The extinction of the dinosaurs at the end of the Cretaceous period, around 66 million years ago, is one of the most well-known mass extinction events in Earth’s history. But what exactly caused the demise of these prehistoric beasts after 165 million years of dominance on land? Many theories have been proposed over the decades, with the leading contender being an asteroid impact that triggered catastrophic changes to the global climate.
When Did the Dinosaurs Go Extinct?
The Cretaceous-Paleogene (K-Pg) extinction event marks the end of the Mesozoic Era and the beginning of the Cenozoic Era. This mass extinction wiped out around 75% of plant and animal species on Earth, including all non-avian dinosaurs. It occurred around 66 million years ago at the end of the Cretaceous period.
The transition between the Cretaceous and Paleogene periods is clearly visible in the geological record as a layer of sediment enriched in iridium and other elements. This layer, known as the K-Pg boundary, can be found all over the world and contains abundant evidence of a sudden catastrophe.
What Animals Went Extinct?
The K-Pg extinction event eliminated a remarkably diverse range of organisms from both marine and terrestrial ecosystems. Here are some of the major groups that completely disappeared:
- Non-avian dinosaurs – All dinosaurs except for the lineage leading to modern birds were wiped out. This includes iconic species like T. rex, Triceratops, Stegosaurus, and Ankylosaurus.
- Plesiosaurs and mosasaurs – These were the dominant marine reptiles of the Cretaceous oceans.
- Pterosaurs – Flying reptiles including Pteranodon and Quetzalcoatlus.
- Ammonites – This group of marine invertebrates with coiled shells had survived multiple extinctions since the Devonian period.
Many other organisms like certain families of bivalves, echinoids, and foraminifera (single-celled protists) also abruptly disappeared from the fossil record at the K-Pg boundary. On land, major losses occurred among plants like ferns, conifers, and flowering plants.
Theories on What Caused the Extinction
Ever since the K-Pg mass extinction was uncovered in the geological record, scientists have debated what factors could have driven so many species to annihilation so rapidly. Here are some of the leading theories that have been proposed over the years:
Asteroid Impact
The dominant theory today is that an asteroid or comet around 6 miles (10 km) wide slammed into Earth around 66 million years ago, triggering catastrophic changes. Evidence for this impact comes from global layers of sediment enriched in iridium and shocked quartz grains at the K-Pg boundary. These materials are rare on Earth but abundant in space rocks. Scientists even identified a massive impact crater buried under Mexico’s Yucatan Peninsula that matches the extinction timeline.
Volcanic Eruptions
Intense volcanic activity, especially large igneous provinces releasing millions of cubic miles of lava, could have caused climate changes leading to ecosystem collapse. The massive Deccan Traps formations in India began erupting just before the K-Pg extinction event.
Climate Change
Global cooling and sea level regression occurred towards the end of the Cretaceous period. Climate shifts related to tectonic plate movements or natural carbon cycle fluctuations may have placed gradual stress on many species.
Combination of Factors
The precise causes are still up for debate, but many scientists believe the asteroid impact was likely the immediate trigger for mass extinction. However, existing climate stresses and volcanic activity over the preceding million years could have made ecosystems more vulnerable when the asteroid struck.
How an Asteroid Impact Caused a Mass Extinction
The asteroid impact hypothesis, first proposed by Walter Alvarez and colleagues in 1980, provides the most direct and catastrophic explanation for what killed off the dinosaurs seemingly overnight in geological terms. Let’s look at how an extraterrestrial impact could have led to ecological collapse 66 million years ago:
Blast and Thermal Radiation
The asteroid or comet would have instantly vaporized upon impact, releasing unimaginable energy equivalent to billions of nuclear weapons. Intense heat and shockwaves would radiate out from the blast site, incinerating anything nearby.
Flying Debris
Molten asteroid fragments and rocky debris from the impact site would have rained down across the globe. This fiery barrage would have sparked widespread wildfires.
Dust and Particles
Tremendous amounts of dust, ash, and other particles would have been launched kilometers up into the atmosphere, enshrouding the planet for years. This would have blocked out sunlight and inhibited photosynthesis.
Acid Rain
Chemical reactions in the upper atmosphere would have produced nitric and sulfuric acid precipitation down to the surface. This acid rain would have poisoned freshwater and marine ecosystems.
Climate Changes
In the long run, the dust and particles in the atmosphere would have caused significant global cooling. Photosynthesis would have decreased, disrupting food chains. Meanwhile, the greenhouse effects of CO2 released from combustion may have later warmed the planet again.
Tsunamis and Earthquakes
The seismic shaking from the impact could have triggered tsunamis, earthquakes, and volcanic activity around the world. Giant waves would have surged inland and disrupted coastlines.
The combined effects of these catastrophic phenomena occurring simultaneously would have overwhelmed Cretaceous ecosystems in a very short time, ultimately leading to the demise of the dinosaurs along with many other groups.
