THE FIVE MAJOR EXTINCTIONS
The Mesozoic era ended in what is almost certainly the most notable catastrophe ever, but the Cretaceous-Tertiary extinction was in fact only the most recent of the five major extinction-level events in Earth’s paleo-history.
First was the Ordovician-Silurian event; up until the Silurian period the only life on Earth lived in the extremely biodiverse seas of the Cambrian and Ordovician, primarily arthropods, but one animal called mikomingia. Mikomingia had no fins, teeth or eyes and was about the size of your thumbnail, but it had a backbone, making it the very first fish, and the earliest vertebrate. During the Ordovician, the seas had much more variety than in the Cambrian and one life form which would lead to the first major extinction. During this period the first plants started growing on land who performed photosynthesis at a remarkable rate which leads the majority of scientists to believe that the great dying happened as a result of the oxidising of the atmosphere. The Silurian period brought great change to the planet, obviously the fish and cephalopods still thrived but the arthropods, mainly land-based, had shrunken to animals recognisable today. Amphibians had also evolved and headed onto land.
The second of the major events was one of the smallest and happened midway through the Devonian period. There is much debate surrounding what caused this extinction because there were lots of possible causes at the time, for instance; asteroid storms, super-volcanoes and the movement of the tectonic plates could all have had a part to play in the Devonian event.
After the Devonian period came, the Carboniferous period came about, whilst not connected to any major extinction level events I still thought it was worth mentioning. During the bulk of the Carboniferous period the oxygen content of the Earth’s atmosphere was over 60% above what it is today, as a result of the extreme climate, plants like ferns and arthropods were able to sustain much larger bodies and the Earth was covered in swampy forests. Then you could’ve found spiders and scorpions the size of your head or, most infamously, creatures like Meganeura and Arthropleura. Meganeura would be recognisable today as a dragonfly, except with a wingspan of over a metre. Arthropleura, on the other hand, was an early ancestor of the millipede which, if it were to stand upright (or as upright as millipede can stand), would be over 2 metres tall.
The largest of the extinction events ended the Palaeozoic era and caused over 90% of animal life to be wiped out. The Permian was hot, and a stark contrast to the Carboniferous period before. By this time, lizards from the Carboniferous and late Devonian had evolved into a strange class of animals called synapsids. Synapsids were somewhere between mammals and reptiles, they moved like a mix between the two, some had tough skin and fur, other scales with smaller hairs. By the end of the Permian almost all large animal life were synapsids, but during the Permian-Triassic extinction event the globe began to heat up. Oceans dried up, drought and desertification brought the end of the synapsids. The only synapsids who survived were small burrowers, capable of extracting water from the last of the desert plants and after the extinction, during the Triassic, they would flourish.
When the Mesozoic era begun, with the Triassic period, the burrowing synapsids evolved into vast herds, spanning the globe. Lystrosaurus were the most successful species ever to have lived, throughout the Triassic period they ventured to every part of the world and were the only large herbivores, there were more of them then there ever have been humans and so their demise, along with of course the numerous carnivores who relied on them for food, made up the Triassic-Jurassic major extinction-level event. The synapsids died out due to the birth of a new species, who, via competition, eliminated such a momentous portion of animal life on Earth. These new reptiles were, of course, the dinosaurs which would dominate the planet for the rest of the era until they had their own extinction at the end of the Mesozoic.
During the next period, the Jurassic, dinosaurs ruled the land, huge marine reptiles the sea, and pterosaurs in the skies. It’s also between this period and the Cretaceous that we can see glimpses of the Earth as we know it today. In the Jurassic, the planet’s sixth super-continent, Pangaea, split up into the seventh super-continent, Gondwana, in the South and Laurasia, which made up Europe, North America and Asia. In the early Cretaceous period the first grasses evolve and later in the period bees and butterflies began pollinating the first flowering plants. During this era, the Himalayas and the Andes would grow, and the Atlantic was still expanding. But the pinnacle of life, the Mesozoic era, would eventually come to a close in the most infamous extinction event ever.
The Cretaceous period was mostly a dangerous and hostile planet, with super-volcanoes, climate change, an increasingly more toxic atmosphere and finally a comet which stuck the Gulf of Mexico and over a few weeks destroyed 75% of animal life. Still the Cretaceous-Tertiary extinction has left its marks on Earth including a boundary deep below the Earth’s surface and, of course, what came next. The Cenozoic era gave rise to some of the first mammals including the first human species, homo habilis.
But I have a concern which I share with many, which is that we are leading out planet towards a sixth major extinction-level catastrophe. It’s not a crazy as you might think that littering and deforestation could have a similar eventual affect to the third great dying. Nature will bounce back, as sad as it is, this isn’t about saving an animal you’ve never heard of in a place, far away in a place you’ve never been. This is about us fixing the problem we made for our own good. We need to learn how to save ourselves.
