Yes, scientists believe the climate turned milder in the Jurassic period and lush, tropical vegetation began to grow. This gave rise to new dinosaur groups, including the long-necked sauropods (plant-eaters). The increase in the amount of vegetation allowed these animals to grow quite large. Their long necks allowed these animals to reach food that was out-of-touch for most other dinosaurs. With abundant food supplies, these plant-eating animals continued to grow from generation to generation.