Triassic Period
The Triassic Period is the first of three periods in the Mesozoic Era, spanning approximately 251 to 201 million years ago. This period is significant for its role in the recovery and evolution of life following the Permian-Triassic extinction event, where an estimated 95% of species vanished. During the Triassic, the supercontinent Pangaea began to break apart, surrounded by the vast Panthalassa ocean and the emerging Tethys Seaway. This geological shift influenced the evolution of both land and marine organisms.
The Triassic is divided into three epochs: Early, Middle, and Late, each characterized by distinct developments in the fossil record. The flora included gymnosperms and cycads, while the fauna saw the emergence of reptiles, including the early dinosaurs and amphibious species. Notably, cynodonts appeared, which are considered ancestors of modern mammals. The period ended with another mass extinction, likely linked to volcanic activity that altered ocean conditions. The Triassic thus represents a crucial time for the rise of reptiles and the evolutionary groundwork for future ecosystems.
Triassic Period
Introduction
The Triassic period is the first of three periods that make up the Mesozoic era, along with the Jurassic and the Cretaceous. It is noted as the age in which the reptiles rose to dominance following the great Permian (P–Tr) extinction, wherein the fossil record shows that as many as 95 percent of all species on the planet perished. The earth's landforms consisted of the supercontinent Pangaea, which was in the beginning stages of separation into smaller continental landmasses. Crescent-shaped Pangaea was surrounded by Panthalassa (from the Greek meaning “all the seas”), a vast ocean with a large bay area called the Tethys Seaway. During the Triassic, creatures evolved to live on the land, and by the end of the period, the dinosaurs had become the dominant land animals.
Time Defined
The Triassic period began about 251 million years ago and lasted for about 50 million years. It is divided into three epochs, Early, Middle, and Late. The Early Triassic is the shortest of the three, at approximately 5 million years; the Middle Triassic has been estimated at 10 million years; and the Late Triassic lasted for about 35 million years. Each epoch is further subdivided into ages of varying duration, based on the fossil record preserved in the geological strata. Dating of fossils and strata from this period has been carried out through isotopic analysis of their mineral content.
Paleogeography
We owe our understanding of the geological evolution of the earth—specifically, the realization that there was once only the supercontinent of Pangaea—to the theory of continental drift, first proposed by Alfred Wegener. People had long noticed that the coastal outlines of the continents seemed to fit together like the pieces of a planet-sized jigsaw puzzle. Wegener was the first to suggest that the continents were not static but rather moved about on the surface of the planet. The theory was given credence by the alignments of magnetic field signatures that are recorded in the ocean floor's geologic record and proven by the observation of seafloor spreading. Extrapolating the movement of the continents over time led to a concept of the structure of Pangaea and the impact of its breakup on evolution. Geological evidence of the Pangaea supercontinent has also since been discovered. The present-day Pacific Ocean is what remains of Panthalassa.
Major Events
The Triassic was a time of extreme change in the world, both geologically and biologically. During the Late Triassic, when dinosaurs first evolved, Pangaea had only just begun the process of breaking apart, a process that continues today. The breakup of Pangaea began when the region known as the Tethys Seaway was still a large bay-like feature of Panthalassa, emerging between the regions of Pangaea that would become the continental landmasses of Laurasia and Gondwana. It is believed to have been a relatively shallow region of the world ocean, and therefore, rich in sea life. As the Triassic began, various oceanic creatures continued to develop, evolving to produce the reptiles that emerged from the water and colonized the land. Pangaea was already covered with a great variety of plant life and biomes, ranging from thick forests to sparse deserts.
The breakup of Pangaea would change the movement of ocean currents and their effect on global weather patterns, and at the same time, initiate more frequent seismic events, such as volcanoes. Separation of the supercontinent into different regions would also affect the course of evolution of the various creatures that dwelt in them.
Life in the Period
Life during the Triassic underwent great changes at a relatively rapid rate. Plant life on land consisted of specialized gymnosperms such as Pleuromeia and Thinnfeldia, conifers such as Voltzia and the still-extant Auricaria, and cycads such as Aricycas paulae. No flowers existed, however, as the angiosperms (flowering plants) had not yet developed. The oceans and waterways contained great numbers of primitive fish. Numerous ammonoids, bivalves, mollusks, snails, and corals also populated the ocean. Crinoids such as Encrinus and gastropods such as Chemnitzia are also commonly found in Triassic limestone deposits. With the development of reptiles came various turtles that colonized both aquatic and terrestrial environments. Amphibious reptiles, such as crocodiles and the Nothosauria, lived on the margins of land and water.
Reptilia branched into two different evolutionary courses during the Triassic, one branch evolving species that would inhabit the land and the other branch evolving species adapted to an aquatic existence. In the waters, ichthyosaurs (fish lizards) breathed air and gave birth to live young. On land, the reptiles developed into the dinosaur species. By the end of the Triassic period, true dinosaurs such as Plateosaurus and Coelophysis had walked the land, while pterosaur reptiles such as Eudimorphodon took to the air.
Perhaps the most significant development of the Triassic Period, from a present-day point of view, was that of Cynognathus and other cynodonts, which are thought to have been the forerunners of the Mammalia that inherited the earth after the dinosaurs disappeared.
The end of the Triassic Period was marked by another mass extinction event. Massive volcanic erruptions in the ocean realeased significant levels of carbon dioxide that created upheavel, rising oceans and a change in oceanic acidity.
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