Merriam-Webster Collegiate Dictionary - mesozoic

 adjective  Date: 1840 of, relating to, or being an era of geological history comprising the interval between the Permian and the Tertiary or the corresponding system of rocks that was marked by the presence of dinosaurs, marine and flying reptiles, ammonites, ferns, and gymnosperms and the appearance of angiosperms, mammals, and birds — see geologic time table  • ~ noun