Our series on mass extinctions has explored Earth’s five mass extinctions: the End-Ordovician, the Late Devonian, the End Permian, the End-Triassic, and the End-Cretaceous (K-Pg) extinction.
We are at the last mass extinction event. What caused the End-Cretaceous mass extinction?
The End-Cretaceous mass extinction was caused by a 6-9 mile (10-14 kilometer) asteroid striking Earth near Mexico’s Yucatán Peninsula 66 million years ago. The impact released energy equivalent to 10 billion Hiroshima bombs, triggering global cooling, wildfires, and ocean acidification. This wiped out 75% of species.
We’ll explore life on Earth during the Mesozoic Era, leading to its final, dramatic chapter. This should be especially fascinating for dinosaur enthusiasts.
Beginnings of Mesozoic Life
The Mesozoic Era began after the End-Permian event, known as the “Great Dying.” This occurred about 252 million years ago. The Great Dying was so severe that it ended the Paleozoic Era and launched the Mesozoic Era or the “Age of Reptiles.”
While this new era began with a depleted biosphere, it would soon give rise to some of Earth’s most remarkable creatures. Britannica delves deeper into the significance of the Mesozoic Era.
Ironically, the “Age of Reptiles” began with few reptiles on Earth. The Permian extinction had wiped out most species.
Surviving species underwent adaptive radiations. By the end of the Triassic Period (252 to 201 million years ago), Earth recovered much of its lost biodiversity.
End-Triassic Shake-Up
At the end of the Triassic Period, the Mesozoic Era was embroiled in another mass extinction. This event paved the way for dinosaurs to evolve and diversify rapidly.
During the Jurassic Period and into the Cretaceous, dinosaurs increased in abundance and diversity. They completely dominated terrestrial ecosystems.
A similar trend occurred in oceans, where aquatic reptiles evolved and diversified. These marine reptiles soon occupied the top positions in their ecosystems.
Terrestrial and aquatic reptiles weren’t the only animals thriving during the Mesozoic. Many other clades of organisms also diversified and thrived.
Pterosaurs ruled the skies and the first birds appeared. Mammals skittered underfoot. Meanwhile, ammonites, sharks, bivalves, and gastropods filled the oceans.
The picture above is a Jurassic ammonite from Bavaria, Germany.
As the dust settled from the End-Triassic extinction, the stage was set for one of the most dramatic periods in Earth’s history: the rise of the dinosaurs.
Mesozoic Plants
Plant life also changed dramatically during the Mesozoic. During the Jurassic Period, 201 to 145 million years ago, cycads, conifers, and other gymnosperms, flowerless, seed-bearing plants, dominated the land. By the late Cretaceous Period, the first angiosperms, or flowering plants, had evolved and diversified.
This image shows a fossil cycad trunk from the Jurassic Morrison Formation of the western United States. Below is a picture of a modern-day cycad. Sights like this would have been much more common during the Jurassic Period.
During the Mesozoic, ferns and horsetails formed the understory beneath the towering gymnosperms. This period also saw the development of key plant adaptations we see today.
These include deeper root systems, more efficient water-conducting tissues, and new pollination methods. When flowering plants emerged in the Cretaceous, they triggered a co-evolution with insects, particularly bees and butterflies, leading to rapid diversification of both plants and their pollinators.
A Cretaceous ancestor of the modern-day Tulip Poplar can be found in this Sci-News article. As you can tell, the Mesozoic was crucial in the evolution of many modern life forms.
Plants revolutionized terrestrial ecosystems and established the foundation for modern plant communities. With diverse plant life providing the foundation, the Late Cretaceous would become a golden age for terrestrial vertebrates.
End-Cretaceous Life
By the end of the Cretaceous Period (66 million years ago) dinosaurs dominated the Earth. Some of the most famous dinosaurs lived during this time.
In the land that would become the western United States, for example, one could find:
- Triceratops
- Tyrannosaurus
- Edmontosaurus
- Ankylosaurus
- Pachycephalosaurus
Not bad for a dino safari! These are some of the most well-known dinosaurs. Many other creatures shared this world. The seas were full of creatures such as:
- Ammonites
- Mosasaurus
- Elasmosaurus
- Atractosteus (gar)
- Sharks
Again, these are some iconic creatures. On land, dinosaurs lived with many diverse animals. Late Cretaceous terrestrial life included:
- Turtles
- Crocodiles
- Champsosaurus
- Early mammals
- Primitive birds
- Diverse insect groups
Pictured below is a fossil Champsosaurus skeleton. More on this creature later.
Together, these creatures formed complex ecosystems that shaped the world of the Late Cretaceous. This thriving world was about to face a catastrophe that would reshape Earth’s history forever.
Asteroid Impact
The age of the dinosaurs didn’t last forever. About 66 million years ago, an asteroid measured 6 to 9 miles wide. 10 to 14 kilometers, struck Earth near the Yucatán Peninsula.
The impact caused worldwide destruction, wiping out the dinosaurs and many other species. Scientists estimate that the energy released was equal to 10 billion Hiroshima bombs.
