The Paleozoic and Mesozoic ERAS are two of the three major divisions of geologic time. The Paleozoic Era, which began about 541 million years ago, is characterized by the emergence of life in the oceans and the colonization of land by plants and animals. The Mesozoic Era, which began about 252 million years ago, is marked by the rise of the dinosaurs and the breakup of the supercontinent Pangaea.