Gallimimus had 64 to 66 vertebrae in its spine, fewer than other ornithomimids.

Fossil bones from the Nemegt Basin, including those of Gallimimus, are more radioactive than fossils from surrounding areas, possibly because uranium accumulated in the bones, transported there by percolating ground water.

In 1965, a pair of large arms, shoulder girdles, and a few other bones of a new dinosaur were first discovered in the Nemegt Formation of Mongolia. In 1970, this specimen became the holotype of the only species within the genus, Deinocheirus mirificus. The genus name is Greek for "horrible hand".