Earth has undergone five major mass extinctions over the past millions of years. During these events, 75% to more than 90% of all species disappeared due to several reasons, including geologic catastrophes, climate change, or even meteor strikes on the Earth’s surface. While most of us see mass extinctions as deadly events, they open up the planet for new forms of life to emerge. Scientists are able to spot them when species go missing from the global fossil record.
Mass extinctions are interesting to study mainly because they allow us to better understand our world and introduce us to species we never have known before. Thus, it’s not surprising that studies about these events, their causes, and the species that lived through them are quite popular.
The Big Five
Mass extinction events caused a very rapid period of speciation among the few species that have survived. Surviving species can freely spread out since there is so much room for them. Many habitats during these periods need to be filled. Eventually, these species adapt to the new environmental conditions and reproduce away from their original populations. At this point, they can be considered a brand new species and biodiversity expand rather quickly.
After mass extinction events, there is increased biodiversity on Earth because the species don’t have to compete for food, shelter, resources, and even mates. These events are nothing new because according to scientists, species go extinct all the time for various reasons. Previous reports estimated that at least 99.9% of all species of plants and animals that ever lived are now extinct. Scientists have discovered at least five different mass extinctions which are referred to as the Big 5, significantly reshaping the levels of life on Earth.
1 - Ordovician-Silurian Extinction: More than 440 million years ago
Scientists suspect that the Ordovician-Silurian Extinction was caused by continental drift and subsequent climate change, eliminating up to 85% of all living species. According to National Geographic, the official magazine of the National Geographic Society that primarily contains articles about science, geography, history, and world culture, this mass extinction involved massive glaciations that locked up much of the world’s water, causing sea levels to massively drop.
2 - Devonian Extinction: 365 million years ago
Studies suggested that around 75% of species were lost in the Devonian Extinction. For instance, trilobites, which survived the first mass extinction, were nearly exterminated during this time. It was caused by quick cooling of air temperatures, lack of oxygen in the oceans, volcanic eruptions and/or meteor strikes.
3 - Permian-Triassic Extinction: 250 million years ago
Scientists considered the Permian-Triassic Extinction as the deadliest of all the mass extinctions. They believe that a comet or asteroid triggered a massive die-off, wiping out more than 90% of all species. It nearly ended life on Earth.
4 - Triassic-Jurassic Extinction: 210 million years ago
The Triassic-Jurassic Extinction was caused by massive floods of lava erupting from the central Atlantic magmatic province, wiping out about 20% of all marine families, including all non-dinosaur archosaurs, many large amphibians, and most mammal-like creatures.
5 - Cretaceous-tertiary Extinction: 65 Million Years Ago
Scientists refer to this as the K-T extinction because it brought on the extinction of dinosaurs. This was caused by a combination of climate change, asteroid impact, and volcanic activity, ending 76% of life on Earth.
Ordovician-Silurian Extinction and Ocean Deoxygenation
About 444 million years ago, the Earth experienced the first of a series of extinction events in the Phanerozoic, which is collectively referred to as the Ordovician–Silurian extinction events. Previously, scientists suggested several scenarios that could explain the massive die-off, which includes volcanic gases poisoning the atmosphere or deadly gamma-ray bursts from a hypernova. However, all evidence gathered is lacking.
To gather more evidence, the researchers created a novel model using published metal isotope data and new data from samples of black shale hailing from the Murzuq Basin in Libya, deposited in the geological record during the mass extinction. According to Phys.org, an internet news portal that provides the latest news on science, the information included 31 different variables related to the metals such as the amounts of uranium and molybdenum that leach off the land and reach the oceans via rivers to settle into the seafloor.
The findings suggest that severe and prolonged ocean anoxia or ocean deoxygenation must have occurred across large volumes of Earth's ocean bottoms. Ocean deoxygenation affects how much oxygen seawater can hold and the circulation patterns that carry oxygen-rich water to the deeper ocean. Previous studies revealed that the amount of oxygen in different parts of the oceans aren’t equal. For instance, several areas have little to no oxygen, which are called “oxygen minimum zones.”
According to Carbon Brief, a UK-based website designed to improve the understanding of climate change, both in terms of the science and the policy response, the oxygen levels in those areas are low enough to be lethal to most marine life. Unfortunately, scientists said that these low-oxygen regions are expanding, which could have “dramatic” biological, ecological, economic and climatic consequences. The risk of deoxygenation not only impacts that affected zones but also all oxygen concentrations in all ocean basins and affects a growing number of coastal regions. Since the 1960s, scientists said that there has been an average 2% decline of oxygen in the ocean.
Researchers from the recent study state that the ocean deoxygenation in the Late Ordovician event occurred after Gondwana, a single super-continent, experienced global cooling that gripped much of it under glaciers. "Our study has squeezed out a lot of the remaining uncertainty over the extent and intensity of the anoxic conditions during a mass die-off that occurred hundreds of millions of years ago," lead author Richard George Stockey said.
Today, marine oxygen loss is also evident mainly due to global warming. Warming affects the ocean and its dissolved oxygen content in several ways. It also influences the solubility of oxygen in the water. The researchers said that increasingly documented deoxygenation in our oceans will put a strain on many organism types. "There is no way that low oxygen conditions are not going to have a severe effect on diversity," Stockey said.