The Jurassic began with the end of the Triassic, at about 201 million years ago, and lasted about 56 million years, until about 145 million years ago. Its three main subdivisions, or epochs, are relatively straightforward as early, middle, and late Jurassic, which span about 27, 11, and 18 million years, respectively. In Europe, the epochs were historically referred to as Lias, Dogger and Malm from the names of packages of rocks, but these informal designations are disappearing because more accurate information shows that, for example, the rocks referred to the Lias were actually partly late Triassic in age as well as early Jurassic. Also, generally, today there’s an attempt to clearly distinguish between periods of time and rock units that can span multiple time periods, so different names are preferred. The epochs of the Jurassic are further subdivided into ages based on particular fossil assemblages, and the ages range from about two to eight million years in length.
The subdivisions of Jurassic time are pretty widely used around the world in part because of the distinctive ammonite fossils that typify each time period. That only works in marine rocks since ammonites lived in the oceans.
—Richard I. Gibson