How the dinosaurs took over

the Mesozoic era like colossi. Literally. The larger sauropods (think , but five times the mass) were the biggest animals ever to walk on land. Even in Mesozoic times, though, it was not always thus. The Mesozoic comprises three periods: the Triassic, the Jurassic and the Cretaceous. Colossus-like bestriding by dinosaurs was restricted to the Jurassic and Cretaceous. So how did it come about?When they first appeared, in the middle of the Triassic, there was nothing obviously special about dinosaurs. Other groups of largish reptiles jostled with them for space on planet Earth. Their supremacy, like that of the mammals which eventually superseded them, was based on surviving a mass extinction that took out the competition. Paul Olsen of Columbia University, in America, and Sha Jingeng of the Nanjing Institute of Geology and Palaeontology, in China, now think they know what gave them their edge when this happened. It was, they write in , an ability to withstand cold.