The Outback interior of the Australian continent is very dry. Almost two-thirds of its area is desert or dry grassland. It is a harsh environment for living things, but not an impossible one. Many animals exist here but the only large one is the gwanna. It is the last survivor of the rhabdodontids, as well as the last of the non-hadrosauroid iguanodonts. These dinosaurs (iguanodonts in general dating back to the Middle Jurassic) are closely related to the basal ornithopods, of which are now found almost all over the world, but the non-hadrosauroid iguanodonts tended to be much larger and had more cheek teeth. On all the other continents of the world the non-hadrosauroid iguanodonts were eventually replaced by the more versatile hadrosaurs, but in Australia they survived in isolation as the hadrosaurs never reached that landmass. The larger number of cheek teeth meant that the non-hadrosauroid iguanodonts were in a better position than the basal ornithopods to evolve into grass-eaters when grassy plains spread in the Miocene. The grasses of the region are not particularly nutritious and a large animal has to range further to find enough to eat. The gwanna lives in small family groups which can move quickly from one area to another seeking fresh pastures. The physical build of the original non-hadrosauroid iguanodont was quite suitable for this way of life and it has not changed dramatically. At rest, the gwanna is on all fours, with its head near the ground. When moving, it is a bipedal animal. Its longer hind legs can bear its full weight, and its body is balanced by the heavy tail as it walks or runs across the open landscape.