The Triassic period is the first period of the Mesozoic Era. It follows the Permian and precedes the Jurassic Period. It is a geologic period and system that extended from 250 – 200 million years ago. Though major extinction events marked the beginning and end of the Triassic period, the exact dates are not clear.
The Permian-Triassic extinction event that marked the start of the Triassic period impoverished the Earth’s biosphere. However, life recovered slowly in the first half and dinosaurs dominated the world in the second half. The Triassic period saw the evolution of the first true mammals and the first flying vertebrates called the pterosaurs.
The major subdivisions of the Triassic period are as follows:-
• Early Triassic: Induan, Olenekian
• Middle Triassic: Anisian, Ladinian
• Late Triassic: Carnian, Norian, Rharetian
In the Triassic period, the Earth’s land mass was concentrated around the equator in a single continent called the Pangaea. The Tethys sea gulf that entered Pangaea from the east opened further during the mid-Triassic and the Paleo-Tethys Ocean that existed during the Paleozoic began to shrink. As all the deep-ocean sediments of the Triassic have disappeared because of subduction of oceanic plates, there is only little information about the Triassic open ocean.
Further, due to the limited shoreline, the Triassic marine deposits are rare around the world. The Triassic was first studied in Europe because of prominent marine deposits in Western Europe. The Triassic stratigraphy is, therefore, based on organisms like Estheria crustaceans living in lagoons and highly saline waters.
Pangaea, the vast supercontinent that existed till the middle part of the period separated into two landmasses called the Laurasia and Gondwana to the north and south, respectively. The climate was mostly hot and dry during the period. As the separation started gradually happening, the Earth started cooling down.
The hot and dry climate led to the formation of red bed sandstones and evaporites. The polar regions remained moist and temperate, a climate suitable for reptile-like creatures. The size of the Pangaea continent hampered the moderating effect and as result the summers were very hot and winters extremely cold.
During the period, there existed three categories of organisms: those that survived the Permian-Triassic extinction, the new organisms that flourished briefly and those that dominated the Mesozoic world.
The holdover plants on land included the lycophytes, the cycads, the ginkgophyta or Ginkgo biloba as it is called in modern times, and glassopterids. The terrestrial flora was dominated by the seed plants. Conifers flourished in the northern hemisphere. In the early periods of the Triassic, the seed fern glossopteris grew well in the southern hemisphere.
There were amphibians, some reptiles and dicynodonts such as Lystrosaurus at the beginning of the Triassic period. Corals appeared during this time and ammonites recovered following the Permian-Triassic extinction.
In the latter half of the Triassic period, about 220 million years ago, the first true mammals like Eozostrodon appeared. According to scientists, the mammals evolved from the extinct reptiles that resembled mammals. The primitive mammals were small and were nocturnal. Frogs, turtles, lizards, salamanders, and pterosaurs first appeared in the Triassic. Insects underwent complete metamorphosis from larva through pupa to adult. Marine reptiles called ichthyosaurs appeared in the seas.
A mass extinction that was specifically severe in the oceans marked the end of the Triassic period. The conodonts and all the marine reptiles, except ichthyosaurs and plesiosaurs, became extinct. The life of invertebrate species including brachiopods, gastropods, and mollusks was very seriously affected. In the case of terrestrial ecosystems, the event was not equally serious everywhere. However, many species of crurotarsans, large amphibians, groups of small reptiles, etc., disappeared. Though some early dinosaurs went extinct, the ones that adapted more survived to evolve in the Jurassic.
The causes of the extinction during the later periods of the Triassic are not clearly known, but it accompanied enormous volcanic activity. Causes like cooling down of the earth or the impact of a meteoroid are also attributed to the extinction event. The dinosaurs expanded into many areas following these extinctions. They dominated the earth for the next 150 million years.
Coal was absent throughout the world at the beginning of the Triassic period. The coal gap is purportedly due to the Permian-Triassic extinction event and sharp drop in seal water level across the Permian-Triassic boundary.
In more than one respect, the Triassic period can be described as the period of transition.