Palaeozoic names a huge stretch of the Earth’s past, when fish, insects, reptiles, and many kinds of plants first became common in the rocks. You can use it as an adjective (Palaeozoic rock, the Palaeozoic era) or as a noun, the Palaeozoic. Learner dictionaries often date it from about 570 million years ago to about 245 million years ago. Though actually, those round numbers are from older textbooks. Specialists now put the Palaeozoic from a little over 500 million years ago to about 252 million years ago, ending with a great dying of life at the close of the Permian. You do not need the exact million; you need the idea: this is the chapter when many animal groups show up, after earlier, stranger life, and before the age of dinosaurs. That later chapter is the Mesozoic. British spelling keeps ae: Palaeozoic. American spelling is Paleozoic. The capital P is usual when it names the era.

古生代（的）（英式拼法；美式作 Paleozoic）