Megalosaurus dinosaur, illustration. Megalosaurus is a genus of extinct meat-eating dinosaurs, therapods, from the Middle Jurassic period in Earth's history, 166 million years ago. It lived in what is now southern England. The first dinosaur fossil ever found, as long ago as 1676, was probably a fragment of thigh bone that belonged to Megalosaurus. This animal was a medium-sized therapod, 6 to 7 metres long and weighing about one tonne.