Gallimimus (meaning “chicken mimic”) is a genus of ornithomimid theropod dinosaurs from the late Cretaceous period (Maastrichtian stage) Nemegt Formation of Mongolia. With individuals as long as 8 m (26 ft), it was one of the largest ornithomimosaurs. Gallimimus is known from multiple individuals, ranging from juvenile (about 0.5 m tall at the hip) to adult (about 2 m tall at the hip). The type species is G.bullatus, which means “capsuled chicken mimic”.
The first fossil remains of this dinosaur were discovered in early August 1963 by a team of Professor Zofia Kielan-Jaworowska at Tsagan Khushu during a Polish-Mongolian expedition to the Gobi Desert of Mongolia. The find was reported by her in 1965. In 1972, it was named and described by paleontologists Rinchen Barsbold, Halszka Osmólska, and Ewa Roniewicz. The only named species is the type species Gallimimus bullatus. The generic name is derived from Latin gallus, “chicken”, and mimus, “mimic”, in reference to the neural arches of the front neck vertebrae which resemble those of the Galliformes. The specific name is derived from Latin bulla, a magic capsule worn by Roman youth around the neck, in reference to a bulbous swelling in the braincase on the underside of the parasphenoid, in the form of a capsule. The holotype specimen, IGM 100/11, consists of a partial skeleton including the skull and lower jaws. It is a larger skeleton; several other partial skeletons have been described, most of them of juveniles, and numerous single bones.
Gallimimus is the largest known member of the family Ornithomimidae. The adult holotype (specimen IGM 100/11) was about 6 metres (20 ft) long and 1.9 metres (6.2 ft) tall at the hip; its skull was 330 millimetres (1.08 ft) long and the femur (thigh bone) was 660 millimetres (2.17 ft). It would have weighed about 440 kilograms (970 lb). In comparison, one juvenile specimen (ZPAL MgD-I/94) was 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 as evidenced by the presence of quill-knobs on the ulna bone of the lower arm, bumps that indicate where feathers would have attached.
A second species announced by Barsbold in 1996, “Gallimimus mongoliensis” based on specimen IGM 100/14 from the older Bayanshiree Formation, has never been formally referred to this genus. In a reanalysis of the nearly complete skeleton of “Gallimimus mongoliensis” Barsbold concluded in 2006 that it is not a species of Gallimimus but may represent a new, currently unnamed ornithomimid genus.
Gallimimus was assigned to the Ornithomimidae in 1972. This is confirmed by recent cladistic analyses.