Extinction – Background extinction, Mass extinction

What is Extinction?

• Imagine a world where the vibrant colors of tropical rainforests are mere echoes of the past, where majestic creatures roam only in ancient tales, and where the intricate web of life has been irreparably damaged. This haunting scenario is the grim reality of extinction—a phenomenon that has shaped Earth’s history and continues to threaten the delicate balance of our planet today.
• An extinction event, also referred to as a mass extinction or biotic crisis, signifies a catastrophic decline in biodiversity. It is characterized by a rapid and widespread reduction in the diversity and abundance of multicellular organisms. These events occur when the rate of extinction surpasses the background extinction rate—the natural rate at which species go extinct—and when the rate of speciation, the formation of new species, fails to keep up.
• Throughout the 540 million years of Earth’s existence, there have been several major mass extinctions. While the exact number is still a subject of debate among scientists, estimates range from five to more than twenty such events. The challenge lies in defining what constitutes a “major” extinction event and selecting appropriate data to measure past diversity accurately.
• These mass extinctions have left an indelible mark on our planet’s history. One of the most well-known examples is the Cretaceous-Paleogene extinction event, which wiped out the dinosaurs around 66 million years ago. This event was likely triggered by a combination of factors, including a massive asteroid impact, volcanic activity, and climate change. The repercussions were catastrophic, causing the loss of numerous species and radically reshaping the Earth’s ecosystems.
• However, mass extinctions are not limited to ancient history. Today, we find ourselves in the midst of what scientists call the sixth mass extinction, often referred to as the Anthropocene extinction. Unlike previous events that were primarily driven by natural factors, the current crisis is primarily a result of human activities. Habitat destruction, pollution, climate change, overexploitation of resources, and invasive species have pushed countless species to the brink of extinction.
• The consequences of extinction are far-reaching and profound. Beyond the intrinsic value of each species, their loss disrupts the intricate web of life, causing imbalances in ecosystems and threatening the stability of the planet. Biodiversity loss can lead to a domino effect, as the disappearance of one species can have cascading effects on others that depend on it for food, pollination, or other ecological services.
• Preserving biodiversity and combating extinction requires urgent and concerted action. Conservation efforts, such as habitat protection, restoration, and species reintroduction, play a crucial role in safeguarding threatened species. Additionally, addressing the root causes of extinction, such as unsustainable practices and climate change, demands collective responsibility and global cooperation.
• As individuals, we also have a part to play. Educating ourselves about the importance of biodiversity, supporting sustainable practices, and advocating for conservation measures are small yet significant steps we can take to make a difference. By recognizing the gravity of the extinction crisis and working together, we can strive to protect and restore the rich tapestry of life that makes our planet truly remarkable.
• In conclusion, extinction events represent dark chapters in Earth’s history, characterized by the loss of countless species and the disruption of fragile ecosystems. From ancient cataclysms to the ongoing human-induced crisis, the toll of extinction serves as a stark reminder of our responsibility to protect and preserve the extraordinary diversity of life on our planet. Let us take action now, for the sake of the species that still grace this Earth and for the generations yet to come.

What is Background extinction?

