Triassic Period

Triassic Period, first of three periods that together make up the Mesozoic Era of the geological timescale. It lasted for 42 million years, from about 248 to 206 million years ago. The Triassic Period is preceded by the Permian, which ended with a mass extinction, and succeeded by the Jurassic, marked by a major marine transgression (a rise in sea level, drowning low-lying coastal areas). It was the German geologist Friedrich August von Alberti who, in 1834, first proposed the Triassic. The name alludes to the tripartite division of rocks that can be recognized across much of Europe from this time, in which there is a middle marine succession (the Muschelkalk), sandwiched between upper and lower non-marine sequences (the Bunter and Keuper Sandstones).

By the end of the Palaeozoic the world’s land masses had coalesced to form a single continental block called Pangaea. During the Triassic, tectonic activity increased markedly and this large land mass started to be torn apart into the continental blocks that we have today. Long rifts developed between North America and the African part of Gondwana. As the Earth's crust stretched, large blocks subsided, creating physiographical basins in which thick beds of red-stained sandstone, siltstone, and shale were deposited. A wedge-shaped ocean separating southern Europe from Africa started to open at this time, initiating the great Tethyan seaway.

The climate during the Triassic was relatively warm compared to today, and Pangaea was so large that much of the land area was far from any ocean and as a result became arid. Thick evaporite deposits formed as a result of intermittent marine flooding of marginal fault-bounded basins during the early stages of rifting. These temporary inland seas became highly saline through evaporation in the arid climate, leading to the creation of thick salt deposits which are now of economic importance.

During the Triassic important changes occurred to the biotas living both on land and in the sea. In the early part of the Triassic, following the end Permian extinction, faunas and floras were characteristically low in diversity. However, by the Middle Triassic many of the major groups that dominate modern marine habitats began to appear. The first coral reefs formed by scleractinian corals date from this time, as do sea urchins and starfish of modern aspect. Molluscs of various sorts thrived, including ammonoids (see Ammonite). Top predators in the Triassic seas were the marine reptiles. Both the dolphin-shaped ichthyosaurs and the longer-necked and paddle-finned plesiosaurs first appeared in the Triassic and preyed on fish and cephalopods.

On land the Triassic flora remained broadly similar to that of the preceding Permian Period and was dominated by gymnosperms. Seed ferns, cycads, and ginkgos were common at this time. Along with this flora there were diverse insects, including the first species to undergo complete metamorphosis from larva through pupa to adult. Initially the land vertebrate fauna consisted of reptiles, lizards, and amphibians of various sorts, but in the later Triassic two very important groups evolved: the first mammals, which arose from mammal-like reptile antecedents; and dinosaurs, from Early Triassic thecodont reptiles. The early mammals were all small, none larger than a house cat, but by the end of the Triassic some dinosaurs had reached lengths of more than 6 m (20 ft). It was also at this time that vertebrates first conquered the air, with the evolution of the first pterosaurs, a group of flying reptiles.

Among the best-known Triassic land faunas are those from the Karoo Basin of southern Africa. In the United States, Triassic deposits are almost exclusively non-marine and include, in Arizona, the famous petrified forest. The most renowned of marine deposits are the Middle Triassic Dachstein reefs of southern Europe, which have yielded a rich diversity of marine invertebrates.