rectangular in shape. Grooves between the spines were formed by ligament scars. The sacrum (between the pelvic bones) consisted of 5 fused sacral vertebrae which were about equal in length. The centra here were spool-like, flattened sideways and had deep, elongated pleurocoels (except for the fifth one). The neural spines here were rectangular, broad, and higher than those in the dorsal vertebrae. They were higher or equal in height to the upper margin of the iliac blade and were separate, whereas in other ornithomimids they are fused together. The tail had 36-39 caudal vertebrae with the centra of those at the front being spool-shaped, while those at the back were nearly triangular, and elongated across. The neural spines here were high and flat, but diminished backwards, where they became ridge-like. The chevrons on the underside of the caudal vertebrae were Y-shaped when viewed from the front and back diminishing in size towards the hindquarters. All the vertebrae in front of the sacrum bore ribs except for the atlas and the last dorsal vertebra. The ribs in the neck were fused to the vertebrae only in adults but not in juveniles.

The scapula was thicker near the torso, thinning down as it curved away from the body. It was connected relatively weakly with the coracoid, which was large and deep from top to bottom. Overall, the forelimbs did not differ much from those of other ornithomimids, all of which were comparatively weak. The humerus (upper arm bone), which had a near circular cross-section, was long, twisted, and both articular surfaces were rough. The triangular cross section ulna was long, slender, and curved slightly towards the radius. The olecranon (the projection from the elbow) was prominent in adults, but not well developed in juveniles. The radius was long and slender with a more expanded upper end compared to the lower. The manus (hand) was proportionally short compared to those of other ornithomimids, having the smallest manus to humerus length ratio of any member of the group, but was otherwise similar in structure. It had three fingers, which were similarly developed; the first was the strongest, the third was the weakest, and the second was the longest. The unguals (claw bones) were strong, somewhat curved (that of the first finger was most curved), and compressed laterally with a deep groove on each side. The unguals were similarly developed, though the third was slightly smaller.

Restoration showing an adult with feathers and wings, based on those known from the related Ornithomimus

The pubis (pubic bone) was long and slender with a "foot" at the end, a common feature of ornithomimids. The hind limbs differed little from those of other ornithomimids, which were proportionally longer than in other theropods. The femur was nearly straight, long, slender, with a laterally flattened shaft. The tibia was straight, long, with two well developed condyles on the upper end and a flattened lower end. The fibula of the lower leg was flat, thin and broad at the upper end narrowing towards the lower end. The lower half of the third metatarsal was broad when viewed end on, partly covering the adjoining two metatarsals to each side, but narrowed abruptly at mid-length, wedging between those bones and disappearing (an arctometatarsalian foot structure). The third toe was proportionally shorter than in other ornithomimids. As in other ornithomimids, the foot had no hallux (dewclaw, the first toe of most other theropods). The unguals of the toes were flat on their lower sides; the outer two declined slightly outwards from their digits. The proportional length of the bones in the hind limbs changed very little with growth.

Skull

Reconstructed skull and neck, NHM

The head of Gallimimus was very small and light compared to the vertebral column. Due to the length of its snout, the skull was very long compared to other ornithomimids, and the snout had a gently sloping upper profile, though this was less distinct in juvenile specimens. Seen from the side, the snout differed from other ornithomimids in not narrowing towards its front half, and the lower front margin of the premaxilla at the front of the upper jaw rose upwards, instead of being horizontal. Seen from above, the snout was almost spoon-shaped, broad and rounded at the tip (or U-shaped), whereas it was acute (or V-shaped) in the North American subspecies. The orbits, or eye sockets, were large and, like other ornithomimids, located on the sides of the skull. The temporal region at the side of the skull behind the eyes was deep. The infratemporal fossa (the cavity below the zygomatic arch) was nearly triangular, smaller than that of the related Struthiomimus, and had deep muscle scars at the back part of the skull roof, along the parietal bone. The parasphenoid bone was thin-walled, hollow, and formed a pear-shaped, bulbous structure. The structure opened at the base of the skull, and had a shallow furrow on its lower edge. The third and fourth cranial nerves had a common exit where they merged with the optic fissure. The internal nares (internal openings for the nasal passage) were large and placed far back on the palate, due to the presence of an extensive secondary palate, which was common to ornithomimids.

