MESOZOIC ERA  , in

geology, the name given to the period of time between the Palaeozoic and Cainozoic eras; it is synonymous with the older and less satisfactory term " Secondary " as applied to the major divisions of geological time and with the " Flozgebirge " of the Wernerian school . This era is sub-divided into a lower, Triassic, a middle, Jurassic, and an upper, Cretaceous period or epoch . The duration of the Mesozoic era was not more than one fourth of that of the Palaeozoic era, measured by the thickness of strata formed during these periods . It was an era marked by peaceful conditions in the earth's crust and by a general freedom from volcanic activity . The sediments as a whole are characterized by the prevalence of limestones as compared with those of the preceding era; they are seldom much altered or disturbed except in the younger mountain regions . Mammals, represented by small marsupials, and primitive forms of birds and bony fishes make their first appearance in rocks of Mesozoic age . Saurian reptiles played an extremely prominent part; ammonites and belemnites lived in extraordinary variety in the seas along with the echinoids and pelecypods, which had to a great extent supplanted the crinoids and brachiopods of the preceding periods . The first clear indications of monocotyledonous and dicotyledonous angiosperms made their appearance, while Cycads and Conifers constituted the bulk of the land flora .