The Triassic Period
In many ways, the Triassic, lasting from 251.0 mya to 199.6 mya,* was a time of transition. It was at this time that the world-continent of Pangea existed, altering global climate and ocean circulation. The Triassic also follows the largest extinction event in the history of life, and so is a time when the survivors of that event spread and recolonized.
The organisms of the Triassic can be considered to belong to one of three groups: holdovers from the Permo-Triassic extinction, new groups which flourished briefly, and new groups which went on to dominate the Mesozoic world. The holdovers included the lycophytes, glossopterids, and dicynodonts. While those that went on to dominate the Mesozoic world include modern conifers, cycadeoids, and the dinosaurs.
Tectonics and paleoclimate
As with almost any other period of the Earth's history, the Triassic had a unique climate and biota indigenous to that time. The paleoclimate was influenced largely by tectonic events that never existed before or since.
At the beginning of the Triassic Period, the land masses of the world were still bound together into the vast supercontinent known as Pangea. Pangea began to break apart in the Middle Triassic, forming Gondwana (South America, Africa, India, Antarctica, and Australia) in the south and Laurasia (North America and Eurasia) in the north. The movement of the two resulting supercontinents was caused by sea floor spreading at the midocean ridge lying at the bottom of the Tethys Sea, the body of water between Gondwana and Laurasia. While Pangea was breaking apart, mountains were forming on the west coast of North America by subduction of the ocean plates beneath the continental plates. Throughout the Middle to Upper Triassic, mountain-forming continued along the coast extending from Alaska to Chile. As mountains were forming in the Americas, North Africa was being split from Europe by the spreading rift. This division of the continents advanced further westward, eventually splitting eastern North America from North Africa.
The climate of the Triassic Period was influenced by Pangea, its centralized position straddling the equator, and the geologic activity associated with its breakup. Generally speaking, the continents were of high elevation compared to sea level, and the sea level did not change drastically during the period. Due to the low sea level, flooding of the continents to form shallow seas did not occur. Much of the inland area was isolated from the cooling and moist effects of the ocean. The result was a globally arid and dry climate, though regions near the coast most likely experienced seasonal monsoons. There were no polar ice caps, and the temperature gradient in the north-south direction is assumed to have been more gradual than present day. The sea level rose as the rift grew between North Africa and southern Europe, resulting in the flooding of Central and South Europe; the climates of terrestrial Europe were hot and dry, as in the Permian. Overall, it appears that the climate included both arid dune environments and moist river and lake habitats with gymnosperm forests.
Some conclusions can be drawn about more specific regional climates and species based on experimental research. The presence of coal-rich sequences in the high northern and southern latitudes, as well as the presence of large amphibians there, indicates that the paleoclimate was wetter in those areas. Living species of some Mesozoic ferns (including the families Osmundacae and Dipteridacae) now live in wet, shady areas under forest canopies, so it is likely that the paleoclimate their Triassic ancestors inhabited were also damp and shaded. The Mesozoic era might also have had large, open areas with low-growing vegetation, including savannas or fern prairie with dry, nutrient poor soil populated by herbaceous plants, such as ferns of the families Matoniaceae and Gleicheniaceae. Thus, despite the union of the continental landmasses, the Triassic vegetation was quite provincial, though this decreased as the Triassic wore on. The northern forests at the beginning of the Triassic were dominated by conifers, ginkgos, cycads, and bennettitaleans, while the forests of Gondwana were dominated by Dicroidium and Thinnfeldia. By the end of the Triassic, both hemispheres gave way to conifer and cycad vegetation.
The Triassic-Jurassic boundary is similar to the Permo-Triassic boundary in that the global climate was not radically altered, though a major extinction of terrestrial vertebrates occurred. With the end of the Triassic and the beginning of the Jurassic, Pangea continued to break apart, inevitably affecting the climate, though not as radically as it had during the Triassic.
Ischigualasto Formation, Argentina: The best-known and best-preserved early dinosaurs come from this Triassic locality in South America.
Resources and references
* Dates from the International Commission on Stratigraphy's International Stratigraphic Chart, 2009.
Brian R. Speer wrote the original text and posted this page, 3/9/1997; the material on tectonics and paleoclimate was added by Manish Asaravala, Hayley Lam, Stephanie Litty, Jason Phillips, and Ting-Ting Wu as part of a Biology 1B project for Section 112 under Brian Speer, 5/1/2000; Sarah Rieboldt updated the pages to reflect the Geological Society of America (GSA) 1999 Geologic Timescale, 11/2002; Dave Smith recombined the content into a single page, adapted it to the new site format and made minor edits, 6/29/2011