Mass Extinction Events

Podcast Transcript

We like to think of the Earth as a very stable place. While there might be seasonal variation in the weather, things don’t really change that much within our lifetimes.

However, if you take a longer perspective, a much longer perspective, things can change a lot.

In fact, there have been five times in the history of the Earth when life on Eath completely changed. When over half of the species on the planet completely disappeared.

Learn more about the Earth’s mass extinction events and what caused them on this episode of Everything Everywhere Daily.

About 550 million years ago, complex multicellular life on Earth appeared.

The path from the Cambrian Explosion to today was not a straight one. In fact, it involved several devastating periods when an enormous amount of life on Earth died.

Although there have been many extinction events, paleontologists have identified five that are known as mass extinction events.

Paleontologists identify mass extinction events by studying the fossil record, where they observe abrupt and widespread disappearances of numerous species within relatively narrow layers of sedimentary rock.

These extinction horizons often coincide globally and are marked by a sharp decline in biodiversity, followed by the appearance of new species in higher layers. Additional evidence includes changes in the types and abundance of fossils, shifts in isotope ratios, such as carbon or oxygen, indicating environmental change, and the presence of geological markers like iridium layers, black shales, or ash deposits. Together, these clues provide a timeline and context for understanding past global crises and their biological impacts.

So, what I want to do is go through these five extinction events and explain what happened and why.

The Ordovician-Silurian extinction event, which occurred around 443 million years ago, is recognized as the first major mass extinction in Earth’s history. It marked the end of the Ordovician Period and the beginning of the Silurian Period. This extinction unfolded in two distinct pulses and resulted in the loss of approximately 85% of marine species, making it one of the most severe biodiversity crises ever recorded.

At the end of the Ordovician period, life on Earth was primarily marine. The seas teemed with invertebrates such as trilobites, brachiopods, bryozoans, crinoids, and mollusks.

The leading hypothesis for the cause of the Ordovician-Silurian extinction is global cooling due to glaciation. Geological evidence indicates that massive glaciers formed over the supercontinent Gondwana, which was located near the South Pole at the time.

As ice sheets expanded, global sea levels dropped significantly, leading to the draining of shallow marine environments that housed diverse ecosystems such as coral reefs and brachiopod communities. This loss of habitat devastated species that were highly specialized for these environments.

Another contributing factor was the disruption of ocean circulation and chemistry. The glaciation may have caused ocean anoxia, a condition in which oxygen levels in the ocean fall dramatically. Evidence for this includes the presence of black shales in sedimentary layers from that time, which are typically deposited in low-oxygen environments.

These changes would have further stressed marine life and caused widespread die-offs, especially among organisms sensitive to changes in temperature and oxygen availability.

Interestingly, there is also evidence that volcanic activity, gamma-ray bursts, and other cosmic events may have played secondary roles.

The second major extinction event was the Late Devonian extinction, which occurred around 372 million years ago, and was a prolonged crisis rather than a single catastrophic event. Spanning several million years, it is considered a series of extinction pulses rather than a singular, sharply defined boundary.

This extinction primarily affected marine life, resulting in the loss of around 75% of all species and significantly reducing biodiversity, particularly among reef-building organisms and armored fish known as placoderms.

The Devonian Period, often called the “Age of Fishes,” was characterized by the proliferation of diverse marine ecosystems and the early colonization of land by plants and arthropods. However, by the Late Devonian, several environmental stresses began to accumulate, contributing to widespread ecological collapse.

One major suspected cause of the Late Devonian extinction is global cooling associated with the expansion of land plants. The rapid evolution and spread of vascular land plants, particularly those with deep root systems, altered Earth’s carbon and nutrient cycles. Roots helped break down rocks, increasing the flow of nutrients like phosphorus into rivers and eventually into oceans.

This influx may have led to eutrophication, an over-enrichment of marine environments that triggered large algal blooms. These blooms, in turn, resulted in oceanic anoxia. The black shales found in Devonian marine sediments around the world provide strong evidence for this hypothesis, just as it did during the first mass extinction event.

This anoxic event, known as the Kellwasser Event, is well-documented in the geological record and marks one of the main extinction pulses during this time.

A second, slightly later pulse called the Hangenberg Event also shows signs of anoxia and possibly indicates another bout of environmental instability. These anoxic events disproportionately affected reef ecosystems, which nearly vanished by the end of the Devonian, and also severely impacted species like trilobites, brachiopods, ammonoids, and jawless fishes.

Another potential cause that has been explored is volcanic activity. Large-scale volcanism, possibly linked to the Viluy Traps in what is now eastern Siberia, could have contributed to atmospheric changes, including climate cooling and disruption of the carbon cycle.

The third major mass extinction event was the Permian-Triassic extinction, which occurred about 252 million years ago. It is the most severe mass extinction event in Earth’s history. The extinction marks the boundary between the Permian and Triassic periods and is often referred to as “The Great Dying.”

This cataclysm wiped out an estimated 96% of marine species, around 70% of terrestrial vertebrate species, and many groups of insects and plants. It reshaped the trajectory of life on Earth, creating ecological vacancies that early dinosaurs and modern marine faunas would eventually fill.

The primary cause of the extinction is believed to have been a massive episode of volcanic activity, specifically the Siberian Traps, a massive area of lava flows in central Siberia covering millions of square kilometers.

More critically, the volcanic activity released vast quantities of greenhouse gases, including carbon dioxide and methane, into the atmosphere. This triggered a sharp rise in global temperatures, leading to intense global warming.