The delicate lower jaw, consisting of thin bones, was slender and shallow at the front, but deep towards the back. The front of the mandible was shovel-like, resulting in a gap between the tips of the jaws when shut. The shovel-like shape was similar to that of the common seagull, and the lower beak may have had a shape similar to that of this bird. The retroarticular process at the back of the jaw was well developed and consisted mainly of the angular bone. The surangular was the largest bone of the lower jaw, which is usual in theropods. It had an elongated mandibular fenestra (opening on the side) which was comparatively small. The Meckelian groove on the inner side of the dentary bone was deep. The lower jaw did not have a coronoid process or a supradentary bone, a common feature of beaked theropods (ornithomimids, oviraptorosaurs, therizinosaurs, and birds), but unusual among theropods in general. The shape and proportions of the skull changed significantly during growth. The rear of the skull and the orbits decreased in size, whereas the snout became relatively longer, similar changes occur in modern crocodiles. The skull was also proportionally larger in the younger specimens. The jaws of Gallimimus were edentulous (toothless), and would have been covered in a keratinous rhamphotheca (horny beak) in life. The beak may have covered a smaller area than in North American relatives, based on the lack of nourishing foramina (openings) on the maxilla. The inner side of the beak had small, tightly packed, and evenly spaced columnar structures, which were longest at the front and shortening towards the back.

History of discovery

Location of the Nemegt Formation in southern Mongolia

Between 1963 and 1965, the Polish Academy of Sciences and the Mongolian Academy of Sciences organised the Polish-Mongolian palaeontological expeditions to the Gobi Desert of Mongolia. Among the dinosaurs discovered in the Nemegt Basin were abundant skeletal remains of ornithomimids at different growth stages, from the Nemegt, Tsagan Khushu, Altan Ula IV and Naran Bulak localities. Three partially complete skeletons, two with skulls, as well as many fragmentary remains, were collected; the largest skeleton was discovered by palaeontologist Zofia Kielan-Jaworowska. A small skeleton without a skull or forelimbs was also discovered in 1967 by the Mongolian Palaeontological Expedition in Bugeen Tsav outside the Nemegt Basin. The fossils were housed at the Mongolian, Polish, and USSR Academy of Sciences. The Polish-Mongolian expeditions were notable for being led by women, among the first to name new dinosaurs. The fossils discovered in these expeditions shed new light on the interchange of fauna between Asia and North America during the Cretaceous period.

Adult holotype skeleton IGM 100/11 exhibited in Experimentarium, Hellerup

In 1972, palaeontologists Halszka Osmólska, Ewa Roniewicz, and Rinchen Barsbold named the new genus and species Gallimimus bullatus, using the largest collected skeleton (specimen IGM 100/11, formerly referred to as G.I.No.DPS 100/11 and MPD 100/10) as the holotype. The generic name is derived from the Latin gallus, "chicken", and the Greek mimos, "mimic", in reference to the front part of the neck vertebrae which resemble those of the Galliformes. The specific name is derived from the Latin bulla, a gold capsule worn by Roman youth around the neck, in reference to the bulbous capsule on the base of the skull. Such a feature had not been described from other reptiles at the time, and was considered unusual. The holotype, excavated from Tsagan Khushu, consists of a near complete skeleton, except for a distorted snout, incomplete lower jaw, vertebral series, and pelvis as well as some missing hand and foot bones. The other two partially complete skeletons were juveniles; ZPAL MgD-I/1 has a crushed skull missing the tip, damaged vertebrae, fragmented ribs, pectoral girdle and forelimbs, and an incomplete left hind limb while the smallest specimen, IGM 100/11, lacks a pectoral girdle, forelimbs and a number of vertebrae and ribs.

At the time it was named, the fossils of Gallimimus represented the most complete and best preserved ornithomimid material yet discovered, and the genus remains one of the best known members of the group. Ornithomimids were previously known mainly from North America, with Archaeornithomimus being the only prior known member from Asia. Since the first discoveries, additional specimens have been found by Mongolian expeditions accompanied by international colleagues. Three of the skeletons later became part of a travelling exhibit of Mongolian dinosaur fossils, touring various countries. Fossil poaching has become a serious problem in Mongolia in the 21st century, and several Gallimimus specimens have been looted. In 2017, Hang-Jae Lee and colleagues reported a fossil trackway deposited in sandstone associated with a clenched Gallimimus foot in mudstone, extending 20 centimetres (0.66 ft) below the layer with the tracks. The rest of the skeleton appeared to have been removed previously by poachers, along with several other Gallimimus specimens. It is unusual to find tracks closely associated with body fossils.

