1911 Encyclopædia Britannica/Mesozoic Era

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 “Flözgebirge” of the Wernerian school. This era is subdivided 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.