The End-Cretaceous Mass Extinction

The end-Cretaceous mass extinction is the last, and probably the most well-known, of the "Big Five" mass extinctions (the others being the End Ordovician, Late Devonian, End Permian, and End Triassic). This event killed all the non-avian dinosaurs (i.e. all dinosaurs except birds). In this article we will discuss what caused this extinction event, how we discovered the cause, and the effect that it had on dinosaurs as well as other plant & animal groups present at the time.

When and What Happened?

Evidence

In the late 1970s/early 1980s, scientists discovered a thin (around 3mm) layer of clay in the rock layers from the end of the Cretaceous period in Italy. This same layer has since been found in several other locations worldwide, and analysis showed that it contained unusually high levels of iridium. Iridium is a metal which is commonly found in comets and asteroids, but is very rare in the Earth's crust. This is because it is a dense and siderophilic (iron-loving) metal - so while the Earth was molten, any iridium would likely have sunk down to the core rather than remained at the surface. The levels found in this layer of clay were far greater than normal levels - around 100 times more in some samples.

The Impact Site

The impact site is now identified as the Chicxulub crater on Mexico’s Yucatán Peninsula, with its center lying offshore. This crater was first discovered in 1978 by Glen Penfield, who was working for an oil company. While analyzing data from the area to look for locations for oil drilling, he noticed magnetic anomalies in a semi-circular shape. After obtaining a gravity map of the area, he found similar anomalies. Penfield reported his findings at a geophysics conference in 1981, but unfortunately attendance was low and so many scientists at the time remained unaware of his findings and continued to search for a crater. One of these was a graduate student from the University of Arizona called Alan R. Hildebrand, who at the time was looking for the crater in Texas. In 1990, a journalist who had attended Penfield's lecture at the 1981 conference told Hildebrand of Penfield's findings. The two researchers then got in contact and began to work together on drill samples from the site. They found samples of the right age with shocked quartz - confirming their suspicions that this was the site of an asteroid impact.

Current evidence suggests that the crater is approximately 300km in diameter. Estimates of the energy released by the impact vary, but it likely released an amount of energy equivalent to around 100 terratons (around 100 trillion metric tons) of TNT. For comparison, this is over a billion times more energy than was released by the atomic bomb dropped on Hiroshima during World War II.

Effects of the Impact

The asteroid impacted the Earth at a speed of around 20 kilometres per second (72,000 km/h - nearly 60 times the speed of sound), and the effects were severe and global. The impact created an enormous cloud of debris and dust which would have covered the Earth for a year or more, blocking out a significant amount of sunlight. This greatly inhibited photosynthesis, causing the loss of many species of plants and plankton. This, in turn, caused the extinction of many other species further up the food chain.

Large quantities of gypsum (a mineral containing sulfate) evaporated at the site of the impact. This released a large amount of sulfur into the atmosphere, forming sulfuric acid clouds which further blocked sunlight, causing a severe decrease in temperatures worldwide which lasted for several years. The resulting acid rain also acidified the oceans, and some researchers have suggested that this may have killed off certain marine species which had calcium carbonate shells (though this is debated).

The impact also created huge tsunamis - a 2022 study estimated waves of around 1.5km in height, and calculated that the initial energy in the impact tsunami was up to 30,000 times that of the December 2004 Indian ocean tsunami (one of the largest tsunamis in modern history, which killed over 230,000 people).

The initial blast created extremely hot plasma - around 10,000 degrees - though this was short-lived. This, along with high-velocity winds (estimated wind speeds of over 1,000 km/h) and shockwaves would have been lethal to any life close to the impact site. The seismic event caused by the impact was roughly equivalent to an earthquake of magnitude 9-11, with some researchers suggesting this may have triggered worldwide volcanic eruptions.

As larger pieces of debris fell back to Earth, they heated up, causing increased temperatures. There is debate around the short-term effects of this, with some models suggesting this may have caused widespread global wildfires. However, recent studies have suggested that the first falling spherules of debris may have formed an opaque cloud which shielded the Earth from some of the heat from further falling debris - and that this "self-shielding" effect may have prevented global wildfires, though some localised fires may well have occurred. The models in this study suggest oven-like temperatures, with Dr. Tamara J Goldin stating to space.com "If you were on the ground, it would feel at a maximum like you're under a broiler in your oven". These temperatures likely lasted for at least 20 minutes and possibly several hours.

