Mantellisaurus dinosaur trackway, illustration. These herbivorous ornithopod dinosaurs lived about 125 million years ago in the Early Cretaceous. It was formerly considered an Iguanodon. The dinosaurs are seen making tracks in mud or sand. Fossilised dinosaur footprints are an example of fossil trackways, a type of trace fossil. Such fossils can be used to study the behaviour of the animals that left them.