Gallimimus was ostrich-like, with a small head, toothless beak, large eyes, a long neck, short arms, long legs, and a long tail. It had a short 'hand' relative to the humerus length, when compared to other ornithomimids. The tail was used for balance. The eyes were placed on the sides of its head and the bottom front part of its beak was shaped like a shovel. Like most modern birds and other theropods, it had hollow bones. Gallimimus had a lot of traits that hint that it could run fast, such as a strong ilium, heavy tail base, long limbs, a long tibia and metatarsus and short toes, but no one knows how fast it could run.
Gallimimus was in 1972 assigned to the Ornithomimidae. This is confirmed by recent cladistic analyses.
The first fossil remains of this dinosaur were found in early August 1963 in the Gobi Desert. In 1972, it was named by paleontologists Rinchen Barsbold, Halszka Osmólska, and Ewa Roniewicz. The only named species is the type species Gallimimus bullatus. The name comes from Latin gallus, "chicken", and mimus, "mimic".
A second species announced by Barsbold in 1996, "Gallimimus mongoliensis", 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, as of yet unnamed ornithomimid genus. 
Beak and Paleoecology
The diet of Gallimimus has been controversial. Suggestions include omnivory or herbivory
A beak is present in a fossilized skull of Gallimimus and the presence of ridges on the beak has been interpreted as part of a filtering mechanism similar to ducks. Ornitomimids were common in moist settings, so Gallimimus a may fed water plants and creatures.
In the Media
Gallimimus was briefly shown in Jurassic Park as a stampeding herd that run by some protagonists, and then attacked and one was eaten by a Tyrannosaurus rex, however, in real life, it would probably have been T. rex's close relative Tarbosaurus. Also, in the Carnivores series, they appear as ambient creatures that run away if the player is to close. They also made a few appearances in The Land Before Time series. It's also a playable dinosaur in the game Dinosaur King.
AnatomyIt is likely that Gallimimus was a very intelligent creature, based on mass and proportion to brain. At least quite smart among most dinosaurs. It shared many features with its relatives like Ornithomimus. It is guessed that Gallimimus was an omnivore, similiar to the Oviraptor. Its main diet consisted of small lizards and bugs, some eggs, and some plants. Gallimimus ran on two slender legs. It's predicted to have been ostrich-like, and most likely had feathers, given that feathers were preserved in it's relatives and more massive feathered dinosaurs did exist. It had a long, flat, toothless beak, with the bottom part shaped like a shovel,a dn probably would've been helpful in crunching hard plants like nuts or roots. It weighed about 970 lbs and was about 8 meters (26 feet) in length, making it one of the largest of ornithomimids. It had three claws on each arm, with three on each foot. The tail, like many other dinosaurs, was meant for balance, especially while they were running. Like other ornithomimids, it was a very fast runner, possibly capable of running up to 43 mph, which is about the same as an ostirich's top speed and over twice as fast as Olympic sprinters! Gallimimus also had large eyes, which possibly suggest that it was a nocturnal dinosaur like Troodon.
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