Evidence for this volcanic origin includes radiometric dating of the Siberian Traps, which coincides closely with the timing of the extinction. Additionally, the carbon isotope record shows a major negative excursion, an abrupt drop in the ratio of carbon-13 to carbon-12, which indicates a massive injection of isotopically light carbon into the atmosphere and oceans.

This is consistent with both CO? from volcanic emissions and the destabilization of methane frozen in ocean sediments, which may have been triggered by warming.

The resulting climate change had cascading effects. The warming of ocean waters led to ocean stratification, preventing the normal mixing of surface and deep waters and contributing, once again, to widespread anoxia. This is supported by the global presence of black shales.

Some models and studies also suggest that anoxic conditions allowed sulfate-reducing bacteria to flourish, producing large amounts of hydrogen sulfide. This toxic gas may have poisoned marine and terrestrial life alike and contributed to the destruction of the ozone layer, increasing harmful ultraviolet radiation at Earth’s surface.

Acid rain likely further compounded the environmental crisis. Volcanic activity’s release of sulfur dioxide would have formed sulfuric acid in the atmosphere, leading to the acidification of terrestrial and marine environments. This affected plant life and aquatic organisms with calcium carbonate shells or skeletons. Fossil evidence indicates a collapse of coral reefs, a significant loss of plant species, and the disappearance of many large amphibians and reptiles.

Recovery from the Permian-Triassic extinction was prolonged, taking millions of years. This unusually slow rebound is another indicator of the event’s severity and is thought to be due to the long-lasting and compounding nature of the environmental problems.

The fourth major extinction event was the Triassic-Jurassic extinction, which occurred approximately 201 million years ago. It marks the transition between the Triassic and Jurassic periods and led to the disappearance of around 70–80% of all species, dramatically altering ecosystems on both land and in the oceans. While not as severe as the Permian-Triassic extinction, it was still a major turning point in Earth’s biological history, paving the way for the dominance of the dinosaurs in the Jurassic.

This extinction event unfolded relatively quickly on a geological timescale and affected a wide range of organisms. On land, many species of large amphibians and early archosaurs, which were reptile relatives of crocodiles and dinosaurs, went extinct. In the oceans, marine reptiles, ammonoids, bivalves, and conodonts suffered major losses. The extinction eliminated many ecological competitors and allowed dinosaurs, previously one of several groups of large terrestrial vertebrates, to become the dominant terrestrial animals.

The most widely supported cause of the Triassic-Jurassic extinction is massive volcanic activity associated with the initial rifting of the supercontinent Pangaea. This tectonic breakup led to the formation of the Central Atlantic Magmatic Province, one of the largest known continental volcanic regions in the Earth’s history. These eruptions released vast quantities of carbon dioxide, methane, and sulfur dioxide into the atmosphere, dramatically altering the global climate.

Geological evidence for this includes extensive lava flows found across what are now parts of North America, South America, Europe, and Africa—regions that once bordered the Central Atlantic Ocean. Radiometric dating of these volcanic rocks shows a close alignment with the extinction, suggesting a causal relationship.

The fifth and final mass extinction event was the Cretaceous-Paleogene extinction event, which occurred approximately 66 million years ago and is perhaps the most famous mass extinction in Earth’s history. It marks the end of the Cretaceous Period and the Mesozoic Era, and the beginning of the Paleogene Period and the Cenozoic Era.

This event led to the extinction of about 75% of all species on Earth, including all non-avian dinosaurs, and drastically reshaped life on the planet. It affected a wide range of organisms across marine, terrestrial, and freshwater ecosystems.

The most widely accepted and well-supported cause of this extinction is the impact of a large extraterrestrial object, most likely an asteroid approximately 10–15 kilometers in diameter. This impact created the Chicxulub crater, a massive structure over 180 kilometers wide, located in the Yucatán Peninsula of modern-day Mexico.

The evidence for this impact hypothesis is substantial. One of the key lines of evidence is a global layer of sediment, known as the K–Pg boundary clay, found at numerous sites around the world. This layer is enriched in iridium, a rare element on Earth’s crust but abundant in asteroids and comets.

The discovery of this iridium anomaly in 1980 by Luis and Walter Alvarez and their colleagues was a pivotal moment in understanding the cause of the extinction.

The impact’s most profound effect likely came from the climate disruption it caused. The collision would have thrown vast quantities of dust, aerosols, and vaporized rock into the atmosphere, blocking sunlight and creating a phenomenon known as “impact winter.”

This global darkening would have drastically reduced photosynthesis for weeks or months, collapsing food chains, especially marine plankton communities and plant life, which formed the base of most ecosystems. This period of darkness and cold was likely followed by a rapid warming due to the greenhouse gases released from the impact site and global wildfires.

While the Chicxulub impact is considered the primary trigger, some scientists suggest that it was not the only factor contributing to the extinction. During the same time frame, there is strong evidence of extensive volcanic activity in the Deccan Traps of present-day India.

The Deccan Traps eruptions released enormous volumes of lava, carbon dioxide, and sulfur dioxide over an extended period. These gases could have caused long-term climate instability, including warming, ocean acidification, and atmospheric toxicity, weakening ecosystems even before the asteroid impact occurred.

The fossil record across the K–Pg boundary shows abrupt extinctions, especially in marine species. On land, dinosaurs disappeared entirely, along with many plant and insect groups. Mammals, birds, crocodiles, turtles, and some amphibians survived and eventually flourished in the aftermath.

These five mass extinction events radically shaped our planet. With the exception of the Chicxulub impact, these events took place over hundreds of thousands or even millions of years. Long spans of time for a human, but brief periods in the history of the planet.

As devastating as these events were, if they hadn’t happened, we wouldn’t be here today.