Ankylosaur, computer artwork. This heavily-armoured dinosaur lived in the early Mesozoic era, in the Jurassic and Cretaceous periods, between about 125 and 65 million years ago. It was a herbivorous dinosaur, and the spiked armour served as protection against carnivorous dinosaurs. Ankylosaurs also possessed a heavy club-like tail, which could be swung at attackers to injure their legs. A full-grown adult could be 6 to 9 metres long, weighing over 6 tonnes..