of, denoting, or relating to an era of geological time that began 250 000 000 years ago with the Triassic period and lasted about 185 000 000 years until the end of the Cretaceous period
noun
2. the Mesozoic
Mesozoic in American English
(ˌmɛsəˈzoʊɪk; ˌ mɛzəˈzoʊɪk)
adjective
[sometimesm-]
designating or of the middle geologic era of the Phanerozoic Eon, subdivided into the Triassic, Jurassic, and Cretaceous periods and characterized by the development and extinction of the dinosaurs and the development of the first birds, mammals, and flowering plants