The development of reptiles marked the beginning of the Mesozoic period (point to the Mesozoic period on the time line of life), the time when dinosaurs ruled the earth for 150 million years! Reptiles in various forms of dinosaurs took over the land, seas and air. So, you see, the law of nature is working itself. First the trilobites covered the earth in great numbers, now the dinosaurs took over and the trilobites died out. But why didn’t they survive? Trilobites spent time on earth and served specific purpose, it became one of the first kind animals on earth and as earth developed, they could not changed and adapt so not only that they were no longer needed, but they couldn’t find a way to survive. As animals began to move and live on the land, dinosaurs began to grow in great numbers and they were very different creatures from the trilobites.