Three skeletons, including the holotype at right, exhibited in CosmoCaixa, Barcelona

In 1988, the palaeontologist Gregory Paul concluded that the skulls of ornithomimids were more similar to each other than previously thought and moved most species into the same genus, Ornithomimus, resulting in the new combination O. bullatus. In 2010, he instead used the combination Struthiomimus bullatus. The species involved have generally been kept separate by other writers. An ornithomimid vertebra from Japan was informally named "Sanchusaurus" in a 1988 magazine, but was assigned to Gallimimus sp. (of uncertain species) in 1990. Barsbold informally referred to a nearly complete skeleton (IGM 100/14) as "Gallimimus mongoliensis", but since it differs from Gallimimus in some details, Yoshitsugu Kobayashi and Barsbold proposed in 2006 that it probably belongs to a different genus. In 2000, the palaeontologist Philip J. Currie proposed that Anserimimus (which is only known from one skeleton from Mongolia) was a junior synonym of Gallimimus, but this was dismissed by Kobayashi and Barsbold, who pointed out several differences between the two.

Classification

Osmólska and colleagues assigned Gallimimus to the family Ornithomimidae in 1972, with the North American Struthiomimus as the closest relative, while lamenting the fact that comparison between taxa was difficult because other ornithomimids known at the time were either poorly preserved or inadequately described. In 1975, Kielan-Jaworowska stated that though many dinosaurs from Asia were placed in the same families as North American relatives, this category of classification was of much wider range than that used for modern birds. She pointed out that while for example Gallimimus had a rounded beak (similar to a goose or duck), North American ornithomimids had pointed beaks, a difference that would otherwise lead taxonomists to place modern birds in different families. In 1976, Barsbold placed Ornithomimidae in the new group Ornithomimosauria (the "ostrich dinosaurs"). In 2003, Kobayashi and Jun-Chang Lü found that Anserimimus was the sister taxon to Gallimimus, both forming a derived (or "advanced") clade with North American genera, which was confirmed by Kobayashi and Barsbold in 2006.

The following cladogram shows the placement of Gallimimus among Ornithomimidae according to Li Xu and colleagues, 2011:

Reconstruction of the holotype skeleton, Museum of Evolution of Polish Academy of Sciences, Warsaw

Ornithomimidae	Archaeornithomimus unnamed Sinornithomimus unnamed Anserimimus Gallimimus unnamed Qiupalong unnamed Struthiomimus Ornithomimus

Ornithomimosaurs belonged to the group Maniraptoriformes of coelurosaurian theropods, which also includes modern birds. Early ornithomimosaurs had teeth, which were lost in more derived members of the group. In 2004, Makovicky, Kobayashi, and Currie suggested that most of the early evolutionary history of ornithoimisaurs took place in Asia, where most genera have been discovered, including the most basal (or "primitive") taxa, while noting that the presence of the basal Pelecanimimus from Europe complicates the issue. The group must have dispersed once or twice from Asia to North America across Beringia to account for the Late Cretaceous genera found there. As seen in some other dinosaur groups, ornithomimosaurs were largely restricted to Asia and North America after Europe was separated from Asia by the Turgai Straits.

Palaeobiology

Reconstructed skull and neck in front view, NHM

The cervical vertebrae of Gallimimus indicate that it held its neck obliquely, declining upwards at an angle of 35 degrees. Osmólska and colleagues found that the hands of Gallimimus were not prehensile nor capable of grasping, and that the thumb was not opposable. They also suggested that the arms lacked strength when compared to, for example, those of the ornithomimosaur Deinocheirus. They agreed with the ornithomimid biological interpretations of palaeontologist Dale Russell from early 1972, which concluded that they would have been very fleet animals, although less agile than the large, modern ground birds, and would have used their speed to escape predators. He also suggested that they had a good vision and intelligence comparable to that of modern ratite birds. Since their predators may have had colour vision, he suggested their colourisation may have been influenced, possibly resulting in camouflage.