• In the grand tapestry of Earth’s history, extinctions have played a significant role in shaping the ever-evolving web of life. While we often focus on mass extinctions and the current biodiversity crisis, it is essential to understand the concept of background extinction—the normal rate of extinction that occurred before human influence became a significant factor.
• Background extinction rate, also known as the normal extinction rate, provides us with a measure of the natural frequency at which species disappear throughout Earth’s geological and biological history. It represents the extinctions that occur during the periods between major extinction events and serves as a baseline for comparison to present-day extinction rates.
• Extinction is a natural part of the evolutionary process. Species emerge, adapt, and eventually, some may disappear, making way for new forms of life. The background extinction rate helps us quantify the “normal” occurrence of extinctions over time. However, it is important to note that background extinction rates have not remained constant, varying over millions of years.
• There are several ways to measure background extinction rates. One approach is to count the number of species that typically go extinct within a specific timeframe. For instance, at the background rate, it is estimated that one species of bird will go extinct approximately every 400 years. This method provides a straightforward perspective on the rate of species loss.
• Another measurement is in terms of million species years (MSY). By calculating the number of extinctions per million species years, we can grasp the relative frequency of extinction events. For example, at the background rate, approximately one extinction occurs per million species years. This means that if there were a million species on Earth, one species would naturally go extinct every year. On the other hand, if there were only one species, it would take around one million years for it to face extinction.
• A third way to examine background extinction is through species survival rates over time. Based on normal extinction rates, species typically persist for 5 to 10 million years before succumbing to extinction. This provides insight into the average lifespan of species in the absence of major catastrophic events.
• Understanding background extinction rates is crucial for evaluating the current state of biodiversity loss. Comparing the present-day extinction rates to the background rates allows us to recognize the alarming increase in extinctions. The accelerated loss of species we witness today is far beyond what can be considered natural or sustainable.
• As we grapple with the consequences of human activities on the planet, it becomes evident that our actions are contributing to an unprecedented wave of extinctions—the sixth mass extinction event. Recognizing the importance of background extinction and the need to preserve Earth’s biodiversity becomes all the more urgent.
• Conservation efforts, habitat protection, sustainable practices, and addressing the root causes of extinction, such as habitat destruction and climate change, are crucial steps in curbing the current extinction crisis. By working together and taking responsible action, we can strive to restore balance, protect vulnerable species, and ensure a future where background extinction rates are once again the norm in a thriving and resilient biosphere.
• In conclusion, background extinction serves as a benchmark for the natural rate at which species have historically gone extinct in the absence of human influence. Understanding this concept allows us to grasp the magnitude of the current biodiversity crisis and the urgent need for conservation actions. By appreciating the value of Earth’s rich tapestry of life and taking steps to protect it, we can strive to restore harmony and safeguard the diversity that defines our planet.

Factors Involved in Background Extinction

Background extinction, the natural rate of species loss throughout Earth’s history, is influenced by various factors that shape the delicate balance of ecosystems. While not as sensational as mass extinctions, background extinction occurs gradually as species struggle to adapt to environmental changes. Understanding these factors can provide valuable insights into the ongoing processes that contribute to the ebb and flow of biodiversity.

Climate change stands out as one of the primary drivers of background extinction. As long as environmental conditions fall within a species’ range of tolerance, they can thrive and reproduce. However, when changes in climate push species beyond their adaptability limits before they can adjust, their chances of survival and reproduction diminish significantly. Rising temperatures, for instance, pose a significant threat to lizard populations in tropical regions. Lizards rely on a delicate balance of sun exposure and shade to regulate their body temperature, ensuring optimal physiological processes. With increasing temperatures caused by human activities, female lizards in certain tropical areas are forced to seek shade before they can gather sufficient nutrients for reproduction. As a consequence, tropical lizard populations are declining, highlighting the vulnerability of species to climate-induced changes.

The introduction of invasive species into new habitats also plays a pivotal role in background extinction. When a new predator invades an ecosystem, its impact often surpasses the ability of native species to adapt or defend themselves. The rapid disruption caused by invasive predators can lead to large-scale extinctions. An infamous example is the potential annihilation of several Australian megafauna species upon the arrival of the first humans. The introduction of these new predators, coupled with other environmental changes, likely contributed to the demise of these ancient giants.

Background extinction unfolds gradually as species experience a decline in reproductive fitness due to environmental changes occurring too rapidly for them to adapt. It is not characterized by dramatic events but rather manifests as a slow and steady loss of species over time. These environmental changes can include shifts in climate, habitat loss, pollution, or the disruption of ecological relationships.

Recognizing the factors involved in background extinction is crucial for managing and mitigating its impacts. Conservation efforts should prioritize protecting habitats, reducing greenhouse gas emissions, and implementing measures to prevent the introduction of invasive species. Additionally, fostering public awareness about the consequences of climate change and promoting sustainable practices can contribute to preserving biodiversity and minimizing the rate of background extinction.

While background extinction may not captivate headlines like mass extinctions, it is an ongoing process that demands our attention and action. By understanding the factors that drive background extinction and taking proactive steps to address them, we can strive to maintain the delicate equilibrium of ecosystems, safeguard species, and preserve the irreplaceable wonders of our natural world.

In conclusion, background extinction is influenced by factors such as climate change and the introduction of invasive species. Environmental changes that occur too quickly for species to adapt disrupt their reproductive fitness, gradually leading to their decline. Recognizing these factors empowers us to implement conservation strategies that protect habitats, mitigate climate change, and prevent the spread of invasive species. By valuing the intricate web of life and taking collective action, we can help maintain the resilience and diversity of our planet’s ecosystems for future generations.

What is Mass Extinction?

