How come we don’t keep ammonites in a tropical fish tank? For the same disappointing reason,
we won’t be digging in to a medium-rare, stegosaurus steak for dinner. This is because they are extinct and Extinction occurs, when the last of any species dies.
The media occasionally mentions some thing about how the earth is going through the 6th great mass extinction, caused by us Homo Sapiens, but no Hey! big deal. Have you heard about Intu shopping centres going bankrupt?
Hold on, did you say 6th?! There have been five more?
Well Yes, they have all come and gone, over earths long history and each mass extinction has brought on a new chapter in the evolution of life.
The extinction of the dinosaurs is the one we all know and love. Popular media portrays a picture of an apocalyptic meteor slamming in to the earth. Which is a great image , but only half of the story. Why the dinosaurs died out is a lot less simple and far more nuance that big fiery ball of death.
And this is how all extinctions boil down. Not one big event, but a series of small events that keep piling on the pressure, until, the eco systems collapse and everything just ends up dead. In other words, those the straw that broke the camels back.
Life that can adapt and survive the extinction event, go on to dominate the new world. So from Dinosaurs lording it over the land, we now live in a world dominated by mammals and its thanks to that meteorite (and other cherish eco system pressures) we have the world as we know it. Podcasts and all.
Yet extinctions are not confined to big events, they happen all of the time and are a very natural thing. 95% of all species, that have become extinct, became extinct due to Back Ground extinction. Now Background extinction, is the technical term for species naturally dying out. That can happen for a number of reasons. Maybe through being out competed by the new animal on the block. Maybe a natural disaster, which could wipe out a local species. or even just a disease. Theoretically there are species today dying out, because of background extinction. The problem we have is humanities reach is so wide, the likely cause of any extinction is down to us.
And sometime that extinction caused by us, can be by complete accident. For example.
The Dodo! We all know the Dodo. As dead as a Dodo! and there is a reason we say that, and it’s because it is dead. Gone for good. It was a bit of an unfortunate species, as it was the bird who forgot how to fly. It lived on the Island of Mauritius, which located just off Madagascar. There it evolved and it was perfectly adapted for sauntering around on the ground. That all changed when humans discovered the island.
Mauritus, became a convenient stop over for humans on the way to India and further east. It soon became settled and people, brought over pigs and the stowaways, rats. The rats and pigs developed a taste for the local delicacy of Dodo eggs, after-all they where just laying on the ground.
And the once abundant Dodo was dead in less that 200 years of the arrival of man. Not our finest moment.
Unintended harm or not. These kind of pressures upon the natural world has caused scientists to declare that we are the cause for earths sixth mass extinction, which has been, is, and shall happen through our life times. I didn’t say this was going to be an optimistic episode.
So if we are in a mass extinction event, what makes a mass extinction? Because to be honest it doesn’t feel like the four horsemen of the apocalypse death, famine, war and conquest are heading down the road…
and yes I am a ware that during 2020, we have seen unprecedented swarms of locust, flooding, fire, plague and protest around out the world.
When 2020 has passed I hope that line is more a little relevant.
. But putting the 2020 to one side. This mass extinction feels very sublime. And that is because mass extinctions take time. They don’t typically happen in the space of a few decades, or even life times. Mass extinctions usually take a few million years. The Devonian extinction took 2-3 million years.
As for an actual definition, well that is a bit of a grey area and has been since the early 1800s when Georges Cuvier suggested life may go through “natural revolutions”. There is no number, such as over 50% and its a mass extinction and under its just back ground radiation. Noo, none of that.
More recently, it has been noted that during these big events, both marine and terrestrial life both appear to be affected about the same amount. In smaller events you seem to get decoupled extinctions, where only the one realm will be affected. However the broad definition we are using is the one you are probably already thinking. A lot of extinctions in a short space of geological time above and below water. And that makes sense, because when you look at a graph of total extinctions. Plotted along side geological time you see large sudden spikes. And those spikes are our mass extinctions.
See Raup and Sepkoski 1982 or 86 for the graph.
So, if we are in the 6th mass extinction what about the other five? well lets dive right in.
First up, we have the `End Ordovician extinction. This was at the end of the Ordovician period, the clue is in the name. This is
– where 60% of life all died out. This started around 445 MYA and this lasted for around two million years and some would say longer. It was caused by the beginning of an ice age, which caused such glaciation, the sea level fell by 70-100 meters. Not bad if your a land animal, but they don’t exist yet. As 99% of life at the time was confined to the sea. So not bad if your a fish, as you can float down, but they didn’t exist yet either. Coral reefs on the other hand, now they did exist, and they where decimated.
A drop in global temperature, mass glaciation and sea level fall, might sound like a mixed bag of misfortune. But live adapted very well. The final nail in the coffin really came when this all happened again, but in reverse. So temperature rose, ice melted and the sea went back up. This was to much change to quickly.
