- An era of geologic time, from the end of the Precambrian to the beginning of the Mesozoic, spanning the time between 544 and 248 million years ago. The word Paleozoic is from Greek and means "old life."
- the geologic era between the preCambrian and Mesozoic, approximately 200 to 550 millions years ago
Source: Noland, George B. 1983. General Biology, 11th Edition. St. Louis, MO. C. V. Mosby