Einiosaurus dinosaurs. Computer artwork of a herd of plant-eating Einiosaurus roaming the plains in what is now the Two Medicine Formation in northwestern Montana, USA. These horned herbivorous dinosaurs were about 20 feet (6 metres) long, 6 feet (2 metres) tall at the hips and weighed about 2 tonnes. They lived during the Late Cretaceous period, around 65 to 100 million years ago. In the distance is an erupting volcano which will bury this scene beneath a massive ash fall, preserving the remains of the flora a fauna for future examination by humans.