In 1988, Paul suggested that the eyeballs of ornithomimids were flattened and had minimal movement within the sockets, necessitating movement of the head to view objects. The side mounted eyes would have improved their rear vision, a survival adaptation that enabled them to see predators approaching from behind, but binocular vision would have been limited. Paul considered the relatively short tails, which reduced body mass, and missing halluxes of ornithomimids to be adaptations for speed. He suggested that they could have defended themselves by pecking and kicking, but would have mainly relied on their speed for escape. In 2015, Akinobu Watanabe, and colleagues, found that together with Deinocheirus, Gallimimus had the most pneumatised skeleton among ornithomimosaurs. Pneumatisation is advantageous for flight in modern birds, but its function in non-avian dinosaurs is not known with certainty. Proposed functions of pneumatisation have included mass reduction of large bones, helped support high metabolic rates, improved balance at speed, or assisted with thermoregulation.

In 2017, Lee and colleagues suggested various post decay, and fossilisation, causes to explain the discovery of a Gallimimus foot in a trackway. However, the numerous hypotheses lacked supporting evidence from the surrounding environment. One proposal suggested that the area was a mass grave following a Gallimimus die off during a famine or other adverse event. The death of a large group over a short period suggests that Gallimimus may have lived in groups, a behaviour that has been described in other ornithomimids.

Development

Juvenile specimen IGM 100/11 exhibited in CosmoCaixa

A 1987 study by the biologists Roman Pawlicki and P. Bolechała showed age-related differences in the content of calcium and phosphorus of Gallimimus specimens. They found that the ratio was highest in young to middle aged animals, decreasing with age. In 1991, these authors found that the bones of old individuals contained the highest amounts of lead and iron, while the values were lower in younger animals. In 2000, the biologists John M. Rensberger and Mahito Watabe examined the bone histology of various dinosaurs, and found that the canaliculi (channels which connect bone cells) and collagen fibre bundles of Gallimimus and other ornithomimids was more similar to that of birds than mammals, whereas ornithischian dinosaurs were more similar to mammals. These differences may have been related to the process and rate in which bones formed. In 2012 the palaeontologist Darla K. Zelenitsky and colleagues concluded that since adult ornithomimosaurs had wing-like structures on their arms whereas juveniles did not (as evidenced by specimens of Ornithomimus), these structures were originally secondary sexual characteristics, which would have been used for reproductive behaviour such as courtship, display, and brooding.

Feeding and diet

Skull cast of the juvenile specimen ZPAL MgD-I/1, Muséum national d'Histoire naturelle, Paris

Osmólska and colleagues pointed out that the front part of the neck of Gallimimus would have been very mobile (the hind part was more rigid), with the neural arches in the vertebrae of that region being similar to chicken and other Galliformes, indicating similar feeding habits. They found the bill of Gallimimus similar to that of a duck or goose, and that it would have fed on small, living pray which it swallowed whole. The mobility of the neck would have been useful in locating prey on the ground, since the eyes were positioned on the sides of the skull. They found it reasonable to assume that all ornithomimids had similar feeding habits, and pointed out that Russel had compared the bills of ornithomimids with those of insectivorous birds. Osmólska and colleagues suggested that Gallimimus was capable of cranial kinesis, a feature which allows individual bones of the skull to move in relation to each other. They also proposed that it did not use its short handed forelimbs for bringing food to the mouth, but for raking or digging in the ground to access food. The hands of Gallimimus may have been weaker than for example those of Struthiomimus, which may instead have used its hands for hooking and clamping, according to the palaeontologists Elizabeth L. Nicholls and Anthony P. Russell in 1985.

In 1988 Paul disagreed that ornithomimids were omnivores that ate both small animals, eggs, and plants, as had previously been suggested. He pointed out that ostriches and emus are mainly grazers and browsers, and that the skulls of ornithomimids were most similar to those of the extinct moas, which were strong enough to bite off twigs, as evidenced by their gut content. He suggested that ornithomimids were adapted for browsing on tough plants, and would have used their hands to pull down branches within reach of their jaws. The palaeontologist Jørn Hurum suggested in 2001 that due to its similar jaw structure, Gallimimus may have had an opportunistic, omnivorous diet like seagulls. He also observed that the tight intramandibular joint would prevent any movement between the front and rear portions of the lower jaw.

