The system of rocks deposited during the middle part of the Mesozoic Era, and encompassing an interval of time between about 200 and 142 million years ago, based on radiometric dating. It takes its name from the Jura Mountains of Switzerland. Its rich marine invertebrate faunas in western Europe have been the subject of intensive study since the pioneering days of geology in the early nineteenth century, and provided the basis for the fundamental stratigraphic concepts of stages and biozones.