The clouds of dust and sulfur covering the planet after the impact caused a large temperature drop worldwide, with freezing and near-freezing temperatures estimated to have lasted for 3-15 years. This would have been devastating for many species, especially as the climate at the end of the Cretaceous was generally quite warm. A 2017 study estimated that global surface temperatures dropped by over 20°C and took around 30 years to recover. Following this long winter, there were several decades (if not longer) of global warming caused by greenhouse gases produced by the impact - with some models estimating this greenhouse warming lasted for thousands of years.

The Extinction Toll

As mentioned earlier, this massive extinction event caused the loss of all non-avian dinosaurs. A few lineages of birds survived including relatives of ostriches, ducks and chickens. However, many species of birds were lost - in the Cretaceous there were several species of toothed bird but these all perished after the asteroid impact. Researchers have suggested this may have been due to differences in diet, with toothless birds faring better because they ate seeds & nuts. This would have given them an advantage after the impact, when there was less light available for plants to grow.

While many plant species died out, especially ones close to the impact site, several lineages of angiosperms (flowering plants) survived as their seeds can survive dormant for many years.

Pterosaurs (flying reptiles, sometimes incorrectly called "pterodactyls") died out along with marine reptiles such as mosasaurs and plesiosaurs. Many species of fish, plankton and mollusks - including ammonites - went extinct. Many, but thankfully not all, species of mammals died out, along with many insects and lizards.

What Survived (and How)?

Despite the widespread devastation, some species made it through. Of course the impact was still a very bad day for most living things, and within the groups that did survive many species, and individuals, did perish. Why some groups survived and others died is a complex and open question, but some factors that researchers have suggested include:

• Diet: Many food chains were devastated. Most living land plants died, which led to extinctions further up the food chain. Animals which could eat seeds, nuts, insects, etc. had a distinct advantage. Groups like crocodilians, which are not picky and would eat many different animals, would also have fared better than more specialized predators. Mammals which survived were probably omnivorous with diets including worms, insects, etc. There is no evidence of any strictly carnivorous or herbivorous mammals surviving.
• Burrowing: The ability to dig and go underground likely helped mammals to survive the extreme temperatures following the impact. Aquatic or semi-aquatic animals likely had an advantage also.
• Size: Smaller animals had an advantage in terms of being able to get underground. Mammals which survived were likely quite small, something resembling a rat. Smaller animals also typically have lower food requirements, and can reproduce and grow more quickly - allowing them to replenish their numbers more quickly.

The Age of the Mammals

Just as the Mesozoic era is known as the Age of Dinosaurs, the following Cenozoic era became the Age of Mammals. The mass extinction event created opportunities for some groups which did not die out to diversify and fill vacant ecological niches. Mammals radiated and diversified into many forms such as primates, whales and bats. Birds filled aerial niches once held by pterosaurs. While marine reptiles such as mosasaurs and plesiosaurs were lost, many other groups survived including fish, turtles, crocodilians, and sharks (which became much more common in the oceans).

Could It Happen Again?

A similar asteroid strike today would be catastrophic. Fortunately, the chances of such an event in your lifetime are extremely low. NASA’s Sentry system currently tracks over 90% of large (>1km) asteroids, and none are on a collision course with Earth.

But if we did find a new large asteroid on a collision course with Earth, what could we do about it? Currently, we are not well prepared - but we're working on it. In 2022 NASA successfully deflected a 177m-wide asteroid, Dimorphos, by kinetic impaction (essentially, hitting it with a high-speed spacecraft). While this mission was a big success, and NASA has another mission planned for 2025, these asteroids are significantly smaller than the one that killed the dinosaurs. Another option is to use nuclear weapons - a spacecraft carrying the nuclear device would be launched and then detonated close to the asteroid. The radiation would vaporize some material on the surface of the asteroid, which would then shoot off - deflecting the asteroid. This method could also be used to simply "blow up" smaller asteroids into small chunks which would burn up in Earth's atmosphere. This method does carry a risk, however, of creating large chunks which could move in unpredictable ways. We are, unfortunately, better at blowing things up than we are at predicting the size and velocity of the resulting pieces.

Whichever method we chose, time would be crucial. NASA has stated it would need at least 5 to 10 years of warning to be realistically able to deflect any large asteroid - while not impossible, deflecting a large asteroid with only a year or two of warning would be extremely difficult. So our best defense is to continue monitoring for these large impacts, as well as research into methods of deflecting them. But for at least the next 100 years, we can be reasonably confident there will be no large impacts.