The tropical grasslands, when they developed and spread halfway through the Miocene, were a totally novel environment for the great dinosaurs. Grass is a remarkably tough substance, and a grass-eating animal needs a number of extreme specializations to allow it to survive. Grass is full of hard silica and so it causes a great deal of wear to teeth. A grass-eating animal needs teeth that are constantly growing or frequently replaced. Complex digestive systems are needed to break down and extract nutrients from the substance of grass. On top of all that, a big grassland-living animal needs to have long legs to allow it to run away from danger. In some places, such as the prairies of the North America, non-avian dinosaurs developed quite happily into grass-eating forms. However, on the savannas of the African continent, herbivorous non-avian dinosaurs were not able to establish much of a foothold. They were primarily beaten to it by their cousins, the pterosaurs. A group of pterosaurs abandoned their powers of flight as soon as the tropical grasslands spread, and they became the main grass-eating animals of Africa. The lank is now the most highly specialized of these, and the one that has become most unlike its flying ancestor. The body has remained short but the forelimbs and hind limbs have become long and equal in length. The neck and face are also long; all plains-living features.