Why Did the Dinosaurs Go Extinct?

The Asteroid Impact of 66 Million Years Ago

All non-avian dinosaurs went extinct at the end of the Cretaceous period (66 million years ago).
"What happened to the Earth 66 million years ago (*1)?" and "What triggered the extinction of the dinosaurs?" are frequently discussed topics in the media, along with general interest in dinosaurs.

The most likely answer is an "asteroid impact."

At the boundary between the geological layers of the Mesozoic Cretaceous, when dinosaurs went extinct, and the subsequent Cenozoic Paleogene, there is a layer with an abnormally high concentration of iridium. Iridium concentrations 20-160 times higher than in other layers have been observed.
Iridium is a substance that is almost non-existent in the Earth's crust but is abundant in meteorites. This layer is called the "K-Pg boundary" (formerly known as the "K-T boundary").

It was previously known that "a huge asteroid collided near Central America at the end of the Cretaceous" because "the K-Pg boundary contains special particles called shocked quartz, and these particles are larger near Central America than in Europe or the Pacific," and "the K-Pg boundary around the Gulf of Mexico and the Caribbean Sea contains many tsunami deposits."
The "asteroid impact as the cause of dinosaur extinction" theory has been widely supported, but there have been various counterarguments, such as the volcanic activity theory and the global cooling theory.

The discovery that definitively confirmed the "asteroid impact as the cause of dinosaur extinction" theory was the finding of a 180-kilometer-diameter crater (the impact site of a meteorite) off the coast of Mexico's Yucatán Peninsula in 1991.
Analysis of samples from this crater, called the Chicxulub crater, revealed that it was the site of the asteroid impact at the end of the Cretaceous and the cause of the K-Pg boundary.
Investigating this has made it easier to scientifically reconstruct the scenario of the dinosaur extinction. Iridium anomalies have been observed at over 300 K-Pg boundary sites worldwide.

The Dinosaur Extinction Scenario

The most widely accepted scenario for the dinosaur extinction is as follows:

1. An asteroid 10-15 kilometers in diameter that struck off the coast of Mexico's Yucatán Peninsula threw up a massive amount of dust.
2. The dust in the atmosphere blocked sunlight for about two years, preventing photosynthetic plants on land and near the sea surface (especially angiosperms, which require a lot of light) from growing.
   Fossils show that fungi, which do not require light, and ferns, which can grow with little light, flourished during this period.
3. The inability of angiosperms to grow led to the extinction of herbivorous dinosaurs that depended on them as their primary food source.
4. The extinction of herbivorous dinosaurs led to the extinction of the carnivorous dinosaurs that preyed on them.
   It is thought that the largest species, which required a large amount of food, disappeared first.

Furthermore, the rocks off the Yucatán Peninsula where the asteroid struck were rich in carbonates and sulfur.
It is known that the impact heated the sulfur in the rocks, causing it to evaporate and generate a large amount of sulfuric acid gas, which then fell as acid rain.
It has also been proposed that this strong acid rain destroyed the food chain and was a direct cause of the extinction.

The blocking of sunlight caused the Earth's climate to cool by 10-28 degrees Celsius, and this rapid cooling would have also affected plants and animals.
The angiosperms that temporarily disappeared were able to survive as pollen and seeds and made a comeback after the global environment stabilized.

The Circumstances of the Asteroid Impact

Many details about what happened in Mexico's Yucatán Peninsula 66 million years ago have also been revealed through analysis.

The 10-15 kilometer-diameter asteroid struck the Earth's surface at an angle of about 30 degrees. It is known that it collided from a north-northwesterly direction, throwing dust mainly towards North America.
The fact that the highest extinction rate of organisms was in North America is one of the pieces of evidence supporting the idea that the asteroid impact caused the mass extinction.

