New research has revealed why it took more than 30 million years for large Triassic dinosaurs to populate the tropics after they first appeared on Earth, ending a mystery that has kept researchers baffled for decades. Using new geological evidence culled from Ghost Ranch, N.M., researchers from the University of Southampton in the U.K. have found that an extremely unpredictable hot and arid climate due to elevated carbon dioxide levels (four to six times of what they are today) kept large herbivorous dinos at bay until after 200 million years ago.