It may come as a shock to the average dinosaur buff originally designated as a way to keep track of long-extinct reptiles. Rather, these vast stretches of historical time were marked out by geologists to distinguish among various types of geologic strata (chalk, limestone, etc.) laid down tens of millions of years ago. The Mesozoic Era lasted about 180 million years, and is divided into three periods, the Triassic, the Jurassic, and the Cretaceous. Each of these periods is divided into many epochs and ages. Mesozoic means "Middle Animal" and is sometimes called the age of reptiles. The term Mesozoic was coined John Phillips in 1840.The world's life forms were very different during the Mesozoic than either before or after. During the Mesozoic, dinosaurs dominated the Earth. After the Mesozoic, the Cenozoic or the "Age of Mammals" began. Forty percent of the known dinosaurs date from the last 15 million years of the Cretaceous period.