Apatosaurus with pterosaurs. Computer illustration of pterosaurs flying around an Apatosaurus (Brontosaurus) sp. dinosaur in an ancient 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. Pterosaurs are the earliest vertebrates known to have evolved powered flight. They lived from the late Triassic to the end of the Cretaceous Period (around 228 to 66 million years ago).