When the asteroid struck, it set off a chain of catastrophic events. First, extreme earthquakes and tsunamis devastated the western half of the globe. Then, the impact sent vast amounts of dust into the atmosphere, leading to a “nuclear winter.”
This drastic cooling slowed photosynthesis worldwide, putting intense pressure on ecosystems. Photosynthetic organisms are the foundation of food chains. Talk about a bad day for life on Earth!
To make matters worse, the larger debris launched into the air from the impact fell back to Earth. This caused warming due to friction with the air.
The sudden increase in temperature from the debris raining down on Earth was devastating. It likely triggered wildfires that literally cooked any creatures unfortunate enough to be caught in the deluge.
Even worse, the dust hurled into the atmosphere during the impact wasn’t just ordinary dust. It carried sulfates and other nasty chemicals.
When these chemicals eventually made their way into the oceans, they caused the water to acidify. This chemical shift was bad news for shell-building organisms.
It made it nearly impossible for them to construct their shells. Unable to adapt quickly enough, many of these species were victims of the aftermath.
All of this sounds very bleak, but what evidence do we have? How do we know it was an impact that brought about this extinction?
The asteroid impact theory required extraordinary evidence to match its extraordinary claim. Scientists found their smoking gun in the K-Pg boundary
The K-Pg boundary is a distinctive layer of rock marking the exact moment between the age of dinosaurs and the age of mammals. This thin stripe in the rock record would reveal the evidence needed to solve the extinction mystery.
Evidence for the Asteroid Impact: the Iridium Anomaly
The strongest evidence for the asteroid impact lies in a peculiar worldwide layer of rock containing trace amounts of iridium, aptly named the “iridium anomaly.” This layer aligns perfectly with the End-Cretaceous extinction at our so-called K-Pg boundary.
Why is that important? Iridium is extremely rare on Earth’s surface. It’s practically nonexistent.
However, it’s a common ingredient in extraterrestrial materials like meteoroids. The unusual presence of this element in a global rock layer strongly suggests an asteroid caused this catastrophic event.
If you want to dig into the chemistry, check out this CNN article on the latest research.
Simply put, iridium is a telltale sign of something from outer space. The next image shows pure iridium, a rare metal on Earth but common in asteroids.
Scientists argue that the iridium anomaly offers compelling evidence of an asteroid impact. Why? Because iridium, being extraterrestrial in origin, is exactly what you’d expect from a giant space rock.
When the asteroid struck Earth and vaporized, it kicked up a massive dust cloud of iridium. As this cloud spread across the globe and eventually settled, it left behind a thin, iridium-rich layer in the rock record. This is our “smoking gun” pointing directly to an asteroid as the culprit.
Evidence for the Asteroid Impact: the Chicxulub Crater
When the asteroid impact hypothesis was first proposed in 1980, there was a problem: if an asteroid struck the Earth, it would have left a massive crater, but no such crater had yet been discovered. So where was it?
The breakthrough came during oil explorations off the coast of the Yucatan Peninsula. Scientists uncovered a colossal crater, stretching 110 miles (177 kilometers) in diameter, spanning parts of the Yucatan Peninsula and the Gulf of Mexico.
If you want to learn more about the Chicxulub impact, I’d suggest this article on nature.com.
The discovery came at the perfect time, aligning with the rise of the asteroid hypothesis. Analysis revealed the crater’s age matched the End-Cretaceous Event, leaving little doubt that this was the impact site.
Despite the strong evidence for an asteroid impact, some scientists propose alternative explanations for this mass extinction.
Competing Hypotheses
Not all scientists agree that an asteroid caused the End-Cretaceous Extinction. Instead, these scientists usually argue that massive volcanic eruptions in India, the Deccan Traps, caused the End-Cretaceous event.
| Potential Cause | Explanation |
|---|---|
| Chicxulub Asteroid Impact | Single massive impact caused rapid global climate changes, triggering worldwide ecosystem collapse. |
| Deccan Traps Volcanism | Extensive volcanic eruptions released greenhouse gases and sulfur, causing long-term climate and ocean chemistry changes. |
| Combined Impact-Volcano Theory | Volcanic activity weakened global ecosystems before the asteroid delivered the final devastating blow. |
| Multiple Impact Theory | Several smaller asteroid impacts over a short time caused cumulative environmental stress. |
| Marine Regression | Dramatic sea level drops exposed and destroyed shallow marine habitats where most species lived. |
| Cosmic Ray Burst | Nearby supernova or gamma-ray burst damaged the ozone layer, exposing life to lethal radiation. |
| Global Disease | A pandemic spread across weakened species due to climate stress. |
| Solar System Oscillation | Earth’s passage through the galactic plane increased cosmic ray exposure and comet impacts. |
This is an extensive list. Let’s get into the two primary alternative hypotheses to the impact-only cause.
The Deccan Traps: A World on Fire
The story of the Deccan Traps represents one of Earth’s most massive volcanic episodes. These eruptions in modern-day India released enormous volumes of lava, covering an area twice the size of Texas.
