The middle era of the three major divisions of the Phanerozoic Eon (Paleozoic, Mesozoic, and Cenozoic eras) of geologic time, encompassing an interval from 251 to 65 million years ago (Ma) based on various isotopic-age dates. The Mesozoic Era is known also as the Age of the Dinosaurs and the interval of middle life. The Mesozoic Erathem (the largest recognized time-stratigraphic unit) encompasses all sedimentary rocks, body and trace fossils of organisms preserved, metamorphic rocks, and intrusive and extrusive igneous rocks formed during the Mesozoic Era.