Apatosaurus dinosaur, fossil thigh bone. Previously known as Brontosaurus, this large herbivorous sauropod dinosaur reached weights of 20 to 30 tonnes. The solid pillar-like femurs (thigh bones), as shown here, helped to support this weight. Apatosaurus grew up to 21 metres in length, and lived 150 million years ago. This specimen (1.5 metres long) is part of the collections held at the Natural History Museum, London, UK.