For roughly 900,000 years, the Deccan Traps spewed greenhouse gases and sulfur compounds into the atmosphere. This could trigger global climate shifts.
The most intense volcanic activity occurred just before the End-Cretaceous extinction. This suggests that these eruptions may have created environmental stresses that weakened Earth’s ecosystems.
Massive flood basalt eruptions triggered both the End-Permian extinction, Siberian Traps, and the End-Triassic extinction, Central Atlantic Magmatic Province (CAMP). The Deccan Traps hypothesis carries significant weight.
However, precise dating of the Deccan Traps reveals a crucial detail: the most intense volcanic activity peaked between 66.4 and 66.1 million years ago, several hundred thousand years before the mass extinction event at 66 million years ago.
The timing presents a challenge for scientists arguing that volcanism was the primary killing mechanism. This would mean that the volcanic eruptions may at most have put some stress on global ecosystems before the asteroid impact, although they didn’t directly lead to the mass extinction.
The One-Two Punch: Combining Volcanoes and Impact
Recent research points to a compelling synthesis of the two major theories. Instead of competing explanations, the Deccan Traps and Chicxulub impact may represent a devastating combination of events.
Think of it as a one-two punch. The volcanic eruptions first weakened global ecosystems through long-term climate changes. Then the asteroid delivered the knockout blow.
This combined theory explains the gradual decline in biodiversity observed in the fossil record and the sudden mass extinction event. It also accounts for evidence supporting both volcanic and impact scenarios, providing the most comprehensive explanation for one of Earth’s most significant biological crises.
While all these theories have been proposed in scientific literature, the Chicxulub impact, Deccan Traps, and Combined Theory have the strongest supporting evidence.
Alternative Extinction Theories: From Space Rays to Global Disease
While the asteroid impact and Deccan Traps dominate scientific discourse, several alternative theories have been proposed to explain the end-Cretaceous extinction.
Some researchers suggest multiple smaller asteroid impacts occurred over time rather than a single devastating strike. Others point to a dramatic drop in sea levels that could have destroyed crucial shallow marine habitats.
More exotic explanations include cosmic ray bursts from nearby supernovae, a global pandemic that spread through weakened ecosystems, and even the solar system’s oscillation through the galactic plane increasing Earth’s exposure to cosmic threats.
There are even more absurd explanations that we won’t get into here. Some involve caterpillars, low dinosaur sex drives, and even alien extermination. Yikes!
However, all of these alternative hypotheses face significant challenges. Most lack substantial geological evidence, struggle to explain the sudden nature of species loss or fail to account for the global distribution of extinction patterns.
While they offer interesting perspectives on potential extinction mechanisms, they remain secondary to the well-documented Chicxulub impact and Deccan Traps volcanism. Whatever the exact cause, the aftermath of this event was undeniable, a world forever changed.
Extinctions Were Devastating
About 70 to 80% of all species on Earth perished in the asteroid impact. All non-avian dinosaurs vanished, leaving only birds to carry on their legacy.
The extinction wasn’t limited to the famous dinosaurs. It wiped out entire branches of Earth’s tree of life. Animals that had dominated the seas and skies for millions of years vanished forever, fundamentally changing Earth’s ecosystems.
Pterosaurs disappeared alongside them, No creatures of their size have ruled the skies since.
In the oceans, marine reptiles vanished, along with ammonites, belemnites, inoceramids, and many other bivalves. The mighty mosasaurs, deprived of their prey, were doomed by the collapse of the food chain.
Here is an Inoceramid bivalve from the Cretaceous of South Dakota. These large, thick-shelled bivalves thrived in ancient seas but vanished in the mass extinction triggered by the asteroid impact. Their disappearance marked the end of a once-dominant group, alongside ammonites and other marine life.
Plants also suffered. Many species of angiosperms, flowering plants that had flourished during the Cretaceous, didn’t survive the devastation. The loss of sunlight from debris and wildfires disrupted ecosystems from the ground up.
Yet even in this global catastrophe, life found a way. Some plants endured, clinging to survival in sheltered environments, waiting for the world to heal.
Yet even in this global catastrophe, some species found ways to endure.
Survivors Took Control
Despite the losses of many marine organisms and the non-avian dinosaurs, many groups of organisms survived the impact of the asteroid. Many mammals survived, as did amphibians, crocodiles, turtles, snakes, and other reptiles.
Interestingly, just as the End-Triassic Event paved the way for dinosaurian hegemony, the End-Cretaceous Event paved the way for the mammals to rise to dominance. Once the dinosaurs were gone, niches opened up for newcomers.
After the impact, mammals underwent rapid evolution and diversification. The effects of this adaptive radiation are still apparent on Earth today.
The End-Cretaceous extinction reshaped life on Earth, highlighting both the fragility of life and its remarkable resilience.
What Caused the End-Cretaceous Mass Extinction and What’s Next?
With the End-Cretaceous extinction now discussed, we close out our series on Earth’s major mass extinctions. But is it truly the end?
Some scholars believe we are currently in the midst of a sixth mass extinction, a topic we will explore in an upcoming article. Stay tuned for more on this pressing issue.
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