Artwork showing the relative scale of humans and Gallimimus. Meaning 'chicken mimic', Gallimimus belonged to a diverse group of similar animals called ornithomimids ('bird mimics'). They were long-legged theropod dinosaurs that lived during the Late Cretaceous period in North America. Gallimimus was fairly large but most likely very agile due to its slender build and long legs. It grew to adult lengths of up to 6 m (20 ft) in length and probably had a substantial covering of primitive feathers.