World of Earth Science | Triassic Period

The Triassic Period, first of the Mesozoic Era's three periods, began about 240 million years ago and lasted for approximately 40 million years. It was preceded by the great Permian-Triassic mass extinction, which destroyed over 90% of living species. This extinction, the worst in Earth's history, was probably caused in part by the merger (late in the late Permian) of all the continental plates into a single huge land mass, Pangaea (pronounced pan-JEE-ah). This destroyed many species by producing a net loss of coastline, while Pangaea's size—one fourth of the Earth's surface—dictated an arid climate over much of its interior. The Triassic was therefore a period of adaptive radiation—the slow filling of vacant ecological niches by species evolved from survivors of the great extinction.

For the most part Pangaea remained geologically stable and volcanically inactive during the Triassic. Erosion proceeded more rapidly than mountain-building. Particles eroded from the Pangaean highlands accumulated in various basins to produce a distinctively Triassic class of reddish sandstones and shales called the red beds. It is not known why the red beds are all red; some geologists argue that the Pangaean climate encouraged iron-concentrating soil bacteria. In the late Triassic, the plates comprising Pangaea began to break up, and continental drift has subsequently distributed the red beds all over the world (North America, South Africa, Europe, Brazil).

Conifers (pine trees) and ferns were common land plants of the Triassic Period. Petrified Triassic conifers, some over 5 ft (1.5 m) across and over 100 ft (30 m) long, are found in Utah.

More than 95% of marine invertebrate species died in the Permian-Triassic extinction. During the Triassic, invertebrates slowly re-evolved diversity. Lobsters and crabs first appeared in this period.

Reptiles increased in number and variety throughout the Triassic Period. Some species took to the sea, evolving into the fish-eating plesiosaurs and ichthyosaurs. The first mammals appeared late in the Triassic Period. These were small and shrew-like, as their descendants would remain until the final elimination of the dinosaurs by an asteroid impact some 120 million years later.

The first dinosaurs also evolved in the late Triassic, but remained unspectacular by modern standards. It was not until the Jurassic Period that the most familiar species (Tyrannosaurus rex, Brontosaurus, etc.) were to evolve.