Apatosaurus dinosaurs mating. Computer illustration of Apatosaurus sp. dinosaurs mating in a prehistoric landscape. Apatosaurs were herbivorous sauropod dinosaurs that lived from about 154 to 150 million years ago, during the Jurassic Period, in what is now North America. They reached an average length of around 23 metres and a mass of at least 16 tons. As such, they are one of the largest land animals known to have ever existed.