The impact speed was 20 kilometers per second (in the more familiar "kilometers per hour," that's 72,000 km/h. The top speed of a Shinkansen bullet train is about 300 km/h, so the speed is astounding).
The energy of the impact was about 1 billion times that of the atomic bomb dropped on Hiroshima. The temperature of the plume that rose from the impact reached 10,000 degrees Celsius, and the surface temperature became 260 degrees. The hot dust that was thrown up would have rained down on forests in a state of ignition, causing wildfires in various places.

The magnitude of the earthquake caused by the impact was over 11. Submarine landslides occurred, and a 300-meter-high tsunami surged. It is understood that this was a catastrophe of such a large scale that it had a global impact.

According to a paper published in the British scientific journal Nature in 2022, the season of the asteroid impact is estimated to have been spring in the Northern Hemisphere (autumn in the Southern Hemisphere).

Why Turtles and Crocodiles Survived

The asteroid impact 66 million years ago, and the temporary die-off of photosynthetic plants it caused, led to the extinction of many organisms, including dinosaurs.

So, what is the reason that "dinosaurs went extinct, while turtles, crocodiles, and small mammals survived"?

It is explained by [habitat], [dependence on photosynthetic plants], and [body size].
In the end-Cretaceous mass extinction, 90% of terrestrial organisms disappeared, whereas only 10% of organisms living in freshwater environments like rivers and swamps went extinct.

In addition to the food chain based on photosynthetic plants, freshwater environments like rivers and swamps have a well-developed detritus food chain. This is an energy chain based on fallen leaves, decaying wood, animal carcasses, insects, and subterranean organisms.
For organisms within the detritus food chain, the "disappearance of fresh plants and animals" did not immediately cause catastrophic damage. This is also evident from the fact that fungi and ferns showed abnormal proliferation immediately after the asteroid impact. For them, the die-off of photosynthetic plants for a few years did not mean an immediate loss of their energy source.

Turtles and crocodiles belonged to this chain.

Some terrestrial organisms, such as small mammals (like rodents), also managed to survive. Their small body size contributed to their survival in the harsh environment. A "small body" means "less food is required" and "allows for reproduction from a young age (faster generation turnover)." Also, as mentioned earlier, since the asteroid impact occurred during autumn in the Southern Hemisphere, mammals and reptiles that were preparing for hibernation in burrows may have been less affected. However, they were by no means unscathed. Some species of turtles, crocodiles, and rodents went extinct, while others survived and were able to flourish afterwards.

Other Extinction Theories

While the "asteroid impact theory" is currently the most widely accepted, there are other theories that attempt to explain the end-Cretaceous mass extinction.
The main ones are the "volcanic eruption theory" and the "gradual extinction theory."

The volcanic eruption theory suggests that environmental destruction was caused by sustained volcanic activity at the end of the Cretaceous.
This theory is based on the fact that iridium anomalies can also be confirmed from volcanic eruptions, and evidence of volcanic activity called flood basalts, formed at the end of the Cretaceous, is found in the Deccan Traps of India. It seems that lava flows occurred at almost the same time as the asteroid impact—about 66.04 million years ago. According to research published in 2015, it is also suggested that the asteroid impact may have been the cause of the lava flows.

The gradual extinction theory suggests that the number of dinosaur species was already gradually decreasing, and that "the asteroid impact was unrelated to the extinction of the dinosaurs (or was just the final blow)."
The research group that proposes this theory bases it on the fact that they have examined the numbers of 30 dinosaur species and found that their populations had declined by the K-Pg boundary. They explain that a change in sea level reduced the land suitable for dinosaur habitats, causing a drastic decrease in the population of species, and that they could not maintain a large enough population to withstand environmental changes such as volcanic activity.

Both the "volcanic eruption theory" and the "gradual extinction theory" are becoming less persuasive due to insufficient evidence and contradictions. The analysis results from the Chicxulub crater off the coast of Mexico's Yucatán Peninsula have given the "asteroid impact theory" a solid standing.