The Mesozoic and Cenozoic ERAS are two of the three major divisions of geologic time. The Mesozoic Era, which began about 252 million years ago and ended 66 million years ago, is often referred to as the Age of Reptiles. During this period, dinosaurs were the dominant land animals and the first birds appeared. The Cenozoic Era, which began 66 million years ago and continues to the present day, is often referred to as the Age of Mammals. During this period, mammals became the dominant land animals and the first humans appeared.