the.com/triassic period

means The geologic period, roughly 252 to 201 million years ago, when life slowly recovered after Earth's largest mass extinction and the first dinosaurs appeared.

from Coined in the 1830s by German geologist Friedrich von Alberti, from Latin/Greek 'tri-' meaning three — because in Germany this period's rock record splits neatly into three distinct layers: sandstone, limestone (shell-bearing), and shale/sandstone again. So the name is really just 'the three-layer rock period,' not anything to do with dinosaurs, who simply happen to show up partway through it.