In the tumultuous history of our planet, there have been catastrophic events that forever altered the course of life on Earth. These events, known as mass extinctions, are harrowing episodes where a staggering 75 percent or more of all species suddenly vanish within a relatively short geological timeframe. These cataclysms unfold like “very awful days” for the organisms that inhabit the Earth at that time.

Over the past 500 million years, geologists and paleontologists have identified at least five major global extinctions. Each of these events marks a significant epoch in Earth’s geological formation. For example, the Mesozoic Era spans from the mass extinction that occurred between the Permian and Triassic periods 251 million years ago to the mass extinction between the Cretaceous and Tertiary periods 66 million years ago, which brought about the demise of the dinosaurs and ammonites, among many others.

Among these extinctions, the Permian mass extinction stands out as the deadliest in the history of life on Earth. Approximately 96 percent of all living forms vanished during this cataclysmic event. Scientists have put forth several theories to explain the causes of this mass extinction. One proposed explanation involves the release of greenhouse gases from a supervolcano eruption. This eruption would have triggered a chain reaction of environmental changes, leading to the destruction of ecosystems worldwide. Another hypothesis suggests that the simultaneous proliferation of certain microorganisms resulted in the release of massive amounts of lethal hydrogen sulfide, rendering most life forms unable to survive.

The mass extinction that wiped out the dinosaurs, on the other hand, has a more well-known cause that cannot be attributed solely to natural factors. An asteroid impact in what is now the Yucatan Peninsula, on the southeastern coast of the Gulf of Mexico, occurred just prior to this extinction event. The catastrophic collision resulted in widespread devastation, including massive fires, tsunamis, and a darkened sky due to debris blocking sunlight. The subsequent environmental disruptions, combined with the simultaneous eruption of the Deccan Traps in contemporary India, which released vast amounts of volcanic gases and ash, likely contributed to the Cretaceous-Tertiary mass extinction.

These mass extinctions, with their immense loss of biodiversity, have shaped the history of life on Earth. They have paved the way for new evolutionary pathways, allowed the rise of new dominant species, and altered the composition of ecosystems. However, it is important to note that these cataclysmic events occurred over long periods of geological time, spanning hundreds of thousands to millions of years.

Today, as we grapple with the ongoing biodiversity crisis, it is crucial to learn from the past. The awareness of how mass extinctions have transformed our planet serves as a reminder of the delicate balance of life and the importance of preserving Earth’s incredible diversity. By understanding the causes and consequences of these events, we can strive to protect vulnerable species, conserve habitats, and mitigate human-induced factors that contribute to the current extinction rates.

In conclusion, mass extinctions are catastrophic events in Earth’s history that result in the rapid loss of a significant percentage of species. They demarcate pivotal epochs and have shaped the trajectory of life on our planet. From the Permian mass extinction to the demise of the dinosaurs, these events have left an indelible mark on the Earth’s biodiversity. By studying and respecting the lessons they offer, we can work towards a future where the wondrous tapestry of life continues to thrive and flourish.

Five Mass Extinctions

Mass extinctions have played a significant role in shaping the biodiversity of our planet. These catastrophic events, marked by a rapid decline in species diversity, have occurred multiple times throughout Earth’s history. In this article, we will explore the five major mass extinctions that have taken place and examine their profound impacts on the evolution of life.

1. Ordovician-Silurian Extinction (450-440 million years ago): The Ordovician-Silurian Extinction stands as the second largest mass extinction in Earth’s history. This event eradicated 27% of all families, 57% of all genera, and approximately 60% to 70% of all species. Predominantly affecting small marine organisms, this extinction event was driven by a combination of climate change, glaciation, and a drop in sea levels.
2. Devonian Extinction (370-360 million years ago): Approximately 19% of all families, 50% of all genera, and at least 70% of all species were lost during the Devonian Extinction. Tropical marine species suffered the most, and the causes behind this event remain uncertain. Proposed factors include global cooling, asteroid impacts, sea-level fluctuations, and a decline in oxygen levels in the oceans.
3. Permian-Triassic Extinction (252 million years ago): The Permian-Triassic Extinction, also known as the “Great Dying,” holds the dubious distinction of being the most devastating mass extinction in Earth’s history. It resulted in the loss of 57% of all families, 83% of all genera, and a staggering 90% to 96% of all species. This event affected a wide range of organisms, including many vertebrates. Possible causes include massive volcanic activity, global warming, ocean acidification, and anoxic conditions.
4. Triassic-Jurassic Extinction (200 million years ago): The Triassic-Jurassic Extinction witnessed the extinction of around 23% of all families, 48% of all genera, and 70% to 75% of all species. The decline of other vertebrate species on land paved the way for the flourishing of dinosaurs. Potential causes include large-scale volcanic eruptions, climate change, and the opening of the Atlantic Ocean, which altered marine ecosystems.
5. Cretaceous-Tertiary Extinction (66 million years ago): The Cretaceous-Tertiary (K-T) Extinction, also known as the K-Pg extinction, marks the end of the non-avian dinosaurs’ reign on Earth. This event resulted in the loss of approximately 17% of all families, 50% of all genera, and 75% of all species. The impact of a massive asteroid or comet near Mexico’s Yucatan Peninsula, combined with volcanic eruptions and climate change, likely triggered this mass extinction.

