As the climate and vegetation on Madagascar changed little over the 145 million years since it became isolated in the Middle Jurassic to Late Cretaceous, the animals that ate the vegetation did not need to change either. The large sauropod dinosaurs have remained practically unaltered over that period of time. The largest is the Madagascan titanosaur that reaches a length of about 18 meters (60 feet) and can reach up to heights of 6 meters (20 feet) to browse from the tops of trees. The body is heavy and supported on stout pillar-like legs. The vertebrae of the neck, body and tail are partially hollowed out to cut down on the weight. The tail is usually carried clear of the ground and ends in a whiplash with which it can defend itself against enemies, such as the megalosaurs. Titanosaurs usually travel through the forests in large family groups, browsing constantly from the trees. The continual chewing of new shoots and buds results in trees of Madagascar having naked trunks rising to a height of about 6 meters (20 feet) before the branches start growing. The food is not chewed but passed down into a voluminous gizzard. There it is ground up by stones that are occasionally swallowed by the animal. From there the food passes into the stomach where it is broken down by bacterial action. The gizzard stones wear away quickly and the worn ones are often vomited up and replaced by fresh ones. The soil of Madagascar is littered with small conical heaps of these rounded, discarded stones. There were many families of sauropod in Jurassic and Cretaceous times but it is now only the titanosaurs that survive, and these are found in South America and Asia as well as in Madagascar (all fragments of the ancient continent of Gondwana).