Mass extinctions

Cretaceous-Tertiary Extinction

This is the most famous extinction event. After the Jurassic ended, dinosaurs continued to proliferate and evolve throughout the subsequent Cretaceous period. They specialized into the forms which are familiar to many children today. More importantly, it was only during the Cretaceous period that life finally recovered from the much earlier Ordovician-Silurian extinction. The number of species at last matched and then exceeded the number from the Ordovician period, over 300 million years prior, for the first time. The synapsids finally evolved into small, rodent-like creatures, which were the first true mammals. 65 million years ago, a huge meteorite impacted the earth at Chicxulub in modern Mexico, disrupting the atmosphere and causing severe global warming, in turn killing 75% of all species. This meteorite contained a high

concentration of iridium, normally rare on Earth, and all around the world rocks which are 65 million years old show a thin layer of iridium left over from the impact. A few small reptiles and mammals were among the survivors of this extinction. Mammals would go on to replace dinosaurs as the dominant terrestrial animal.

 Carbonif

erous Rainforest Collapse

After the Devonian period came the carboniferous period. A few land animals developed terrestrial eggs, which allowed them to live almost anywhere on land rather than being confined to shores where they could lay their eggs, as turtles still do today. Winged insects appeared and prospered. Sharks enjoyed a golden age and the few trilobites who had survived the last extinction became increasingly rare. Gigantic trees appeared and vast rain forests covered much of the land, increasing the air oxygen content to 35%. For comparison, today 21% of the air is oxygen. Conifers from the Carboniferous period remain almost unchanged today. 305 million years ago, a short sudden ice age caused carbon dioxide levels to become the lowest in the known history of Earth. The great forests died and with them, many of the land animals. Nearly 10% of all the species on Earth disappeared at this time. The trees rotted, condensed, and are now our main source of carbon fuels, after which this period was named.

Permian-Triassic Extinction

After the rain forests fell, the most successful animals left on land were those who laid eggs. These quickly dominated before other species had a chance to recover and they diversified, producing a huge variety of reptiles and dominant synapsids, which were mammal-like reptiles and the ancestors of mammals. 252 million years ago, a disaster occurred which the Earth had never seen before and has never seen since. It was caused by a meteorite impact or volcanic activity which changed the air composition radically. Between 90% and 99% of all life became extinct. This is the biggest mass extinction in history, and is known as the ‘Great Dying’.

For reference, let us look at the extinction of animals caused by humans. During our tenure, high estimates suggest that we have wiped out nearly 1000 species of animal. There are about 8 million species alive today, meaning that even according to the most pessimistic estimates, we have obliterated 0.01% of all animal life. Although this is nothing to be proud of, it is infinitesimal when compared to the gargantuan extinctions nature herself casually puts forward.

Triassic-Jurassic Extinction

After the desolation caused by the end of the Permian period, reptiles again became dominant and the dinosaurs appeared. Dinosaurs were not dominant above other reptiles, and at this stage were not much larger than horses. It was their descendants who became the famous and fearful creatures we know so well. All the larger dinosaurs, tyrannosaurus, stegosaurus, triceratops, and the giant long-necked sauropods, came in the Jurassic or the Cretaceous periods. 205 million years ago, 65% of Triassic life died out, including all the large land animals. Many of the dinosaurs were spared due to their small size. Most mass extinctions last a million years or so, but this one took only ten thousand years. It was likely caused by massive volcano eruptions which disgorged huge amounts of carbon dioxide or sulphur dioxide, resulting in sudden climate change.

End Jurassic Extinction

During the Jurassic period, gigantic sea reptiles such as the famous plesiosaur dominated the oceans. Pterosaurs ruled the skies and dinosaurs ruled the land. Stegosaurus, the long diplodocus, and the great hunter allosaurus became common. Conifers, cycads, ginkgoes, and ferns provided lush forests. Smaller dinosaurs evolved feathers and birds began to appear. 200 million years ago, 20% of life suddenly vanishes from the fossil records, mostly marine species. Shellfish and corals had been widespread, yet they almost completely vanished. The few who survived managed to repopulate the seas gradually over the coming millions of years. This extinction did not greatly affect land animals, and only a few species of dinosaurs were lost. The cause of this almost marine-exclusive extinction is a matter of debate, but one possibility is that the ocean tectonic plates sank slightly and made the oceans deeper. Most marine life was adapted for shallow water, and it perished as it crept further and further away from the surface.