Evidence Supporting an Asteroid Impact
Let’s examine some of the key pieces of evidence that support the theory that an extraterrestrial impact triggered the Cretaceous-Paleogene mass extinction:
Iridium Anomaly
There is a thin global layer of sediment enriched in the element iridium precisely at the K-Pg boundary that contains shocked mineral grains and other impact markers. Iridium levels are 30 times higher than normal in this layer compared to the rest of the geological record. Since iridium is rare in the Earth’s crust but more abundant in asteroids and comets, this suggests that material from an extraterrestrial object suddenly rained down onto the planet at this time.
Chicxulub Crater
An ancient impact crater measuring 110 miles (180 km) wide buried under the Yucatan Peninsula in Mexico has been dated to approximately 66 million years ago, matching the timing of the extinction. Core samples from the crater contain high levels of iridium and melted rock formations, indicating a massive impact from space.
Shocked Quartz
Sand-sized grains of quartzite in the K-Pg layer show microscopic deformations indicative of sudden, intense pressures. These shocked quartz crystals can only form under extreme conditions like a massive impact or nuclear explosion. Volcanic eruptions do not generate enough pressure to produce shocked quartz.
| Evidence | Description | Implication |
|---|---|---|
| Iridium anomaly | Thin global layer enriched in rare element iridium at K-Pg boundary | Asteroid or comet impact sprinkled extraterrestrial material over Earth |
| Chicxulub crater | 110 mile wide impact crater in Mexico dated to 66 million years ago | Massive asteroid or comet struck Earth at extinction time |
| Shocked quartz | Sand grains show deformations from intense pressure | Only massive impact or nuclear blast creates enough force to shock quartz |
Fern Spike
There is a sudden proliferation of fern pollen grains in the fossil record precisely at the K-Pg boundary layer across numerous sites worldwide. Ferns are often the first plant species to colonize disturbed environments, so this “fern spike” indicates widespread habitat destruction and loss of forest species.
Species Selectivity
The extinction patterns show certain groups, like ammonites, mosasaurs, and larger animals, suffered higher losses than smaller or more widespread generalist species. This is consistent with an abrupt catastrophe.
The agreement between multiple independent lines of evidence from sedimentology, geochemistry, paleontology, and crater geology makes a compelling case that an asteroid or comet impact triggered the dinosaur extinction 66 million years ago.
Alternative Theories
While the extraterrestrial impact theory is widely accepted by most scientists, some challenges and alternative possibilities have been proposed over the years:
Deccan Traps Eruptions
The massive flood basalts of the Deccan Traps in India began erupting just before the K-Pg extinction. Proponents of this theory argue that volcanic outgassing and ash could have caused climate changes. However, others counter that the Deccan eruptions occurred over hundreds of thousands of years, too gradually to explain such an abrupt extinction event.
Multiple Impacts
In addition to the Chicxulub crater, some scientists have proposed that multiple large asteroid or comet impacts may have contributed to the K-Pg extinction. Candidate craters have been found in Ukraine and Texas around the extinction boundary, but there is ongoing debate about these sites.
Gradual Climate Change
Some scientists think the extinction was more gradual, citing fossil evidence of ecosystem decline during the last 1-2 million years of the Cretaceous. They argue that shifting climate zones and sea levels caused a slow attrition of species vulnerable to environmental changes.
While these factors may have created background stresses, the evidence indicates that the K-Pg mass extinction was geologically abrupt, occurring within a few decades at most. This suggests a more cataclysmic trigger like an asteroid impact dealt the final blow to finish off the dinosaurs.
Why Did Some Animals Survive?
The K-Pg extinction eliminated all dinosaurs except for the birds. However, some other animal groups like mammals, snakes, crocodilians, turtles, amphibians, and certain fish managed to survive. Why did these organisms live on while others perished?
Some key factors that helped determine survival:
- Small size – smaller animals required less food and resources to sustain viable populations.
- Wide distribution – widespread generalist species avoided complete annihilation.
- Ability to burrow underground or hide in water – this provided shelters from the initial blast, heat, and debris.
- Flexibility – adaptable omnivorous or scavenging diets proved advantageous in the post-impact disturbed habitats.
- Freshwater habitats – freshwater ecosystems were less devastated than exposed marine ones.
Mammals in particular were typically small, generalized foragers and burrowers. This allowed them to find shelter in burrows and subsist on dead organic matter in the aftermath. During the Paleocene recovery, mammals diversified into open niches once occupied by larger dinosaurs and marine reptiles.
Conclusions
In summary, the Cretaceous-Paleogene extinction 66 million years ago was one of the most dramatic mass extinction events in Earth’s history, famously killing off all dinosaurs except birds. An asteroid or comet impact, with its catastrophic effects like dust clouds, acid rain, climate changes, and tsunamis, appears to have triggered this rapid loss of biodiversity. The Chicxulub impact crater and global iridium layer provide strong evidence for this impact scenario. Gradual climate shifts or volcanic activity may have also stressed ecosystems, but the agreement of paleontological, sedimentological, and geochemical data points to an extraterrestrial impact as the final death blow. In just a short time, the reign of the non-avian dinosaurs after 165 million years came to a sudden end, paving the way for the rise of mammals.