K-T Extinction

Sixty-six million years ago, a cataclysmic event forever altered the course of life on Earth. Known as the K-T mass extinction, this phenomenon occurred at the boundary between the Cretaceous (K) and Tertiary (T) periods. It wiped out more than three-fourths of all plant and animal species, including iconic creatures like the dinosaurs. In this article, we delve into the details of the K-T extinction, its far-reaching consequences, and the survival of a select few lineages.

1. The Great Extinction: The K-T extinction marked the end of the Mesozoic Era, eradicating numerous animal lineages that thrived during this period. Dinosaurs, once dominant on land, were among the most notable casualties, with nearly all species vanishing. Marine invertebrates also suffered greatly, with a significant decline in diversity. The event brought about the demise of many key elements of the Mesozoic Era, reshaping the trajectory of life on our planet.
2. Surviving Archosaurs: While most archosaurs perished during the K-T extinction, a few lineages managed to survive. These surviving archosaurs eventually gave rise to modern birds and crocodilians. These resilient creatures, bearing evolutionary ties to their prehistoric ancestors, provide a glimpse into the past and offer clues about the world before the mass extinction.
3. Impacts on Marine Life: The K-T extinction had a profound effect on marine flora and fauna. Among the planktonic organisms, such as coccolithophores and planktonic foraminifera, only about 13 percent of genera survived. Free-swimming mollusks experienced a significant decline, with many species becoming extinct. Larger foraminifers, specifically the orbitoids, vanished, while hermatypic corals dwindled to just one-fifth of their original genera. Rudist bivalves also disappeared from the Earth’s oceans.

Causes of K-T extinction

The K-T extinction event, also known as the Cretaceous-Tertiary extinction event, was a catastrophic event that resulted in the mass extinction of numerous plant and animal species, including the dinosaurs. Several theories have been proposed to explain the causes of this event, but two major theories stand out: the Extra-planetary impact theory and the Deccan Traps flood basalts hypothesis.

Extra-planetary impact theory

The Extra-planetary impact theory gained significant attention when a group of scientists led by Luis Alvares, Walter Alvarez, Frank Asaro, and Helen Michel discovered a global layer rich in iridium at the K-T boundary. Iridium is rare on Earth’s crust but abundant in asteroids and comets. This finding led the team to suggest that an asteroid struck the Earth, causing the K-T extinction event. Subsequent research identified the Chicxulub crater buried beneath the Yucatán Peninsula as the source of the K-T boundary clay. This massive impact generated an enormous amount of energy, equivalent to billions of atomic bombs, resulting in a global catastrophe.

The impact caused immediate devastation, including a brief but intense pulse of infrared radiation that likely cooked exposed organisms. It also resulted in long-term disruptions to the Earth’s geochemistry and climate, leading to widespread ecological devastation. The impact generated a cloud of soot, dust, and debris that blocked sunlight for years, severely impacting photosynthesis. This lack of sunlight led to the death of phytoplankton and land plants, subsequently affecting herbivores and carnivores.

Furthermore, the impact released sulphuric acid aerosols into the stratosphere, leading to acid rain. This acidification of the seas resulted in the death of many organisms, particularly those with calcium carbonate shells. The impact also caused a significant drop in sea surface temperatures, affecting marine ecosystems.

The worldwide fires ignited by the impact released large amounts of carbon dioxide, which, combined with the dust and debris cloud, caused global cooling. However, after the dust settled, the increased concentration of CO2 led to global warming, further exterminating the already vulnerable organisms that survived the initial impact.

