Not many think of extinction events without the dinosaurs crossing their mind. In case you didn't know, the dinosaurs were killed off around 65 million years ago when an asteroid around 6-9 miles wide struck in the Gulf of Mexico, causing a series of events that killed any animal heavier than 25 kilograms.
The event that killed off the dinosaurs was only one of the 'big 5' mass extinction events that happened on our planet and, despite its severity, it wasn't even the worst...
What is the The Great Dying?
At the end of the Permian period (the period of time just before the reign of the dinosaurs, which spanned approximately from 300 MYA to 250 MYA) a mass extinction occurred that nearly put life on the planet back to square one. Around 98% of all life on the planet went extinct. You read right, planet Earth was just 2% away from being completely lifeless, meaning we most certainly wouldn't be here. This is still the worst extinction event that planet Earth has seen so far...
This has become informally known as 'The Great Dying'. Despite the severity of the event, the exact cause has long been a mystery, with hypothesises only coming up in the last 10-15 years.
So what exactly caused 'The Great Dying'? And why don't we talk more about it? Well once we see what was the cause, we'll see why we should be talking more about it, as it's not just uncontrollable asteroids that threaten this planet...
What effects did the The Great Dying have?
Marine invertebrates were the ones who felt the effects of The Great Dying the most. Groups that completely disappeared were the eurypterids (sea scorpions), trilobites and blastoids (sea buds). Other groups that came dangerously close were ammonites (with only certain groups barely surviving), gastropods, brachiopods and crinoids.
Marine vertebrates also faired pretty poorly, with groups such as acanthodians (the group very closely related to cartilaginous fish) completely disappearing and all other groups seeing rapid decline. Having said that, they did fair better (barely) than the invertebrates, but the reason for that is something I'll discuss further down this post.
Thus far, terrestrial invertebrates proved to be fairly resilient to all other previous mass extinctions, however, not even they could escape the severity of this one. To date, this is still the worst mass extinction that terrestrial invertebrates went through (some say the only one). Most groups that superficially resembled modern counterparts went extinct, including the groups that contained the giants from the previous period, the carboniferous. After the Permian, all insects seen in the fossil record show that they more or less stayed the same throughout time.
Just over two thirds of terrestrial vertebrate groups became extinct. There were barely any amphibians, reptiles and proto-mammals left to carry on the lineages, with pelycosaurs and anapsids dying out completely. Herbivores suffered the most, with almost no herbivorous animals seen at the beginning of the Triassic and Lystrosaurus being the most abundant land vertebrate during the early Triassic. Overall, terrestrial vertebrates didn't recover for a staggering 30 million years.
If there is one group more resilient than terrestrial invertebrates, it's plants. Even when there are low swings, it's usually due the difficulty plants have with fossilising. However, remarkable changes have been observed in the ecosystems after the event, namely with the plants. Such sudden changes in plant morphology can only be explained by the fact that even the flora suffered great losses. This has been supported by the coal gap seen in the early Triassic.
What caused The Great Dying?
So, as stated before, the cause of this had long been a mystery. Even now, we aren't too sure, since most surface rocks are subducted and recycled back into the Earth every 200 million years or so (remember this extinction occurred 250 MYA). What is seen in the rock record is massive spikes in levels of methane and carbon dioxide. Not only would this have severely disrupted things on land, but would have caused massive surges of ocean anoxia (where water is low or devoid of oxygen).
Earlier, I mentioned that the biggest victims were marine invertebrates, especially those with shells. These animals have hard parts made of calcium carbonate, which they take in from the surrounding waters to build it. For this, they must rely on stable carbon dioxide levels in the ocean. Once this spikes, they're pretty screwed.
Many causes of The Great Dying have been hypothesised over the years. The main, generally accepted cause is volcanism in the Siberian traps. A series of flood basalt events (in which large volumes of basaltic lava cover vast expanse of land through volcanic eruptions) meant that around 2 million square kilometres were covered with volcanic lava across 2 million years. That's a lot of lava and is still the largest flood basalt event to occur on this planet.
With events of this magnitude, comes our old friend carbon. Massive amounts of carbon would be released into the atmosphere from such events across 2 million years, changing global temperatures via the greenhouse effect, as well as bleeding into the oceans and disrupting that delicate carbon ratio. This would also explain the vast areas of oceanic anoxia seen from this time.
What can we learn about our own climate change?
So, carbon dioxide is the cause of severe extinction and climate change... sound familiar? Events like the Permian extinction event really do bring home the severity of our current situation. Remember that events like this don't happen in a day, they occur across thousands of years and considering that we have only really been affecting the atmosphere and Earth's ecosystems as much as we do in the past 200 years or so, we have lost a scary amount already. If this continues, we will see at least Permian levels of loss in a geological instant, maybe more. Life on Earth just about recovered by the skin of its teeth, but if we see that again in a much shorter amount of time, I don't think it would recover again...