The Triassic period stands out in Earth’s history as the time when dinosaurs first evolved. It was followed by the Jurassic and Cretaceous periods – at the end of the latter, the dinosaurs were wiped out in a mass extinction event along with the majority of all other life.
As a period of geologic time, the boundaries of the Triassic are defined on the basis of rocks and the fossil record. It was the German geologist Friedrich August von Alberti who first marked the Triassic out. He discovered in 1834 that rocks throughout Europe of a certain age contained a unique fossil fauna, bookended with layers of rock that contained far fewer fossils, which clearly signified extinctions.
When did the Triassic period start?
The Triassic began 252 million years ago and ended 201 million years ago. The period of time before the Triassic was called the Permian. This was a time when a wide variety of animals lived, including a group of animals called the synapsids, which would later evolve into mammals. One member of this group was a large, sail-backed animal called dimetrodon, which looks like it could be a dinosaur but isn’t.
Then 252 million years ago came the Permian-Triassic extinction event. This is the biggest extinction event our planet has ever seen, in which 70 per cent of species on land disappeared along with 95 per cent of those in the oceans. This not only marked the end of the Permian period and the start of the Triassic, it was such a serious catastrophe that it is used as the marker of the end of a geologic era, the palaeozoic era.
Which dinosaurs lived during the Triassic period?
Out of the ashes of this extinction, several new groups of reptiles began to evolve in the Triassic. One particularly successful group was called the pseudosuchians. This is the lineage to which present-day crocodiles and alligators belong. They are a paltry bunch today, about 25 species all told. But back in the Triassic, there were scores of them, including armoured ones that ate plants, toothless omnivores that sprinted on their hind legs and apex predators called rauisuchians that were 9 metres from nose to tail and had teeth like steak knives.
Alongside these terrestrial vertebrates, the first dinosaurs began to appear. They were mostly much smaller and meeker creatures, many not much bigger than a cow. One example is Tanystropheus, which had an extraordinary neck that was twice as long as its body. Another is Coelophysis, a predator that looked a bit like a miniature T-rex.
Pangaea
When these animals lived, Earth’s land was all mushed together into one big C-shaped supercontinent called Pangaea. Around the back of the “C” was a vast ocean called Panthalassa and the expanse of water nestled in its curve is called the Tethys Sea.
The interior of Pangaea would have been an intensely hot and inhospitable desert. It’s thought that the dinosaurs mostly stuck to its coastal edges while the pseudosuchians were better able to endure the inner regions.
How did the Triassic differ from the Jurassic?
The pseudosuchians remained the dominant group for the whole of the Triassic, with the dinosaurs skulking in the shadows. It was only in the next geologic period, the Jurassic, that the powerful pseudosuchians died back and the dinosaurs really took over, evolving into huge animals like the stegosaurus and the long-necked diplodocus. It’s always been a major mystery how the dinosaurs went from pipsqueaks to rulers.
The end Triassic extinction event
About 200 million years ago, during a period known as the Triassic-Jurassic boundary, Pangaea started to crack. North America separated from Europe; South America from Africa. As they parted, Earth haemorrhaged lava from volcanoes in what is now the bottom of the Atlantic Ocean. Released with the lava were huge volumes of carbon dioxide and sulphur dioxide. These gases continually warmed the atmosphere, while bursts of ash from the eruptions shut out the sun’s light.
As you might expect, this change in global climate killed off a huge number of species and decimated the pseudosuchian populations. Dinosaurs, somehow, seem to have emerged from this time relatively unscathed and gone on to prosper in the gaps left by the ancient crocs. Exactly how they managed still remains unclear, though recent evidence points to their unusual lungs as a clinching factor.
Aucun commentaire:
Enregistrer un commentaire