Meaning "bird mimic," this extremely fleet ostrich dinosaur was about the size of or slightly larger than today's flightless ratite ostriches. At 15 feet or so in length and over 300 pounds, these Cretaceous omnivores were perhaps the fastest family of dinosaurs ever seen on this planet. Close relatives were Struthiomimus. Ornithomimus had a toothless jaw and what can only be described as a horny bird like beak. Because of their large brain size relative to other dinosaurs and their large eyes, it is thought that they were among the most advanced dinosaurs ever..