Gallimimus is what is called an ornithomind dinosaur, which is a group is specially adapted lightweight theropods that became the more common dinosaur types toward the end of the Cretaceous period. Of these Gallimius was one of if not the biggest of the group, often credited at being six meters long but with other fossil material indicating a larger size of up to eight meters long. Mongolia seems to have had the right conditions for freak giant dinosaur genera with another example being Gigantoraptor, a giant oviraptorid when compared to other dinosaurs of its kind. To date the only other contenders for an even larger ornithomimids are Beishanlong and Deinocheirus.
Key to the survival strategy of Gallimimus, and other ornithomimids, was speed. The legs especially the lower portions were long and capable of covering a lot of ground with each stride. The bones like in all theropods were hollow in order to reduce the overall weight of the body so that less energy was required to move and faster running speeds could be maintained. The main body however was proportionately longer than other related genera, and this may have had an impact upon how tight Gallimimus could turn. This may not have been so much of a hindrance however as much of Mongolia during the Cretaceous was open arid plain and realistically all Gallimimus would have to do to survive would be to outpace their potential predators.
Like with all ornithomimids, there is no universal agreement upon what Gallimimus actually ate. The overall size of the skull is small in proportion to the body, but the snout is still longer than most other ornithomimids. The jaws were fashioned into a toothless keratinous beak, and without teeth of a certain kind, it is impossible to establish what Gallimimus ate without the evidence of stomach remains to give further clues. One thing that can be established is that the ‘beak’ could have been used to either pick out plant parts, or even pick up small animals. The size of the skull while small was still large enough to pick up small lizards, snakes and even mammals that realistically would have had to be swallowed whole since there were no teeth to process the bodies. Additionally the point of articulation in the jaws was so rigid that Gallimimus could do little more than open and close its mouth. If Gallimimus and other ornithomimids did indeed hunt small animals then they may have killed them by picking them up and throwing them hard against the ground in a similar way to how South American seriema birds do today (which are also seen to being analogous to the phorusrachid terror birds that lived after the dinosaurs). A final dietary option for Gallimimus would be to raid the nests of other dinosaurs for eggs.
Gallimimus also had long arms and fingers, features that could have been used for raking up plants from the ground or perhaps even grasping other things such as eggs so that Gallimimus could run and eat away from the attentions of angry parents.
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