On a quick side note, a sea level rise of 100 meters would flood London, Cardiff, Manchester, Glasgow and most of Edinbrough.
Next up, we have the Devonian extinction. And very little is understood about it. What we do know is that it started around 382 MYA lasted in the region of 2-3 million years.
Nobody can agree on a cause – volcanoes, meteorites and sea level rise are all thrown their hats in to the ring on this one.
But later on, during the extinction crisis, the super continent of Gondwana land, moved down to the south pole… Oh, what was that? Super continent of Gondwana land. Yes, it was Africa, Australia, Antarctica and india all combined in to one giant land mass.
Anyway the, result of Gondwana occupying the worst real estate on the planet was it caused an ice age. No sooner had it moved in, it was covered in a layer of ice and cooling things down for everybody else. What a bad neighbour. This was another straw for the camels back.
Extinction number 3. I should probably quickly mention life recovered between all these extinctions. Things have to get better again, so they can get much worse. And much worse they became. The Permian extinction was the biggest of all. It occurred 254 MYA and it killed 82% of all life. That, is pretty phenomenal.
At the time all of the continents of the world where joined up and it was called Pangea. Which means “One world”. Because it was so vast, it was difficult for rain to penetrate so deep in to the super continent. Imagine being a rain cloud, it is far easier for you to rain coastally that travel inland for tens of thousands of miles. and so, most of Pangea was a vast dry hot desert. Not idea for life. But life did exist, and on land this time. Dimetrodons where the dominate land animal. Almost like a giant lizard with a sail on its back.
What caused the extinction this time was volcanoes and subsequently global warming.
The Siberian traps, which where based in Siberia funnily enough. Were the main culprit. It was the largest volcano field ever in earth history.
During its time is was estimated to have extruded 1.8 million cubed miles of lava, covering 2.5 million squared miles. Or in kilometers, 2-3 million cubic Km of Lava. Covering 4 million km squared. To a depth of 400-3000 meters. With all of this taking place in possibly the short span of 600,000 years.
Now the volcano was not the only cause, in fact there are many other factors too, but the Siberians Traps, coupled with low sea levels and an extremely hot climate. You have a recipe for disaster. Which is how we hit the record breaker.
Extinction number 4. The Triassic. Now the Triassic is where the fun begins and we see the evolution of dinosaurs.
Beginning around 228 MYA the triassic extinction, killed out 56% of life and nobody is even sure why. It is the least well understood mass extinction. At the time, Pangea was breaking apart in to the beginnings of the contents we know today. This caused some volcanism. The sea at the was falling and there may have been a meteorite. Who knows!?
Finally the one we have all been waiting for, extinction number 5. The Cretaceous extinction. This is the one with the big meteorite and the dinosaurs. In comparison to the Permian, where 82% of all life died out, the cretaceous extinction only killed a cool 47% of life. That is still just under one out of every two animals dying out. So in a hypothetical world where on one can survive, which would you chose cats or dogs? … I will also allow for “bring back the Woolly Rhino” in which case, it bins the other options.
Now to turn your attention away from the woolly rhinos and back to the podcast. The cretaceous is famous for the big meteorite impact. And that is partially correct, only palaeontologist believe that explanation is a bit to simplistic to cause the complete ecological collapse of the entire planet.
Instead they prefer the idea of one or more meteors, more sea level fall and more super massive volcanoes. Combined, it may seem over kill to us outsiders, as a meteor the size of the UK hitting the earth would certainly put stop to your weekend plans, but it wasn’t enough for the Palaeontologists! So they opted for over kill.
The super volcano involved this time was The Deccan traps. It is believed they erupted so much it began producing acid rain, killing any local vegetation. Along with this, the gasses exhumed being to cause a long term cooling affect on the planet. And now enter top stage right, a giant meteor, causing a nuclear winter.
And that is how the dinosaurs died out.
There are no more mass extinctions to mention. You maybe glad to hear, but others did occur on a lesser scale. There was a small extinction event in humanities early history, which wiped out the woolly-mammoths and saber-toothed cats.
What I like most about these mass extinctions are that they follow a kind of a pattern. Out with the old in the with new. As each time we see life evolve in a new and more arguably more advanced way. At the end of the Ordovician we see the evolution of fish and jaws for chewing food.
At the end of the Devonian we see the rise of Amphibians and those first steps on land.
The permian ushered in the age of the reptile, replacing the amphibians as the dominant land family. And then the cretaceous replaced the reptiles with the mammals.
You may have also noticed it was the same few reoccurring environmental changes that caused the mass extinctions. And thats because they are all linked. But we will take a deeper look in another episode at how these climate mechanisms works and bounce off one other.
Thanks for listening
The great extinctions, what caused them and how they shape life – Norman Macleod. Published by the Natural History Museum, London.
https://www.co2levels.org/ – A good graph showing how Co2 in the atmosphere has changed historically. You can also compare other greenhouse gases on the site, such as oxygen.
Raul and Sepkoski 1982.