The first period of the Mesozoic Era is called the Triassic. Its name comes from the distinctive three-fold division of these rocks in central Germany. Red beds in the lower part of the geologic section were overlain by marine limestone, with terrestrial sandstones on top of that. The period was named by geologist Friedrich von Alberti in 1834, and while the Triassic in much of the rest of the world lacks the distinctive triple division, the name is used everywhere.
The subdivisions of Triassic time are simple at the first level, with three epochs – early, middle, and late, but those epochs are not the same length – they represent about 4, 12, and 34 million years, respectively, which adds up to a total length of about 50 million years. The ages, the subdivisions of those epochs, are based on the appearance and disappearance of various faunal packages, and they vary around the world, but the general internationally agreed ages total seven in the whole Triassic. Five of the seven ages were defined and named for localities in the Austrian and Italian Alps, and the other two come from sites in Russia.
The Triassic began with the end of the Permian at 251 million years ago, with the great extinction event. The Siberian basalt flows that started toward the end of the Permian continued into the Triassic, and the period ended at about 201 million years ago with yet another mass extinction event – not as severe as the Permian-Triassic devastation, but bad enough. We’ll talk about it at the end of this month.
—Richard I. Gibson