The beak of Gallimimus contained structures which have been compared to the lamellae of for example the Northern shoveller, or the ridges in the beaks of turtles and hadrosaurids

In 2001, the plaeontologists Mark A. Norell, Makovicky, and Currie reported a skull of Gallimimus (IGM 100/1133) and of Ornithomimus preserving soft tissue structures on the beaks. The beak of the Gallimimus had columnar structures which the authors found similar to the lamellae in the beaks of anseriform birds, which use these for manipulating food, straining sediments, filter-feeding by segregating food items from other material, and in cutting plants while grazing. They found the Northern shoveller, which feeds on plants, molluscs, ostracods, and foraminiferans, to be the modern anseriform with the most similar condition to Gallimimus. The authors noted that ornithomimids probably did not use their beaks to prey on large animals, and that they were abundant in mesic environments, while rarer in more arid environments, suggesting that they may have depended on waterborne sources of food. If this interpretation is correct, Gallimimus would have been one of the largest known terrestrial filter-feeders ever.

The palaeontologist Paul Barrett pointed out in 2005 that the lamella-like structures of Gallimimus did not appear to have been flexible bristles like those of filter-feeding birds (as there is no indication of these structures overlapping or being collapsed), but were instead more like the thin, regularly spaced vertical ridges in the beaks of turtles and hadrosaurid dinosaurs. In these animals, such ridges are thought to be associated with herbivorous diets which use them to crop tough vegetation. Barrett suggested that the ridges in the beak of Gallimimus represented a natural cast of the internal surface of the beak, indicating that the animal was a herbivore that fed on material high in fibre. The fact that ornithomimids have been found with gastroliths (gizzard stones) also indicate a herbivorous diet, as these are used to grind food of animals that lack the necessary mastication apparatuses. Barrett also calculated that a 440 kilograms (970 lb) Gallimimus would have needed between 0.07 and 3.34 kilograms (0.15 and 7.36 lb) of food per day (depending on whether its metabolism was ectothermic or endothermic), which he found would be infeasible if it was a filter-feeder. He also found that ornithomimids were not only abundant in formations that represent mesic environments, but also arid environments, and that the ponds and streams in such environments would not provide enough water to sustain filter-feeders. The palaeontologist Espen M. Knutsen concluded in 2007 that the beak shape of ornithomimids in comparison with those of modern birds was consistent with omnivory or high-fibre herbivory.

Palaeoecology

Juvenile skeleton, Mongolian Natural History Museum, Ulaanbaatar

Gallimimus is known from the Nemegt Formation in the Gobi Desert of southern Mongolia. This geologic formation has never been dated radiometrically, but the fauna present in the fossil record indicate it was probably deposited during the early Maastrichtian stage, at the end of the Late Cretaceous about 70 million years ago. The rock facies of the Nemegt Formation suggest the presence of stream and river channels, mudflats, and shallow lakes. Such large river channels and soil deposits are evidence of a far more humid climate than those found in the older Barun Goyot and Djadochta formations. However, caliche deposits indicate at least periodic droughts occurred. Sediment was deposited in the channels and floodplains of large rivers. The environment was similar to the Okavango Delta of present-day Botswana.

The habitats in and around the Nemegt rivers where Gallimimus lived provided a home for a wide array of organisms. Occasional mollusc fossils are found, as well as a variety of other aquatic animals like fish and turtles. Nemegt crocodylomorphs included several species of Shamosuchus. Mammal fossils are rare in the Nemegt Formation, but many birds have been found, including the enantiornithine Gurilynia, the hesperornithiform Judinornis, as well as Teviornis, a possible anseriform. Herbivorous dinosaurs of the Nemegt Formation include ankylosaurids such as Tarchia, the pachycephalosaurian Prenocephale, large hadrosaurids such as Saurolophus and Barsboldia, and sauropods such as Nemegtosaurus, and Opisthocoelicaudia. Predatory theropods that lived alongside Gallimimus include tyrannosauroids such as Tarbosaurus, Alioramus, and Bagaraatan, and troodontids such as Borogovia, Tochisaurus, and Saurornithoides. Theropod groups with both omnivorous and herbivorous members include therizinosaurs, such as Therizinosaurus, oviraptorosaurians, such as Elmisaurus, Nemegtomaia, and Rinchenia. Other ornithomimosaurs include Anserimimus and Deinocheirus, but Gallimimus is the member of the group most commonly found in the Nemegt.

See also

• Timeline of ornithomimosaur research