Triassic Period

THE TRIASSIC PERIOD is the geologic time period that extends from about 251 to 199 million years ago. This is the first period of the Mesozoic era, following the Permian and preceding the Jurassic period. Both the start and end of the Triassic are marked by major extinction events. During the Triassic period, both marine and continental life showed an adaptive radiation, beginning from the starkly impoverished biosphere that followed the Permian-Triassic extinction. The first flowering plants may have evolved during the Triassic, as did the first flying vertebrates, the pterosaurs. The Triassic period is further separated into Early, Middle, and Late Triassic epochs.

During the Triassic period, almost all the Earths land mass was concentrated into a single superconti-nent centered more or less on the equator, known as Pangaea. This supercontinent began to rift during the Triassic period but had not yet separated.

The Triassic climate was generally hot and dry, forming typical red bed sandstones and evaporites. There is no evidence of glaciation at or near either pole. The polar regions were moist and temperate—a climate suitable for reptile-like creatures. Pangaeas continental climate was highly seasonal, with very hot summers and cold winters. It probably had strong, cross-equatorial monsoons. The interior of Pangaea was hot and dry during the Triassic period. This may have been one of the hottest times in Earth history. Rapid global warming at the very end of the Permian may have created a super hothouse world that caused the great Permo-Triassic extinction.

The Permian-Triassic extinction event, also known as the Great Dying, was an extinction event that occurred 251.4 mya (million years ago). This was the Earths most severe extinction event, with up to 96 percent of all marine species and 70 percent of all terrestrial vertebrate species becoming extinct. There are several proposed mechanisms for the extinction event, including both catastrophic and gradualistic processes, similar to those theorized for the Cretaceous extinction event. The former include large or multiple impact events, increased volcanism, or sudden release of methane hydrates from the seafloor. The latter include sea-level change, anoxia, and increasing aridity. Evidence that an impact event caused the Cretaceous-Tertiary extinction event has led naturally to speculation that impact may have been the cause of other extinction events, including the Permian-Triassic extinction. Several possible impact craters have been proposed as possible causes of this extinction event, including the Bedout structure off the northwest coast of Australia and the so-called Wilkes Land crater of east Antarctica. In each of these cases, the idea that an impact was responsible has not been proven and has been widely criticized. If impact was a major cause of this extinction event, it is possible or even likely that the crater no longer exists. Seventy percent of the Earth's surface is [Page 981]sea, so an asteroid or comet fragment is over twice as likely to hit sea as to hit land. There is evidence that the oceans became anoxic toward the end of the Permian. There was a noticeable and rapid onset of anoxic deposition in marine sediments around east Greenland near the end of the Permian. The most likely causes of the global warming that drove the anoxic event were a severe anoxic event at the end of the Permian, causing sulphate-reducing bacteria to dominate the oceanic ecosystems and causing massive emissions of hydrogen sulfide, which poisoned plant and animal life on both land and sea. These massive emissions of hydrogen would have severely weakened the ozone layer, exposing much of the life that remained to fatal levels of ultraviolet radiation.

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