Ornithomimus was a small, feathered dinosaur. Adults measured no more than 10-13 feet long and weighed about 250-350 pounds. What Ornithomimus lacked in size, it made up for in speed. It's estimated that Ornithomimus could hit a top speed of 35-40 miles an hour. This would have been useful when escaping predators like Dromaeosaurus and Tyrannosaurus. Ornithomimus lived in North America about 65-70 million years ago.