Deccan Traps flood basalts Hypothesis

The second major theory, the Deccan Traps flood basalts hypothesis, suggests that the massive volcanic eruptions in the Deccan Traps region of India played a role in the K-T extinction event. These eruptions began around 66.25 million years ago and lasted for about 30,000 years. The volcanic activity released dust and sulphuric aerosols into the air, potentially blocking sunlight and reducing photosynthesis in plants. Additionally, the emissions of carbon dioxide from the Deccan Traps volcanism could have intensified the greenhouse effect once the dust and aerosols cleared from the atmosphere.

Some geophysical models propose a combination of the impact and the Deccan Traps eruptions as contributing factors to the K-T extinction event. These models, supported by high-precision radiometric dating, suggest that the Chicxulub impact may have triggered some of the largest Deccan eruptions and even influenced eruptions in other active volcanoes worldwide.

In conclusion, the causes of the K-T extinction event are still a subject of scientific investigation, and while the Extra-planetary impact theory and the Deccan Traps flood basalts hypothesis provide plausible explanations, it is likely that a combination of both factors, along with other environmental and ecological factors, contributed to the mass extinction event that reshaped life on Earth.

Causes of extinctions

Extinctions throughout Earth’s history have been caused by a variety of factors. Building on the ideas from the provided content, here are some of the major causes of extinctions:

1. Volcanic eruptions or Flood basalt events: Massive volcanic eruptions, known as flood basalt events, can release large amounts of dust, particulate aerosols, sulphur oxides, and carbon dioxide into the atmosphere. These emissions can inhibit photosynthesis, cause acid rain, and contribute to sustained global warming, ultimately leading to the collapse of food chains and extinctions.
2. Sea-level falls: Global cooling or the sinking of mid-ocean ridges can result in sea-level falls. This can disrupt weather patterns, leading to extinctions both on land and in the oceans. Sea-level falls have been associated with several mass extinctions throughout history.
3. Impact events: The impact of large asteroids or comets can cause widespread devastation. The energy released from such impacts can cause food chains to collapse by producing dust and particulate aerosols, inhibiting photosynthesis. Impact events can also result in mega tsunamis, global forest fires, and the emission of sulphur oxides, further contributing to extinctions.
4. Global cooling: Sustained and significant global cooling can lead to the death of many polar and temperate species. This can reduce the available habitat for tropical species and result in arid conditions due to the locking of water in ice and snow. Global cooling has been linked to several mass extinction events, including the End-Ordovician, Permian-Triassic, and Late Devonian extinctions.
5. Global warming: The opposite of global cooling, sustained global warming can expand the habitat for tropical species while causing severe extinctions of polar species. It can also lead to the melting of polar ice and snow, increasing the volume of the water cycle and potentially causing anoxic events in the oceans. Global warming has been implicated in mass extinctions such as the Paleocene-Eocene Thermal Maximum and the Triassic-Jurassic extinction.
6. Clathrate gun hypothesis: Methane clathrates, formed when methane is trapped in water cages, can release large amounts of methane due to sudden temperature rises or pressure drops. This release, known as a “clathrate gun,” can cause rapid global warming or intensify existing warming due to the potent greenhouse effect of methane. Clathrate gun eruptions have been suggested as potential causes of the end-Permian extinction and the Paleocene-Eocene Thermal Maximum.
7. Anoxic events: Anoxic events occur when oxygen becomes deficient or absent in the middle and upper layers of the ocean. This can be caused by sustained global warming or massive volcanism. Anoxic events lead to a drop in the bio-availability of essential elements like selenium, resulting in lethal conditions for many organisms. Anoxic events have been associated with several mass extinctions, including the Ordovician-Silurian, late Devonian, Permian-Triassic, and Triassic-Jurassic extinctions.
8. Hydrogen sulphide emissions from the seas: During the Permian-Triassic extinction event, global warming disrupted the balance between photosynthesizing plankton and deep-water sulphate-reducing bacteria. This disruption caused massive emissions of hydrogen sulphide, a poisonous gas that weakened the ozone layer and exposed life to fatal levels of UV radiation.
9. Oceanic overturn: Disturbances in thermo-haline circulation, the vertical circulation of water in the ocean, can cause surface water to sink and bring anoxic deep water to the surface. This sudden influx of anoxic water can kill oxygen-breathing organisms that inhabit the surface and middle depths of the ocean. Oceanic overturn has been suggested as a cause of the late Devonian and Permian-Triassic extinctions.
10. Gamma ray burst: A nearby gamma-ray burst can destroy the Earth’s ozone layer, leaving organisms vulnerable to harmful ultraviolet radiation from the Sun. While gamma-ray bursts are rare, occurring only a few times per million years in a given galaxy, they have been proposed as a possible cause of the End-Ordovician extinction.
11. Plate tectonics (Movement of the continents): The movement of continents can cause extinctions in various ways. It can initiate or end ice ages, alter climate through changes in ocean and wind currents, and expose previously isolated species to new competition. The fusion of continents can destroy the continental shelf, the species-rich part of the ocean, while converting coastal regions to interior land with different climatic conditions and extreme seasonal variations.

