Triassic

Concise Oxford English Dictionary © 2008 Oxford University Press:
Triassic /trʌɪˈasɪk/

▶adjective Geology relating to or denoting the earliest period of the Mesozoic era (between the Permian and Jurassic periods, about 245 to 208 million years ago), a time when the first dinosaurs, ammonites, and primitive mammals appeared.

– origin C19: from late L. trias (see triad), because the strata are divisible into three groups, + -ic.