Triassic Period—251.9 to 201.3 MYA

small footprint in stone
Triassic age trace fossil of a horseshoe crab (Kouphichnium isp.), Petrified Forest National Park, Arizona.

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In 1834 Fredrich von Alberti, a longtime official in the German salt-mining industry, introduced “Triassic” as a descriptive term for a sequence of rocks within a striking threefold division: red beds, topped by chalk, followed by black shale. The term is thus essentially descriptive. In the type area of southern Germany, the strata are widely traceable but poorly fossiliferous. In the Alps to the south, however, a complete sequence of marine faunas provides the standard of reference for worldwide correlation today (Eicher 1976).

Significant Triassic Events

The Mesozoic Era begins with the Triassic Period. This era is popularly known as the “Age of Reptiles” and for good reason: reptiles, and particularly dinosaurs, were the dominant land-dwelling vertebrate animals at the time. During the Triassic, the first dinosaurs walked on the land, the first pterosaurs sailed through the skies, and the first ichthyosaurs and plesiosaurs swam in the oceans. The lineage of many modern-day reptiles began in the Triassic Period, including crocodiles, lizards, and turtles. Other now-extinct reptiles first appeared during the Triassic: the fish-like ichthyosaurs and plesiosaurs with their long limbs and necks and small heads. On the tectonic side of things, Pangaea began to break apart, a process that continued into the Cenozoic Era. The Triassic ended with the world’s fourth, but not last, mass extinction.

Learn more about events during the Triassic Period

Reptiles were probably derived from an amphibious ancestor during the Pennsylvanian Period. Once established they underwent several evolutionary bursts in which diverse kinds of reptiles occupied a variety of habitats. Among the most spectacular were the terrestrial dinosaurs, see Jurassic Period—the Golden Age of Dinosaurs.

The marine plesiosaurs were formidable hunters of Mesozoic fish. Their limbs were modified into efficient paddles, perfectly adapted for sculling through the water. Even more streamlined for marine life were the ichthyosaurs (“fish lizards”), which as their name implies, include remarkably fish-like species, though perhaps the better analogy would be with the porpoises, which may fill a similar role in modern seas to that of the ichthyosaurs (Fortey 1991).

Though pterosaurs (flying reptiles) had their origin in the Triassic, they became widespread and varied during the following Jurassic and Cretaceous periods. The pterosaurs had membranous wings like present-day bats. Unlike bats, however, some of them grew to enormous size, and the front support for the wing was provided by only one extremely long, extended finger.
Pangaea began to break apart during the Triassic but dispersed mostly during the Jurassic. Just as the formation of Pangaea influenced geologic and biologic events during the Paleozoic Era, the breakup of this supercontinent profoundly affected geologic and biologic events during the Mesozoic Era. The movement of continents affected ocean circulation and climatic regimes. Populations became isolated and were brought into contact with other populations, leading to evolutionary changes in the biota.

The breakup of Pangaea can be separated into three phases. The first phase formed two resultant continents: Gondwana (South America, Africa, India, Antarctica, and Australia) in the south and Laurasia (North America and Eurasia) in the north. When Pangaea broke up, the re-formed Gondwana continent was not precisely the same as before Pangaea formed; for example, areas that had been part of Gondwana remained attached to North America, such as the land that became Florida.

Seafloor spreading at divergent plate boundaries along the mid-ocean ridge of the Tethys Ocean, the body of water between Gondwana and Laurasia, caused the movement of the two continents. Moreover, rifting separated North America (part of Laurasia) and Africa (part of Gondwana), which were sutured together at the time, initiating the opening of the Atlantic Ocean, continuing westward into the Gulf of Mexico. The rifting was accompanied by igneous activity along the new margins, producing features like the Palisades of New Jersey and New York. The rifting also caused the Tethys Ocean to close, though the final closing did not occur until about 35 million years ago when India collided with Asia, forming the Himalayan orogeny.

The second, major phase in the break-up of Pangaea began in the Early Cretaceous, when Gondwana separated into four continents (Africa, South America, India, and Antarctica/Australia). The third major and final phase of the breakup of Pangaea occurred in the Paleogene Period. North America/Greenland broke free from Eurasia, and Australia split from Antarctica and moved northward.
Mass extinctions are major losses of biota, typically marked by the loss of 10% or more families and 40% or more species, in a geologically short time. By comparison to the preceding Permian extinction event, the Triassic extinction may not seem to be “massive.” However, 23% of families disappeared from both marine and terrestrial environments, qualifying the Triassic extinction as a “mass extinction.”

By comparison to the following mass extinction at the end of the Cretaceous—which was, geologically speaking, instantaneous—the Triassic extinction may seem to have taken a long time. Organisms such as certain groups of cephalopods began dying out during the mid-Triassic; others like the conodonts disappeared during the Late Triassic; still others such as certain gastropods (e.g., Murchisoniaceae) and some groups of reptiles (e.g., aetosaurs and phytosaurs) became extinct at the TriassicJurassic boundary. Hence, the Triassic extinction may in fact be a cluster of smaller extinction events that occurred throughout this geologic period.

Visit—Triassic Parks

Every park contains some slice of geologic time. Here we highlight a few parks associated with the Triassic Period. This is not to say that a particular park has only rocks from the specified period. Rather, rocks in selected parks exemplify a certain event or preserve fossils or rocks from a certain geologic age.

More about the Mesozoic

Part of a series of articles titled Geologic Time Periods in the Mesozoic Era.

Arches National Park, Canyon de Chelly National Monument, Canyonlands National Park, Capitol Reef National Park, Colorado National Monument, Dinosaur National Monument, Glen Canyon National Recreation Area, Petrified Forest National Park, Wrangell - St Elias National Park & Preserve, Wupatki National Monument, Zion National Park more »

Last updated: April 28, 2023