It’s important to note that these causes of extinctions can often interact and have cumulative effects, leading to more severe ecological disruptions and higher extinction rates. Additionally, the exact causes and mechanisms of past extinctions are still areas of ongoing scientific research and investigation.

Role of Mass extinctions in evolution

Mass extinctions play a significant role in the process of evolution. Drawing ideas from the provided content, here are some ways in which mass extinctions influence evolutionary patterns:

1. Stimulating the Evolution of New Lineages: Mass extinctions result in the elimination of specific lineages and their descendant species. This reduction in diversity creates opportunities for new lineages to emerge and evolve. With fewer competitors and vacant ecological niches, surviving species have the chance to diversify and occupy new evolutionary trajectories.
2. Opening Up New Habitats and Niches: The sudden disappearance of plants and animals that previously occupied specific habitats creates vacant spaces and resources. Surviving species, which may have previously occupied different habitats or ecological roles, can adapt and specialize to exploit these newly available resources. This can lead to the emergence of new adaptations and the evolution of species specifically adapted to these vacant habitats.
3. Evolutionary Pathways Divergence: Following a mass extinction event, surviving species may evolve in different directions than the lineages that existed before the extinction. With reduced competition and altered environmental conditions, evolutionary pressures can lead to the emergence of novel traits, behaviors, and ecological strategies. This can result in the evolution of species that are distinct from their predecessors and may explore new evolutionary pathways.
4. Expanding Diversity through Vacant Niches: Mass extinctions create a significant reduction in the number of species within ecosystems. This reduction in species competition leaves behind numerous vacant ecological niches. These vacant niches serve as opportunities for the surviving lineages to diversify and occupy diverse ecological roles. The availability of vacant niches allows for adaptive radiation, where species rapidly diversify to fill various ecological niches and exploit available resources.

A notable example is the impact of the end-Cretaceous mass extinction event on mammalian evolution. Prior to this event, mammals were a relatively small group of rodent-like organisms. However, with the extinction of dominant reptilian groups, mammals had the opportunity to occupy new ecological roles and exploit vacant niches. This led to a rapid diversification and expansion of mammals into various forms and lifestyles, resulting in the great diversity of mammalian species we see today.

In summary, mass extinctions reshape ecosystems by eliminating certain lineages and opening up new opportunities for surviving species. These events create ecological vacuums, reducing competition and enabling the evolution of new lineages, adaptations, and ecological strategies. Mass extinctions have played a crucial role in shaping the trajectory and diversity of life on Earth throughout evolutionary history.

Differences Between Background Extinction and Mass Extinction

Characteristics	Background Extinction	Mass Extinction
Definition	Slow and gradual extinctions	Sudden and devastating events
Classification	Prevalent throughout geological time	Occur at intervals of roughly 100 million years
Types	Result from factors affecting reproduction	Often not directly caused by cataclysmic events
Symptoms	Responsible for the majority of extinct species	Small number of species affected by major extinctions

FAQ

References

• https://www.tutorialspoint.com/difference-between-background-extinction-and-mass-extinction
• https://ourworldindata.org/mass-extinctions
• https://www.thoughtco.com/the-5-major-mass-extinctions-4018102
• https://newcollege.ac.in/CMS/Eknowledge/e67541dd-8b64-480c-b043-a2e21cf66fa7Mass%20Extinctions.pdf
• https://flexbooks.ck12.org/user:planetfinder/cbook/origins-and-the-search-for-life-in-the-universe/section/8.2/primary/lesson/five-major-mass-extinctions/
• https://courses.lumenlearning.com/wm-biology2/chapter/mass-extinctions/
• https://www.nettelhorst.org/ourpages/auto/2018/1/29/55776071/Five%20Major%20Mass%20Extinctions.pdf