What qualifies a mass extinction?
A mass extinction can be defined as a period where a high percentage of life on earth (and their ecosystems) dies out rapidly. They are defined by both the scale – the percentage of species that are lost – and how quickly this happens.
These details are linked, but both are relevant in defining a mass extinction. Broadly around 75% of species become extinct in a period less than two million years. This may sound like a long time but is considered a short period geologically!
What are the five mass extinctions and their impact?
1. End Ordovician
The oldest mass extinction that we know of is the End Ordovician mass extinction, around 445 million years ago. The Ordovician period saw a large increase in marine life of many different forms, as coral and algae thrived as the climate warmed. Tribolites are some of the most characteristic life forms of the Ordovician period, crab-like creatures that geologists have discovered in fossil records from modern day Baltic regions.
However, an Ice Age followed making around 85% of this population extinct over a 1.5million year timespan, so it is considered the second largest extinction. Glaciers formed across the globe, disrupting the ways in which nutrients like oxygen, carbon and nitrogen, cycled through the ocean.
The cause is debated, but scientists believe the reduced sea levels, reduction in C02 and movement of the continent (Gondwana) causing the onset of an ice age.
2. Late Devonian
Another known mass extinction occurred in the Late Devonian period, around 375 million years ago. The impact of this extinction was more pronounced in tropical environments, with corals suffering losses, and brachiopods – a type of shelled sealife – becoming extinct.
The Devonian era was also known as the Age of Fishes and there were a huge variety of them during this time period, including distinctive jawed-fish.
The cause for this mass extinction is still debated, but the concentration in loss of life in tropical conditions suggests that climate cooling was again a factor. One suggestion for cause and effect is that in the Late Devonian period, vegetative life on land evolved and formed the first forests as the first trees appeared. As plant life expanded, they used up more carbon dioxide in photosynthesis, depriving the existing sea life of energy.
3. End Permian
The End Permian mass extinction occurred around 250 million years ago, with geological evidence suggesting it eliminated around 90% of all life in only 200,000 years. It was a time of fearsome sabre-toothed predators, giant insects and bizarre-looking sharks.
The ancestor of modern mammals lived through the Permian period, the Synapsid. Some synapsids looked a lot like what we would think of as a dinosaur, like the Pelycosaur. Some of our discoveries about synapsids are still being made as the fossil record is incomplete, so we’re still understanding this evolution.
Again, climate warming is considered the best explanation for this mass extinction. At the end of the Permian Period, massive volcanic eruptions in Siberia and what is now South America, led to a huge outpouring of lava. The eruptions produced high levels of carbon dioxide, warming the planet and perhaps prompting such rapid and vast loss of life. There were also massive emissions of hydrochloric acid and sulpher which would have blocked life from thriving. There are also theories about large amounts of methane being released from melting permafrost.
4. End Triassic
The Triassic period ended with the rift of the Pangea supercontinent, breaking into the continents as we know them today around 200 million years ago.
The period was dominated by reptiles, and dinosaurs first appeared towards the end of this era. It also saw the first mammals. Some fantastically large creatures lived during this time, like the Shastasaurus – a type of Ichthyosaur that could grow up to 69 ft long, weighing over 80 tons.
Similar to the End Permian mass extinction, volcanic eruptions introduced carbon dioxide into the atmosphere, causing climate change and extinction on a large scale. This had an impact on both marine life and on land, as many different types of life became extinct, like Phytosaurs.
5. End Cretaceous
Perhaps the most famous mass extinction around 65 million years ago, the End Cretaceous mass extinction ended dinosaur domination on land.
Geological evidence suggests this was caused by a meteor hitting the Yucatan Peninsula of Mexico. This caused an enormous dust cloud to encircle the entire planet. The dust cloud reduced the amount of sunlight reaching the Earth’s surface, an obstacle for photosynthesis and disrupting food chains. Dust falling back to Earth may have also caused wildfires.
There is some debate around this meteor hit as the sole cause of the extinction, as there is also evidence of significant volcanic activity in India.
The End Cretaceous mass extinction is interesting in that it ended the dominance of non-avian dinosaurs. Intelligent, lethal and carnivorous creatures such as the Tyrannosaurus, Velociraptor and Spinosaurus ruled the continents. As these dinosaurs existed at the top of the food chain, mammals had likely evolved as mainly nocturnal creatures in their avoidance of predatory dinosaurs, such as shrews and mice. The ensuing extinction of dinosaurs on land allowed mammals to evolve and thrive.
Have there only been five extinctions?
There have been other smaller extinction events, but these five are most referenced because of the huge amount of life that was lost across different species as a consequence.
There is evidence for an estimated 18 extinction events in the last 4.5 billion years, including the mass events mentioned above. However, smaller is also a bit of a misleading term for them. Some extinctions in the Cambrian period possibly saw up to 80% loss of life, but this was mainly within one species (triobites). The diversity of life at the time was much less, so the number of species that went extinct is lower than other mass extinctions.
How do we know about mass extinctions?
Mass extinctions were first identified by geologists and palaeontologists who analysed fossils. Fossils are like a time capsule for different time periods on Earth, and we call this the fossil record.
When looking at different layers of rock showing fossils from these time periods, older rock layers contain a greater diversity of fossil life forms. In comparison, there is an immediate dearth of life form in the more recent layer. This gives us a clue as to when different species died out suddenly. The transition in fossils from one period to another reflects a striking loss of species and the gradual introduction of new species in more recent fossils.
Will there be another mass extinction?
Yes. On average they happen every 33 million years. Some experts believe we are currently living in a sixth mass extinction, this time driven by human activity.
As we’ve seen in previous mass extinctions, too much carbon dioxide or methane can be catastrophic for life on Earth. While the previous mass extinctions were all prompted by natural phenomena, humans use of land, water and energy is generating an unsustainable level of carbon dioxide. We may be prompting another mass extinction ourselves as the planet and different species can’t keep up with the rate of change that would usually happen over millions of years.
Currently, 40% of land on Earth has been harvested for food production. Agriculture is accountable for 90% of deforestation and responsible for 70% of the planet’s freshwater use. This has a huge impact on the species that inhabit earth by significantly altering their habitats. A good example of this is the eradication of prairie land in the US, and the diverse range of plants that inhabited prairies.
Unsustainable consumption is a significant contributor to greenhouse gases causing global warming. This is causing extinction of species up to 10,000 times the natural rate. While some extinction is a natural part of the evolutionary cycle, the human driven sixth mass extinction poses worrying questions about balance and human coexistence with the natural world.